B.S.ABDUR RAHMAN UNIVERSITY (B.S. ABDUR RAHMAN INSTITUTE OF SCIENCE & TECHNOLOGY)

(Estd. u/s 3 of the UGC Act. 1956) www.bsauniv.ac.in

GROWTH AND INVESTIGATION OF THIRD

ORDER NONLINEAR OPTICAL PROPERTIES

OF PURE AND DOPED META-NITROANILINE,

POTASSIUM DICHROMATE, L-ALANINE SINGLE

CRYSTALS

A THESIS

Submitted by

THILAK T.

Under the guidance of

Dr. M. BASHEER AHAMED

in partial fulfillment for the award of the degree of

DOCTOR OF PHILOSOPHY

in

PHYSICS

February 2015

ii

iii

iv

B.S.ABDUR RAHMAN UNIVERSITY

(B.S. ABDUR RAHMAN INSTITUTE OF SCIENCE & TECHNOLOGY) (Estd. u/s 3 of the UGC Act. 1956)

www.bsauniv.ac.in

BONAFIDE CERTIFICATE

Certified that this thesis report GROWTH AND INVESTIGATION OF

THIRD ORDER NONLINEAR OPTICAL PROPERTIES OF PURE AND

DOPED META-NITROANILINE, POTASSIUM DICHROMATE, L-ALANINE

SINGLE CRYSTALS is the bonafide work of THILAK T. (RRN: 0990201)

who carried out the thesis work under my supervision. Certified further, that

to the best of my knowledge the work reported herein does not form part of

any other thesis report or dissertation on the basis of which a degree or

award was conferred on an earlier occasion on this or any other candidate.

SIGNATURE SIGNATURE

Dr. M. BASHEER AHAMED DR. I. RAJA MOHAMED

RESEARCH SUPERVISOR HEAD OF THE DEPARTMENT

Professor Professor and Head

Department of Physics Department of Physics

B.S. Abdur Rahman University B.S. Abdur Rahman University

Vandalur, Chennai-600 048 Vandalur, Chennai-600 048

v

ACKNOWLEDGEMENT

I express immense gratitude to my research supervisor

Dr. M. Basheer Ahamed, Professor, Department of Physics,

B.S. Abdur Rahman University, Vandalur, Chennai 600 048, for his valuable

guidance and kind attention during all stages of this research work.

I sincerely thank Dr. P. Murugakoothan, Associate Professor,

PG & Research Department of Physics, Pachaiyappa’s College, Chennai-

600030, for his inspiring guidance and moral support during my research

work.

My sincere thanks to Dr. G. Vinitha, Assistant Professor, Division of

Physics, S.A.S, V.I.T. University, Chennai, for her help and encouragement.

I sincerely thank the Secretary, Correspondent, Principal, First year

HOD and the Staff of the Basic Department of Science and Humanities,

Sri Ramanujar Engineering College, Kolapakkam, Chennai-600 127.

I also sincerely acknowledge the assistance extended to me by the

Library staff members of the B.S. Abdur Rahman University, Sri

Ramanujar Engineering College and Anna University of Chennai, for their

time bound help to collect primary and secondary sources in writing up of this

research work.

I wholeheartedly acknowledge my Doctoral Committee members:

Professor Dr.R. Jayavel, Director, Centre for Nano science and Technology,

Anna University, Chennai-600 025, and Dr. S. Rajasekaran, Professor,

Department of Mathematics, B.S. Abdur Rahman University, Vandalur,

Chennai-600 048.

I gratefully acknowledge Dr. I. Raja Mohamed, Professor and

Head, Department of Physics, B.S. Abdur Rahman University, Vandalur,

Chennai-600 048.

vi

I sincerely thank Dr. V.M. Periasamy, Pro-Vice Chancellor,

B.S. Abdur Rahman University, Vandalur, Chennai-600 048, for his

motivation and encouragement throughout this research work.

My profound thanks to Professor Dr. R. Raja Prabu, Dean (AR) and

Professor Dr. I. B. ShameemBanu, Dean (SPCS) for their support, and

valuable suggestions for my research work.

I would like express my sincere thanks to Dr.S.Krishnan Assistant

professor, Department of Physics, B.S. Abdur Rahman University, Vandalur,

Chennai-600048.Dr.M. Anbuchezhiyan, Assistant Professor, Department of

Physics, Valliammai Engineering College, Kattankulathur and Dr.M.Vadivelu

Assistant Professor, Department of Physics, G.K.M College of Engineering

and Technology, Perungalathur, for the favorable help in my research work.

I gratefully acknowledge the use of the characterization facilities at

SAIF, IIT - Madras; Department of Nuclear Physics, University of Madras;

Department of Physics St.Joseph’s Engineering College, Trichy; and

Department of Nano Technology, B.S.Abdur Rahman University, Vandalur,

Chennai-600 048.

My heartfelt thanks to my family and my friends for their care, affection

and encouragement.

THILAK .T

vii

ABSTRACT

In this research work, an intensive analysis is done in the field of

crystal growth related to crystallography and material science. In the past,

there has been a growing interest in crystal growth process, particularly in

view of the increasing demands for materials for technological applications.

In recent years, nonlinear optical materials have attracted many researchers

owing to their application to high-speed all-optical switching devices. The

development of nonlinear optical materials led to its suitability for various

applications in frequency conversion, optical telecommunication, image

processing, optical computing and data storage devices. The nonlinear

optical (NLO) crystals are the important materials for the development of

laser science and technology because it is almost the only kind of materials

to change frequency of laser beam, modulated it in amplitude and phase.

Second harmonic generation (SHG) is a nonlinear optical process which

converses input optical wave into an output. The third order nonlinear optical

properties, such as nonlinear refractive index 2,n

nonlinear absorption

coefficient , nonlinear susceptibility 3 can be determined using Z-scan

techniques. In the present analysis, the growth and characterization of pure

m-Nitroaniline (mNA), and potassium doped m-Nitroaniline (mNAK) single

crystals, potassium dichromate single crystal (KDC), L-alanine single (LA)

crystals are examined. The grown crystals were subjected to structural,

optical, mechanical and thermal studies. The third order nonlinear optical

studies were done using continuous wave Nd: YAG Laser.

Pure and potassium doped meta-Nitroaniline single crystal were

grown by slow evaporation technique under room temperature using

methanol as a solvent. The presence of potassium in the crystal was

identified by energy-dispersive X-ray diffraction (EDAX). The single crystal X-

ray diffraction (SXRD) and powder X-ray diffraction (PXRD) studies reveal

the growth of mNA, mNAK crystals belong to the orthorhombic system having

non-centrosymmetric with space group Pbc21. The presence of functional

groups is estimated qualitatively by using Fourier transform infrared (FT-IR)

analysis. The thermal stability of the crystal is examined by thermo

viii

gravimetric analysis (TGA) and differential thermal (DTA) analysis. The

mechanical properties of the crystals are analyzed using Vicker’s micro

hardness study. The linear optical properties of the grown crystals are

studied using UV-vis-NIR spectrometer. The examined UV-vis-NIR spectrum

analyses the UV cut-off wavelength and optical band gap. The refractive

index and extinction coefficient is calculated from the transmission data of

mNAK crystal. The second harmonic generation (SHG) efficiency is

determined by using Kurtz and Perry powder technique and is compared with

the standard Potassium dihydrogen phosphate (KDP) crystal. The third order

nonlinear optical properties were determined using the Z-scan technique.

The single crystal of potassium dichromate (KDC) is also successfully

grown from an aqueous solution using slow evaporation technique. The

single and powder X- ray diffraction studies confirm the grown crystal triclinic

structure with centrosymmetric space group 1P . The UV cut-off wavelength is

found to be 240 nm. The thermal stability of the crystal is studied by thermo

gravimetric analysis (TGA) and differential thermal (DTA) analysis. The

mechanical strength of the grown crystal is carried out by Vickers micro

hardness test and the crystal perfection is confirmed by etching studies. The

absence of second harmonic generation (SHG) in the material confirms the

centrosymmetric nature. The closed aperture Z-scan studies reveal the

negative nonlinearity in the crystal and the open aperture Z-scan reveals

saturation absorption. The third order nonlinear parameters values are

evaluated for the grown KDC single crystal.

In a similar method, nonlinear optical single crystals of L-alanine have

been grown from an aqueous solution of potassium dihydrogen phosphate

(KDP). Single crystal X-ray diffraction confirms the orthorhombic structure of

the grown crystal. Fourier transform infrared (FT-IR) studies reveal the

presence of functional groups present in the grown crystal. The optical

transmitance study reveals the very good transparency of the crystal and its

optical band gap is found to be 4.9 eV. The thermal stability of the grown

crystal is found to be 288.7 °C. The second harmonic generation (SHG) of

the material was investigated using Nd: YAG laser. The Z-scan

ix

measurements further confirm that the material exhibit large third order non-

linear optical properties. The third order non-linear parameters are highly

encouraging for the grown crystal.

x

TABLE OF CONTENTS

CHAPTER NO.

TITLE PAGE NO.

ACKNOWLEDGEMENT v

ABSTRACT vii

LIST OF TABLES xvi

LIST OF FIGURES xvii

LIST OF SYMBOLS xx

LIST OF ABBREVIATIONS xxiii

1. INTRODUCTION 1

1.1 INTRODUCTION OF CRYSTAL GROWTH 1

1.2 GENERAL CRYSTAL GROWTH METHODS 2

1.2.1 High- temperature solution growth 3

1.2.2 Hydrothermal growth 3

1.2.3 Gel growth 4

1.3 LOW-TEMPERATURE SOLUTION GROWTH 4

1.3.1 Slow cooling method 5

1.3.2 Slow evaporation method 5

1.3.3 Temperature gradient method 6

1.4 FUNDAMENTAL OF SOLUTION GROWTH 6

1.4.1 Nucleation 6

1.4.2 Ostwald’s contributions 7

1.4.3 Supersaturation 7

1.5 OPTIMIZING SOLUTION GROWTH METHOD 11

1.5.1 Solvent selection 11

1.5.2 Preparation of solutions 11

xi

CHAPTER NO.

TITLE PAGE NO.

1.5.3 Seed preparation 12

1.5.4 Agitation 12

1.5.5 Crystal habit 13

1.5.6 Factors that influence the perfection of

crystal

13

1.6 SCOPE OF THE THESIS 14

1.7 OUTLINE OF THE THESIS 15

2. LITERATURE SURVEY 17

2.1 INTRODUCTION OF NONLINEAR OPTICS 17

2.2 THEORETICAL EXPLANATION OF

NONLINEAR OPTICS

18

2.3 THE TYPES OF NONLINEAR OPTICAL

EFFECTS

21

2.3.1 Second harmonic generation 22

2.3.2 Sum frequency generation 23

2.3.3 Difference frequency generation 23

2.3.4 Optical parametric generation 24

2.3.5 Linear electro optic effect 24

2.3.6 Optical Rectification 24

2.4 LINEAR AND NONLINEAR OPTICAL

PHENOMENA

25

2.4.1 Linear optical phenomena 25

2.4.2 Nonlinear optical phenomena 25

2.5 NONLINEAR OPTICAL CRYSTALS

27

xii

CHAPTER NO.

TITLE PAGE NO.

2.6 THE CLASSIFICATION OF NONLINEAR OPTICAL CRYSTALS

29

2.6.1 Inorganic crystal 29

2.6.2 Organic crystals 30

2.6.3 Semi organic crystal 32

2.7 REVIEW OF Z-SCAN STUDIES 32

3. CHARACTERIZATION TECHNIQUES 34

3.1 INTRODUCTION 34

3.2 X-RAY DIFFRACTION ANALYSIS 35

3.2.1 X-ray diffraction and Bragg’s Law 36

3.2.2 Single crystal X-ray diffraction(SXRD)

analysis

37

3.2.3 Powder X-ray diffraction (PXRD)

analysis

39

3.3 FOURIER TRANSFORM INFRARED

SPECTRAL (FT-IR)ANALYSIS

40

3.4 OPTICAL STUDIES 42

3.5 THERMAL ANALYSIS 44

3.5.1 Thermo gravimetric analysis (TGA) 45

3.5.2 Differential thermal analysis (DTA) 45

3.6 MECHANICAL STUDY 46

3.7 SECOND HARMONIC GENERATION (SHG)

MEASUREMENT TECHNIQUE

48

3.8 Z-SCAN TECHNIQUE 50

3.8.1 Z-scan technique experimental setup 52

xiii

CHAPTER NO.

TITLE PAGE NO.

3.8.2 Closed aperture Z-scan 53

3.8.3 Open aperture Z-scan 53

3.8.4 Nonlinear absorption 53

3.8.5 Nonlinear Optical Susceptibility 3 54

3.9 ENERGY-DISPERSIVE X-RAY (EDX)

ANALYSIS

55

3.10 ETCHING STUDIES 55

4. GROWTH, THERMAL, MECHANICAL, LINEAR AND

NONLINEAR OPTICAL PROPERTIES OF PURE,

POTASSIUM DOPED META-NITROANILINE

SINGLE CRYSTALS

57

4.1 INTRODUCTION 57

4.2 CRYSTAL GROWTH 58

4.3 RESULTS AND DISCUSSIONS 59

4.3.1 Energy-dispersive X-ray (EDX)

Spectroscopy

59

4.3.2 Single and powder X-ray diffraction

studies

60

4.3.3 FT- IR analysis 64

4.3.4 Thermal analysis 65

4.3.5 Mechanical study 66

4.4 LINEAR AND NONLINEAR OPTICAL

STUDIES

68

4.4.1 UV-vis-NIR transmittance study 68

xiv

CHAPTER NO.

TITLE PAGE NO.

4.4.2 Determination of optical band gap 69

4.4.3 Determination of optical constants 70

4.5 NONLINEAR OPTICAL STUDIES 73

4.5.1 Second harmonic

generation(SHG)analysis

73

4.5.2 Z- scan study 74

4.6 CONCLUSION 80

5. GROWTH, AND CHARACTERIZATION OF

POTASSIUM DICHROMATE (KDC) SINGLE

CRYSTAL

81

5.1 INTRODUCTION 81

5.2 CRYSTAL GROWTH 82

5.3 RESULTS AND DISCUSSION 82

5.3.1 Single crystal and powder X-ray

diffraction studies

82

5.3.2 Optical absorption study 84

5.3.3 Thermal analysis 85

5.3.4 Mechanical study 86

5.3.5 Etching analysis 87

5.4 NONLINEAR OPTICAL (NLO) STUDIES 88

5.4.1 Second harmonic generation (SHG)

study

88

5.4.2 Z-Scan study 88

5.5 CONCLUSION 91

xv

CHAPTER NO.

TITLE PAGE NO.

6. EFFECT OF KDP ON THE GROWTH, THERMAL

AND OPTICAL PROPERTIESOF L-ALANINE

SINGLE CRYSTAL

92

6.1 INTRODUCTION 92

6.2 EXPERIMENTAL PROCEDURE 93

6.3 RESULTS AND DISCUSSION 94

6.3.1 Single crystal X-ray diffraction 94

6.3.2 FT-IR spectral analysis 94

6.3.3 Transmittance spectral study 95

6.3.4 Thermal analysis 97

6.4 NONLINEAR OPTICAL STUDIES 97

6.4.1 Second harmonic generation (SHG)

study

97

6.4.2 Third harmonic optical studies 98

6.5 CONCLUSION 101

7. SUMMARY AND SUGGESTIONSFOR FUTURE

WORK

102

7.1 SUMMARY 102

7.2 SUGGESTIONS FOR FUTURE WORK 105

REFERENCES 106

LIST OF PUBLICATIONS 126

TECHNICAL BIOGRAPHY 128

xvi

LIST OF TABLES

TABLE NO.

TITLE PAGE NO.

2.1 Optical effects of nonlinear materials 21

4.1 Elemental composition present in mNAK crystal 59

4.2 Structural data for mNA and mNAK single crystals 61

4.3 Indexed powder X-ray diffraction data of mNA crystal 62

4.4 Indexed powder X-ray diffraction data of mNAK

crystal

63

4.5 Comparisons of different NLO crystals compound

(SHG) efficiency

74

4.6 The nonlinear parameters values of mNA and mNAK

crystals

80

5.1 Structural data for KDC single crystal 84

5.2 Nonlinear optical parameters of KDC single crystal 89

6.1 Nonlinear optical parameters of L-alanine single

crystal

99

xvii

LIST OF FIGURES

FIGURE

NO. TITLE

PAGE

NO.

1.1 Solubility diagram showing different levels of the

saturation

7

1.2 Change in free energy due to the formation of the

nucleus

10

2.1 Schematic diagram of SHG 22

2.2 Schematic diagram of sum frequency generation 23

2.3 Schematic diagram of difference frequency generation 24

2.4 Schematic diagram of optical parametric generation

i

24

3.1 Diffraction of X-rays from the planes of atoms in a

crystalline solid

36

3.2 Bruker AXS diffractometer 38

3.3 Perkin Elmer FT-IR instrument 42

3.4 UV -vis- NIR spectrometer 44

3.5 TGA-DTA analyser 46

3.6 Functional block diagram for SHG determination 50

3.7 Experimental set up for Z-scan technique 52

4.1 As grown crystals of (a) pure mNA (b) mNAK 58

4.2 EDX spectrum of mNAK crystal. 59

xviii

FIGURE

NO. TITLE

PAGE

NO.

4.3 Indexed powder XRD patterns of mNA and mNAK

crystals

61

4.4 FT-IR spectrum of mNA and mNAK crystals 64

4.5 TG- DTA curve of mNAK crystal 65

4.6 Microhardness value vs Load for mNA and mNAK

crystals

67

4.7 Log( P) vs log (d) of mNA and mNAK crystal 67

4.8 Transmittance spectrum of mNA and mNAK crystals 68

4.9 The plot of h vs (h)2 (eV) 70

4.10 Extinction coefficient and Refractive index vs

wavelength of mNAK crystal

71

4.11 Plot of optical conductivity against absorption

coefficient for mNAK crystal

72

4.12 (a)Closed aperture curves (s = 0.035) of mNA crystal

(b) Open aperture curve (s = 1) of mNA crystal

(c) Ratio curve for mNA crystal

77

(d) Closed aperture curves (s = 0.035) of mNAK

crystal

(e) Open aperture curve (s = 1) of mNAK crystal

(f) Ratio curve for mNAK crystal

79

5.1 As grown single crystal of KDC 82

5.2 Powder X-ray diffraction spectrum of KDC crystal 83

xix

FIGURE

NO. TITLE

PAGE

NO.

5.3 UV-vis-NIR spectrum of KDC crystal 85

5.4 TG-DTA curves for KDC crystal 86

5.5 Vicker’s hardness number (HV) vs Load (P) of KDC 87

5.6 Photograph of the etching pattern for (a) 10s, (b) 30s 88

5.7 (a) Closed aperture Z-scan curve for KDC crystal

(b) Open aperture curve for KDC crystal

(c) Ratio curve of KDC crystal

90

6.1 As grown L-alanine single crystal 93

6.2 FT-IR Spectrum of L- alanine crystal 95

6.3 Transmittance spectrum of L-alanine crystal 96

6.4 Plot of h vs 2( )h for L-alanine crystal 96

6.5 TG-DTA curve for L-alanine crystal 97

6.6 (a) Closed aperture curve (s=0.035) of

L- alanine crystal

(b) Open aperture curve(s=1) of L-alanine

crystal

(c) Ratio curve for L-alaninecrystal

100

xx

LIST OF SYMBOLS

S - Super saturation

C - Actual concentration

C* - Equilibrium saturation concentration

C - Concentration driving force

sG - Surface free energy

vG - Volume free energy

- Interfacial tension

K - Boltzmann constant

V - Molar Volume

n2 - Third order nonlinear refractive index

- Nonlinear absorption coefficient

(3) - Nonlinear susceptibility

E - Electric field vector

0 - Permittivity of free space

K - Phase-mismatch

- Conversion coefficient

xxi

effd - Effective nonlinear coefficient

Hv - Vickers hardness number

P - Applied load

d - Diagonal length

- Linear absorption coefficient

(3)Im - Imaginary part third order nonlinear susceptibilities.

(3)Re - Real part third order nonlinear susceptibilities.

Eg - Optical band gap

n - Change in refractive Index (Tp-Tv)

p vT - Change in normalized transmittance between peak

and valley

- Molar extinction coefficient

0 - Permittivity of free space

- Wavelength

- Change in absorption coefficient

0 - Gaussian beam spot radius at focus (half width at

1/e2

of maximum of the irradiance)

I - Irradiance

xxii

I0 - Peak on axis irradiance at focus

K - Wave Vector

effL - Effective sample thickness

Z - Position of the sample with respect to the focal

position

ZR - Rayleigh length

0 - Variation of this quantity as a function

Esu - Electro static unit

cm/W - Centimeter per Watt

cm2/W - Centimeter square per Watt

°C - Celsius

mm - Meli meter

- Linear susceptibility

- Frequency

p - Pumping wavelength

s - Signal wavelength

xxiii

LIST OF ABBREVIATIONS

NLO - Nonlinear optics

SHG - Second harmonic generation

XRD - X-ray diffraction

FT-IR - Fourier transform infrared

mNA - meta-Nitroaniline

mNAK - Potassium doped meta-Nitroaniline

KDC - Potassium dichromate

LA - L-alanine

UV - Ultraviolet

IR - Infrared

TGA - Thermo gravimetric analysis

DTA - Differential thermal analysis

Nd:YAG - Neyodimium yttrium aluminium garnet

NLA - Nonlinear absorption

NLR - Nonlinear refraction

EDX - Energy dispersive X-ray spectroscopy

LiNbO3 - Lithium niobate

OPA - Optical parametric amplification

OPO - Optical parametric oscillation

xxiv

SXRD - Single crystal X-ray diffraction

PXRD - Powder X-ray diffraction

KDP - Potassium dihydrogen phosphate

KTP - Potassium titanyl phosphate

ADP - Ammonium dihydrogen phosphate

BBO - Beta barium borate

KNbO3 - Potassium niobate

LBO - Lithium tri borate

LAP - L-arginine phosphate monohydrate

NTF - Nicotinium tri fluroacetate

LACC - L-alanine cadmium chloride

KB5 - Potassium pentaborate

LiIO3 - Lithium iodate

DLAP - Deuterated L-arginine phosphate

2APPTS - 2-Aminopyridinium p-Toluenesulfonate

TGS - Tri glycine sulphate

1

1. INTRODUCTION

1.1 INTRODUCTION OF CRYSTAL GROWTH

Crystal growth is one of the most important fields of material science,

which involves controlled phase transformation. Fundamental experimental

aspects of crystal growth were derived from early crystallization in the

Eighteenth and Nineteenth Century. The phenomena of under cooling,

supersaturation and the heat of crystallization were recognized in the

Eighteenth Century [1]. Theoretical understanding started with the

development of thermodynamics in late Nineteenth Century and with the

development of nucleation and crystal growth theories. The critical one to

achieve higher quality crystal and the role of transport phenomena was a

unique feature of the Twentieth Century. In the past, there was a growing

interest in crystal growth process, particularly in view of the increasing

demand for materials for technological applications [2-4]. Therefore,

researchers worldwide have always been in search of new materials through

their single crystal growth. The methods of growing crystals are very wide

and mainly dictated by the characteristics of the material and its size [5, 6].

The single crystal technology is the mother of all the recent technologies and

modern science. In the modern world, the large-scale uses of crystals are

brought about mainly by the demands of solid state materials for research

and developments in physics. Varieties of crystals are needed to meet some

very important gaps in conventional production in engineering. The several

kinds of single crystals and its applications are found in the development of

technologies such as; laser, semiconductor, high and low energy particles in

physics, nuclear fusion, medical diagnostics, display, and thermal imaging

[7].The contribution of crystals in electrical industry, photonic industry, optical

fiber communications, which depends on materials/crystals, such as;

semiconductors, superconductors, transducers, polarizer‟s, ultrasonic

amplifiers, radiation detectors, ferrites, magnetic garnets, solid state lasers,

photosensitive, nonlinear optics, piezo-electric, acoustic-optic, electro-optic,

refractory of different grades, crystalline films for microelectronics and

computer industries. The crystal growth is an interdisciplinary subject which

2

covers physics, chemistry, material science, chemical engineering,

metallurgy, crystallography, mineralogy, etc. In the past, there had been a

growing interest in crystal growth processes, particularly in view of the

increasing demand of materials for technological applications. [4,8].

The new solid state and the single crystals explosion lead to the

invention of the transistor in 1948. Many new crystals are grown and

fabricated in order to assess the device properties. The application of

semiconductor based on electronics created an enormous demand for high

quality of semiconductor, ferroelectric, piezoelectric, oxide single crystal [9].

The crystal growth plays an important role in the area of immense

technological excellence which is useful for many crystals in important areas

of service to the humanity, medicine, engineering, technology and also

strategic areas of defense and space science. In addition, many crystals are

useful as elements in acoustic-optic, piezoelectric, photo-refractive, photo-

elastic applications and also as radiation detectors, laser hosts, transducers,

parametric amplifiers, Bragg cells, harmonic generators etc., [10-12].

1.2 GENERAL CRYSTAL GROWTH METHODS

The mode of selecting a particular method of crystal growth

techniques depends on the properties of the materials, such as melting point,

vapour pressure, decomposition, solubility in solvents etc. The growth

methods depend on growth kinetics, crystal size, shape and nature of the

application of the crystals. The method should also be economically feasible

and the crystal formed should be free from defects.

The consistency and the characteristics of devices fabricated from the

crystals depend on the homogeneity and defect contents. The process of

producing single crystals, which offers homogeneous media on the atomic

level with directional properties, attracts more attention than any other

process. The methods of growing crystals are wide and mainly dictated by

the characteristics of the material and its size [5].

The methods of growing single crystals may be classified according to

their phase transformation as given below [13].

3

Solid state process Solid- solid phase transformation

Solution growth process Liquid- solid phase transformation

Vapour growth process Vapour-solid phase transformation

Based on the phase transformation processes, crystal growth

technique are classified as solid growth, vapour growth, melt growth and

solution growth.

1.2.1 High- temperature solution growth

Hydrothermal and flux growths from the category of high temperature

solution growth. They are studied at a large scale in this method, a solid

molten salt / flux is used as the solvent instead of liquid. The growth takes

place below the melting point of the solute [14]. This technique could be

applied to incongruently melting materials. Mixed crystals of solid solution

could also be grown by the choice of optimum growth parameters. The same

techniques could be used for the crystallization of oxide compounds, which

generally have the high melting point as well as for materials which undergo

phase transitions below the melting point. The crystals were grown from

these methods usually have the lower concentration of equilibrium defects

and lower dislocation density.

1.2.2 Hydrothermal growth

The hydrothermal crystal growth may be defined as the use of an

aqueous solvent at high-temperature and pressure to crystalline the sparingly

soluble materials at low-temperatures. The substances like calcite, quartz

etc., are considered insoluble in water, but high-temperature and pressure

these substances are soluble. This method of crystal growth at high-

temperature and pressure is known as hydrothermal method. Temperature

typically in the range of 400 °C to 600 °C and the pressure involves is about

hundreds of atmosphere. Growth is carried out in steel autoclaves with silver

or gold linings. Depending on the pressure the autoclaves are grouped into

low, medium and high-pressure autoclaves. The concentration gradient

4

required to produce growth is provided by a temperature difference between

the nutrient and growth areas. In this technique, very few crystals are grown

the good quality and large dimensions. Quartz is an outstanding example of

industrial hydrothermal crystallization.

1.2.3 Gel growth

Gel growth is a very convenient laboratory process [15]. It gave an

excellent survey of this process. Only small crystals can be grown by this

technique. Gels are two-phase systems comprising a porous solid with liquid

filled pores. The pore dimensions depend on the concentration of the gel

material. The most frequently used gels are based on silica. But gels based

on gelatin, various soft soaps and pectin are also used. Seed crystals should

be used to reduce flaws at the center of the crystals. It is very important to

provide constant ambient temperature.

1.3 LOW-TEMPERATURE SOLUTION GROWTH

The growth of crystals from aqueous solution is one of the classical

methods of crystal growth. The low- temperature solution growth is one of the

popular methods to produce technologically important crystals. This method

demands that the materials must crystallize from solution with prismatic

morphology. In general, this method involves seeded growth from a saturated

solution. The driving force i.e, the supersaturation is achieved either by

temperature lowering or by solvent evaporation. This method is widely used

to grow bulk crystals, which have high solubility and have variation in

solubility with temperature [16]. After many modifications and refinements,

the process of solution growth now yields good quality crystals for a variety of

applications. The growth of crystals from solution at room temperature has

many advantages over other growth methods though the rate of

crystallization is slow. The mechanism of crystallization from solutions is

governed, in addition to other factors, by the interaction of ions or molecules

of the solute and the solvent, which is based on the solubility of substance on

the thermodynamical parameters of the process pressure, temperature, and

solvent concentration [17].

5

Low-temperature solution growth could be sub-divided into the

following methods:

(i) Slow cooling method

(ii) Slow evaporation method

(iii) Temperature gradient method

1.3.1 Slow cooling method

The slow cooling method is the best way to grow crystals by solution

growth technique. The main disadvantage of slow cooling is the need to use

a particular range of temperature. The possible range of temperature is

usually narrow and hence, much of the solute remains in the solutions at the

end of the growth run. To compensate this effect, large volumes of the

solution is needed. A wide range of temperature may not be described

because the properties of the grown crystal may vary with temperature. Even

though the method has the technical difficulty of requiring a programmable

temperature control, it is widely used with great success. In the recent time,

many crystals grew from a slow cooling technique.

1.3.2 Slow evaporation method

The slow evaporation method is similar to slow cooling method in

terms of the apparatus concerned. In this method, the saturated solution is

kept at a particular temperature and provision is made for evaporation. If the

solvent is non-toxic like water, it is permissible to evaporate into the open

atmosphere. The typical growth conditions involve a temperature stabilization

of about 0.05°C and rate of evaporation of a few mm3/hour. The evaporation

technique has an advantage that the crystals grow at a fixed temperature.

But inadequacies of the temperature control system still have a major effect

on the growth rate. This method can effectively be used for materials having

very low- temperature coefficient of solubility. But the crystals tend to be less

pure than the crystals produced by slow cooling technique, as the size of the

crystal increases more impurities find the place in the crystal faces. This

6

method can effectively be used for materials having a very low-temperature

coefficient of solubility.

1.3.3 Temperature gradient method

The temperature gradient method involves the transport of materials

from a hot region containing a source material to be grown to a cooler region

where the solution is supersaturated and the crystal grows. The advantage of

this method is

(i) Economy of solvent

(ii) The crystal grows at fixed temperature

1.4 FUNDAMENTAL OF SOLUTION GROWTH

1.4.1 Nucleation

Nucleation is an important phenomenon in crystal growth and is the

precursor of the overall crystallization process. It is the process which

generates a metastable mother phase, the initial fragments of a new and

more stable phase capable of developing spontaneously into gross

fragments of the stable phase. It is consequently a study of the initial stages

of the kinetics of such transformations [18].

Nucleation might occur spontaneously or it might be induced

artificially. These two cases are referred to as homogeneous and

heterogeneous nucleation respectively. Both these nucleation‟s are called

primary nucleation and occur in systems that do not contain crystalline

matter. On the other hand, nuclei are often generated in the vicinity of

crystals present in the supersaturated system. This phenomenon is referred

to as secondary nucleation [6]. The growth of crystals from solutions could

occur a certain degree of supersaturation or super cooling has first been

achieved in this system. There are three steps involved in the crystallization

process.

Achievement of supersaturation or super cooling

Formation of crystal nuclei

7

Successive growth of crystals to get distinct faces

1.4.2 Ostwald’s contributions

While the concept of a definite super solubility is contained in the

earliest writings on the crystal growth subject, Ostwald appears to be the first

to explain the relationship between supersaturation and spontaneous

crystallization. The relationship between the concentration and temperature

is schematically shown in Figure1.1 [19].

Meirs solubility diagram consists of three zones, which are termed as

region I, II and III respectively. Region I correspond to the under saturated

zone, where crystallization is not possible. This region is thermodynamically

stable. The region II between the super solubility curve and solubility curve is

called as metastable zone, where spontaneous crystallization is not possible,

but seeded growth could be initiated in this region. In the region III,

spontaneous nucleation is more probable, called as unstable or labile zone.

Figure. 1.1: Solubility diagram showing different levels of saturation.

1.4.3 Supersaturation

The solution in which the concentration of the solute exceeds that of

the equilibrium condition at a given temperature is known as supersaturated

8

solution. The supersaturated solution is thermodynamically unstable. The

supersaturation required for crystallization process could be achieved by (i)

slow evaporation (ii) slow cooling and (iii) by adding any external impurity.

The supersaturation achieved by slow cooling is the best method to grow

bulk size crystals by solution technique. In this case, crystallization takes

place by lowering the temperature of the solution under a controlled cooling

rate. The supersaturation can be achieved by slow evaporation of the solvent

for materials, which are having a very small temperature coefficient of

solubility. Here crystallization could take place at a constant temperature.

The supersaturation could also be achieved by the addition of some

impurities. The growth proceeds by the reduction of solubility of the solute

due to the presence of impurity. The selection of suitable growth methods

depend mainly on the shape of the solubility curve.

The degree of supersaturation (S) of a solution could be expressed by the ratio:

*

CS

C (1.1)

where, C is the actual concentration of the solution and C* is the equilibrium

saturation concentration of the solution at a given temperature. Thus, for a

saturated solution, S = 1, S < 1 denotes under saturation and S> 1 indicates

supersaturation.

The concentration driving force C is given by

*C C C (1.2)

The relative supersaturation is defined as,

C C*

C*

(1.3)

C

1C*

(1.4)

S 1 (1.5)

9

The crystallization process is initiated by the formation of embryos or

nuclei with number of micro size solid particles present in the solution,

termed as centers of crystallization. The mechanism of formation of such

embryos is the simple collision process of a single molecule A1 with a cluster

Ai-1 consisting of (i -1) molecules and thus giving rise to a cluster Ai i.e.,

i 1 1 iA A A (1.6)

The total Gibb‟s free energy change, G of the embryo between the

two phases associated with this process is given as

S VG G G (1.7)

where SG is the surface free energy and VG is the volume free energy

For the spherical nucleus of radius, r

2 3

V

4G 4 r r G

3 (1.8)

where is the interfacial tension and VG is the free energy change per unit

volume and is a negative quantity. The quantities G , SG and VG are

represented in Figure 1.2. Since the surface free energy increases with 2r

and volume free energy decreases with 3r , the total net free energy change

increases with increase in size and attains a critical size after which it

decreases.

At critical condition, the free energy of formation obeys the condition,

d G

0dr

(1.9)

Hence, the radius of the critical nucleus is expressed as

v

2r*

G

(1.10)

10

v

kT1nSG

V

(1.11)

V = molar volume of the crystal

k = Boltzmann constant

T = Temperature of the solution in K

Hence, the critical free energy barrier

3 3

2

16 VG

3

(1.12)

Figure. 1.2: Change in free energy due to the formation of the nucleus.

The number of molecules in the critical nucleus is expressed as

34 ( *)

i*3V

(1.13)

The crucial parameter between a growing crystal and the surrounding

mother solution is the interfacial tension (). This complex parameter could

be determined by conducting the nucleation experiments [3].

11

1.5 OPTIMIZING SOLUTION GROWTH METHOD

1.5.1 Solvent selection

The solution is a homogeneous mixture of a solute in a solvent. The

solute is the component present in a smaller quantity. For a given solute,

there may be different solvents. Apart from high purity starting materials

solution growth requires good solvents. The solvent must be chosen taking

into considering the following factors.

Moderate solubility for the given solute

Must not react with the solute

Good solubility gradient

Low viscosity

Low correction

If the solubility is too high it is difficult to grow bulk single crystal and if

the solubility is too small, it restricts the size and growth rate of the crystals.

The solubility data at various temperatures are essential to determine the

level of supersaturation. Hence, the solubility of the solute in the chosen

solvent must be determined before starting the growth process [5]. If the

solubility gradient is very small, slow evaporation of the solvent is the other

option for crystal growth to maintain the supersaturation in the solution. The

growth of crystal from solution is mainly a diffusion-controlled process the

medium must be less viscous to enable faster transport of the growth units

from the bulk solution by diffusion. Hence, a solvent with less viscosity is

preferable [20].

1.5.2 Preparation of solutions

Preparation of the solution to grow the desired crystal is an important

stage in solution growth. The saturated solution is filtered using the filter

paper. The filtered solution is transferred into the beaker. Extreme care is to

be taken to avoid under saturation, which results in the dissolution of the

seed crystal. Similarly, high supersaturation is also to be avoided in order to

12

prevent the formation of spurious nucleation. The growth vessel is

hermetically sealed in order to avoid the evaporation of the solvent. The

solution is tested for saturation by suspending small test seed crystal in the

solution. By adjusting the temperature, the necessary equilibrium condition is

achieved and the test seed crystal is removed and a fresh seed crystal is

introduced for crystal growth.

1.5.3 Seed preparation

The quality of the grown crystal very much depends on the quality of

the seed crystal used. Small seed crystals could be obtained by spontaneous

nucleation in the labile region of the supersaturated solution. The seed used

to grow a large uniform crystal must be a single crystal free of inclusions,

cracks, block boundaries, sharp cleaved edges, twinning and any other

obvious defects. It should be of minimum size, compatible with other

requirements. When larger crystals of the same material are already

available, they could be cut in the required orientation to fabricate the seed

crystal. Since the growth rate of the crystal depends on the crystallographic

orientation, the seed crystal must be cut in such a way that it has larger

cross-section in the fast growing direction.

1.5.4 Agitation

In order to get a regular and even growth, the level of supersaturation

is to be maintained equally on the surface of the growing crystal. An uneven

growth leads to localized stress at the surface, generating imperfection in the

bulk crystals. Moreover, the concentration gradients that exist in the growth

vessel cause fluctuations in supersaturation at different faces of the crystal,

which seriously affect the growth rate of individual faces. The degree of

formation of concentration gradients around the crystal depends on the

efficiency of agitation of the solution. This is achieved by agitating the

saturated solution in either direction at an optimized speed using a stirrer

motor.

13

1.5.5 Crystal habit

The growths of a crystal in equivalent rates along the crystallographic

directions are the prerequisite for its accurate morphology with developed

faces. This will result in a large bulk crystal from which samples of any

desired orientation could be cut. Further, such large crystals should also be

devoid of dislocation and other defects. These imperfections become isolated

from defective regions surrounded by large volumes of high perfection when

the crystal grows with a bulk habit. In the crystals, which grow as needles or

plates, the growth dislocations propagate along the principal growth

directions and the crystals remain imperfect. The needles like crystals are

very limited applications and plate-like crystals need to be favorably oriented.

Changes of habit in such crystals, which naturally grow as needles or plates

could be achieved by any one of the following ways:

Changing the temperature of growth

Changing the pH of the solution

Adding a habit modifying agent and

Changing the solvent

The achievement in this area is of great industrial importance where

such morphological changes are induced by crystallization to yield crystal

with better perfection and packing characteristics.

1.5.6 Factors that influence the perfection of crystal

In order to grow large size, well-faceted optically clear crystals there

are five basic factors need to be optimized:

The purity of the starting material

The precision of the seed

Cooling or evaporation rate

pH of the growth solution

14

The efficiency of agitation of the seed and the solution

Hence, good quality single crystals could be grown from quality seed

in an efficiency stirred the solution.

1.6 SCOPE OF THE THESIS

In the field of single crystal growth, several important contributions

have been made to grow crystals due to their potential applications in

optoelectronics, Infrared detector (IR) and nonlinear optical (NLO)

applications.

The solution growth method at room temperature and at slightly

elevated temperatures offers simple equipment to grow good quality single

crystals. The present investigation of the thesis is aimed towards the

development of nonlinear optical single crystals of organic and inorganic

materials by low-temperature solution growth method. To study the various

properties of the grown crystals, characterization studies are the assessment

technique to study the chemical composition, structure, optical properties,

physio-chemical properties etc., The single crystals chosen for the present

investigation were characterized by single crystal X- ray diffraction (SXRD)

and Powder X-ray diffraction (PXRD) studies in order to confirm the

crystalline nature and lattice parameters. The functional groups present in the

crystals were analyzed using Fourier transform infrared (FT-IR) spectral

analysis. The optical properties were examined by UV-vis-NIR spectral

analyzes and the thermal stability of the grown crystals was analyzed using

TG-DTA studies. Second harmonic generation (SHG) efficiency of the

crystals was confirmed by Kurtz and Perry powder technique. The third order

nonlinear optical properties, such as nonlinear refractive index 2n , nonlinear

absorption coefficient , and nonlinear susceptibility (3) were studied using

the Z-scan technique. The studies are carried out to determine the sign and

magnitude of nonlinearity and to investigate nonlinear refractive index and

nonlinear absorption coefficient. The magnitude and response of third order

nonlinear susceptibility are important parameters in characterizing and

determining the applicability of any materials as a nonlinear optical device.

15

In the present analysis, the growth and investigation of the third

order nonlinear optical properties of pure and potassium doped meta-

Nitroaniline, potassium dichromate, L-alanine single crystals are examined.

1.7 OUTLINE OF THE THESIS

This thesis is divided into seven chapters

Chapter I: The Chapter I discusses the brief introduction of crystals

growth. The various methods of crystal growth are explained in this chapter.

The scope of the thesis is presented in brief manner.

Chapter II: The Chapter II analyzes the theoretical concept of

nonlinear optics. The nonlinear optical (NLO) crystals built from inorganic,

organic and semi organic materials and third order nonlinear optical crystal

are included in this chapter. The literature survey is made in elaborate

manner.

Chapter III: The Chapter III illustrates an overview of various

principles and instrumentation techniques, which are used to characterize the

grown crystals.

Chapter IV: This Chapter deals with growth, thermal, mechanical,

linear and nonlinear optical properties of pure and doped meta-Nitroaniline

single crystals. The crystals are grown using solution growth technique, using

methanol as a solvent. The structure of the grown crystal were confirmed by

single crystal and powder X-ray diffraction technique, The EDX spectrum is

used to identify the doped material, potassium presentation. The linear and

nonlinear optical properties of pure and potassium doped meta-Nitroaniline

crystals. The optical transmission studies reveal that the mNA and mNAK

crystals have UV cut-off wavelength around 345 nm and 352 nm

respectively. The nonlinear optical property (NLO) of the crystal was tested

by pulsed Nd: YAG laser as a source. The third order nonlinear optical

properties were measured using single beam Z-scan technique using

continuous wave Nd: YAG laser.

16

Chapter V: The Chapter V presents the growth and characterization

of pure potassium dichromate (KDC) single crystal. The single crystal of

potassium dichromate (KDC) is grown from aqueous solution by slow

evaporation technique. The lattice parameters of the grown crystal were

determined by single and powder X-ray diffraction analysis. The optical

absorption studies reveal that the crystal has UV cut-off wavelength around

240 nm. The thermal stability of the grown crystal analyzed using TG/DTA

studies. The mechanical strength of the grown crystal was carried out by

Vickers micro hardness test. The crystal perfection was confirmed by etching

study. The third order nonlinear optical study was performed using by a

single beam Z-scan technique using continuous Nd: YAG laser. The closed

aperture Z-scan studies reveal the negative nonlinearity in the crystals and

open aperture Z-scan reveals the saturation absorption. Also, various

parameters such as nonlinear refractive index 2n , absorption co-efficient

and nonlinear optical susceptibility 3 were calculated for the grown crystal.

Chapter VI: The Chapter VI studies the effect of potassium di

hydrogen phosphate on L-alanine single crystal. The lattice parameters of the

grown crystal were determined by X-ray diffraction analysis. The optical

transmission study reveals very good transparency of the grown crystal. The

thermal stability of the grown crystal is found to be 288.7°C. The nonlinear

optical property (NLO) of the crystal was tested by pulsed Nd: YAG laser as a

source. The third order nonlinear optical studies were performed using single

beam Z-scan technique using continuous Nd: YAG laser. The closed

aperture Z-scan studies reveal the negative nonlinearity in the crystals and

open aperture Z-scan reveals the saturation absorption. The various

parameters such as, nonlinear refractive index 2n , absorption co-efficient

and nonlinear optical susceptibility 3 were calculated for the grown crystal.

Chapter VII: The Chapter VII illustrates the concluding remarks and

presents the prospect of the research work.

17

2. LITERATURE SURVEY

2.1 INTRODUCTION OF NONLINEAR OPTICS

This chapter analyzes the literature survey of the nonlinear optics in

detail. The high speed and ease of production of photons (light), the area of

photonics are the active fields of research in view of modern society‟s

demand and also for improved telecommunication, data storage, retrieving,

processing and transmission. The design of devices and utilization of

photons instead of electrons for the transmission of information has created a

need for new materials with unique nonlinear optical (NLO) material [21]. The

nonlinear optics is the field which includes all phenomena in which optical

parameters of materials are changed with the interaction of intense coherent

source of light. The nonlinear optical phenomenon leads to the enhancement

of understanding light-matter interactions. The searches for new molecular

materials with nonlinear optical properties are the subject of considerable

importance investigations in their potential applications in photonic devices

[22]. The invention of the laser has created a revolution in research and

development in the area of photonics and the investigations on nonlinear

optical properties enhanced and widened the horizon of application of lasers.

The drawback of most of the laser materials is their inability to generate the

radiation in a wide spectral region, such as for some applications the

appropriate laser materials exist and thermal properties of source materials

the emitted power is restricted. The nonlinear optics makes it possible to

transfer the energy from wavelength to another and hence provide a solution

for the radiation sources in the spectra range of radiation [23]. The nonlinear

optical (NLO) single crystals therefore, could be employed to generate the

sources of different wavelengths for which the lasers are not available. The

output radiation beam from a nonlinear optical device is the similar properties

of a laser source that could be directly employed as laser source. For certain

applications highly powerful laser radiation is used in inertial confinement

fusion research [24].The field of nonlinear optics emerged nearly five

decades ago with the development of the first operating laser and the

demonstration of frequency doubling phenomena. These milestone

18

discoveries not only created much interest in laser science, but also set the

scope for future research work in nonlinear optics. The extraordinary growth

and development of nonlinear optical materials during the past decade had

rendered photonic technologies, an indispensable part of daily life. With the

emerging demand for information systems, nonlinear optical materials are

considered as the key elements for the future photonic technologies of optical

computing, telecommunications, optical interconnects, high density data

storage, sensors, image processing, switching etc., The second harmonic

generation (SHG) is a nonlinear optical process that results in the conversion

of an input optical wave into an output, twice the input frequency. The

process occurs within a nonlinear medium, usually a crystal. The light

propagated through a crystalline solid, which lacks a centre of symmetry,

generates light at second and higher harmonics of the applied frequency.

Such frequency doubling processes are commonly used to produce green

light (532 nm), for example, an Nd: YAG (Neodymium yttrium-aluminium-

garnet) laser operating at 1064 nm. This important nonlinear property of

non-centro symmetric crystals is called second harmonic generation (SHG)

and this phenomenon and the materials in which it occurs are the subjects of

intense study [25].

2.2 THEORETICAL EXPLANATION OF NONLINEAR OPTICS

The explanation of the nonlinear effect lies in the way in which a beam

of light propagates through solid, nuclei and associated electrons of the

atoms in the solid form the electric dipoles.

The electromagnetic radiation interacts with the dipoles causing them

to oscillate, the classical laws of electromagnetism results in the dipoles that

acts as sources of electromagnetic radiation. The phase velocity and

wavelength of this electromagnetic wave are determined by 2,n the refractive

index of the doubled frequency. To obtain high conversion efficiency, the

phase vectors of input beams and generated beams are to be matched. If the

amplitude of vibration is small, the dipoles emit radiation of the same

frequency as the incident radiation. As the intensity of the incident radiation

increases, the relationship between irradiance and amplitude of vibration

19

becomes nonlinear resulting in the generation of harmonics in the frequency

of radiation emitted by the oscillating dipoles. The frequency doubling or

second harmonic generation (SHG) and indeed higher order frequency

effects occur as the incident, intensity is increased. In a nonlinear medium,

the induced polarization is a nonlinear function of the applied field. A medium

exhibiting second harmonic generation (SHG) is a crystal composed of

molecules with asymmetric charge distributions arranged in the crystal in

such a way that a polar orientation is maintained throughout the crystal.

At very low fields, the induced polarization is directly proportional to

the electric field [4].

0P E (2.1)

where is the linear susceptibility of the material, E is the electric field

vector, 0 is the permittivity of free space. At high fields, the polarization

becomes independent of the field and the susceptibility becomes field

dependent. Therefore, this nonlinear response is expressed by writing the

induced polarization as a power series in the field.

(1) (2) (3)

0( . . . ...)P E E E E E E (2.2)

In nonlinear terms, the product of two or more oscillating fields give

oscillation at combination of frequencies and therefore, the above equation

could be expressed in terms of frequency as:

(1) (2)

0 0 0 1 0 0 1 2 1 2

(3)

0 1 2 3 1 2 3

( ) {( ( ; ). ( ) ( ; , ). .

( ; , , ). . . ...)}

P E E E

E E E

(2.3)

where 2 3, …. are the nonlinear susceptibilities of the medium 1 is the

linear term responsible for material‟s linear optical properties like, refractive

index, dispersion, birefringence, and absorption 2 is the quadratic term

which describes second harmonic generation in non centrosymmetric

materials 3 is the cubic term responsible for third harmonic generation,

stimulated Raman scattering, phase conjugation and optical instability.

20

Hence, the induced polarization is capable of multiplying the fundamental

frequency to second, third and even higher harmonics. The coefficients of

2 3, ) and 3 give rise to certain optical effects, which are listed in

Table 2.1. If the molecule or crystal is centrosymmetric, then 20 , If a

field E is applied to the molecule (or medium), equation 2.3 predicts that the

polarization induced by the first nonlinear term is predicted to be 2 ,E yet if

the medium is centrosymmetric the polarization should be 2E . This

contradiction could only be resolved if 20 in centrosymmetric media. If

the same argument is used for the next higher order term, E produces

polarization 3E and E produces 3

E , so that 3 is the first non-zero

nonlinear term in centrosymmetric media.

1 22 (2.4)

During this process, a polarized wave at the second harmonic

frequency 12 is produced. The refractive index 1n is defined by the phase

velocity and wavelength of the medium. The energy of the polarized wave is

transferred to the electromagnetic wave at a frequency 2 . The phase

velocity and wavelength of this electromagnetic wave are determined by 2 ,n

the refractive index of the material corresponding to the doubled frequency.

To obtain high conversion efficiency, the phase vectors of input beams and

generated beams are to be matched.

1 2

2K 0

(n n )

(2.5)

where K represents the phase-mismatch. The phase–matching could be

obtained by angle tilting, temperature tuning or by other methods. Hence, to

select a nonlinear optical crystal, for a frequency conversion process, the

necessary criterion is to obtain high conversion efficiency. The conversion

efficiency , is given by

21

Table 2.1 Optical effects of nonlinear materials

Order Susceptibility Optical effects Applications

1 1 Refraction Optical fibers

2 2

SHG ( 2 )

Frequency mixing

1 2 3( )

Pockel effect

( 0 )

Frequency doubling

Optical parametric oscillators

Electro-optic modulators

3 3

4 wave mixing Phase gratings

Kerr effect

Optical amplitude

Raman coherent spectroscopy Real- time holography

Ultra high- speed optical gates

Amplifiers, choppers etc.

2

2 effd sin K.LPL

K.L

(2.6)

where effd is the effective nonlinear coefficient, L is the crystal length, P is

the input power density and K is the phase -mismatching. In general, higher

power density, longer crystal, large nonlinear coefficients and smaller phase

mismatching would result in higher conversion efficiency [26].

2.3 THE TYPES OF NONLINEAR OPTICAL EFFECTS

Some nonlinear optical processes are familiar to physicists, chemists,

and other scientists because they are in common use in the laboratories. The

second harmonic generation is a nonlinear optical process that results in the

conversion of an input optical wave into an output wave of twice as that of the

input frequency. The process occurs within a nonlinear medium, usually a

22

crystal (KDP-Potassium di hydrogen phosphate, KTP-Potassium Titanyl

Phosphate, etc.)., Such frequency doubling processes are commonly used to

produce green light (532nm) using, a Nd:YAG (Neodymium:Yttrium

Aluminum Garnet) laser operating at 1064 nm [27].

Some of the NLO processes are given below:

(a) Second harmonic generation

(b) Sum frequency generation

(c) Difference frequency generation

(d) Optical parametric generation

(e) Linear electro- optic effect or Pockel‟s effect

(f) Optical rectification

2.3.1 Second harmonic generation

The process transformation of light with frequency „ ‟ into the light

with double frequency 2 and half the wavelength (Figure 2.1) are referred

to second harmonic generation (SHG).The process is spontaneous and

involves three photon transitions. A second harmonic generation has been of

practical interest ever since after it was demonstrated because of its efficient

conversion from fundamental to second harmonic frequencies. This could be

achieved by the available powerful sources of coherent radiation at higher to

attainable wavelengths [28].

Figure. 2.1: Schematic diagram of SHG.

23

The most extensively studied conversion process of all has been the

doubling of the 1.064 μm line obtained from the neodymium ion in various

hosts. In particular, the doubling of the continuous wave Nd:YAG laser

source is recently the subject of intensive study, because the laser light itself

is efficient and powerful when the green light obtained by doubling is placed

spectrally for detection by photomultipliers

2.3.2 Sum frequency generation

It is a nonlinear optical process. Crystal materials with inversion

symmetry could exhibit nonlinearity. In such NLO materials, the sum

frequency generation could occur. Figure 2.2 illustrates the sum frequency

generation.

1 2 3 (2.7)

when two electromagnetic waves with the frequency 1 and 2 interaction an

NLO medium, a nonlinear polarizability could be induced. The NLO material

generates an optical wave of frequency 3 which is equal to the sum of the

two input wave frequency 1 and 2 .The energy of output wave is

represented by the equation 2.7.

Figure. 2.2: Schematic diagram of sum frequency generation.

2.3.3 Difference frequency generation

The process of difference-frequency generation is described by the

following equation. Figure.2.3 illustrates the difference frequency generation.

Here the frequency of the generated wave is the difference of those of the

input frequencies.

1 2 3 (2.8)

24

Figure. 2.3: Schematic diagram of difference frequency generation.

2.3.4 Optical parametric generation

The optical parametric generation (Figure. 2.4) is an inverse process

of sum frequency generation. It splits one high-frequency photon (pumping

wavelength p ) into two low-frequency photons (signal wavelength s and

idler wavelength i )

s i p

(2.9)

Figure. 2.4: Schematic diagram of optical parametric generation i

2.3.5 Linear electro-optic effect

The Pockel‟s effect is a linear change in the refractive index of a

medium in the presence of an external electric field. Here a dc field is applied

to a medium through which an optical wave propagates. The change in the

polarization is present in the two interacting field components effectively

alters the refractive index of the medium.

2.3.6 Optical Rectification

The optical rectification is defined as the ability to induce a dc voltage

between the electrodes placed on the surface of the crystal when an intense

laser beam is directed into the crystal.

25

2.4 LINEAR AND NONLINEAR OPTICAL PHENOMENA

2.4.1 Linear optical phenomena

Nonlinear effects can also occur in gases and liquids but are mostly

common in crystals. The electrons present in nonlinear crystals are bound

“potential wells”, which act very much like tiny springs holding electrons in the

crystal to lattice points. If an external force pulls an electron away from its

equilibrium position, the spring pulls it back with a force proportional to

displacement: The spring‟s restoring force increases linearly with the

electron‟s displacement from its equilibrium position. The electric field in a

light wave passing through the crystal exerts a force on the electrons that

pulls them away from their equilibrium positions.

In an ordinary (i.e., linear) optical materials, the electrons oscillate

about their equilibrium positions at the frequency of this electronic field. The

fundamental law of physics says an oscillating charge would radiate at its

frequency of oscillation, so these electrons in the crystal “generate” light at

the frequency of the original light wave. Part of the energy in the light wave is

converted to the motion of the electrons, and this energy is subsequently

converted to light again. But the overall effect is to retard the energy as it

moves through the crystal because it takes a detour into the motion of the

electrons.

2.4.2 Nonlinear optical phenomena

The nonlinear materials are one in which the electrons are bound by

short springs. If the light passing through the material is intense enough, its

electric field could pull the electrons to reach the ends of their springs. The

restoring force is no longer proportional to the displacement; it becomes

nonlinear. The electrons are jerked back roughly rather than pulled back

smoothly and they oscillate at frequencies other than the driving frequency of

the light wave. The electrons radiate at the new frequencies, generating the

new wavelengths of light. The exact values of the new wavelengths are

determined by conservation of energy. The energy of the new photons

generated by the nonlinear interaction must be equal to the energy of the

26

photons used. The energy of the two 1.064-μm photons is equal to the

energy of the single 532-nm photons.

The latter half of the Twentieth Century is marked by the development

of electronics based on the semiconductor industry and technology. The

speed and memory of the present devices used in communication,

information processing, and other areas would reach their limit there by not

being able to cater the needs of the future. It is expected that the coming

century would be an era for wide application of all optical technologies since,

in an information society, it is necessary to rapidly transmit, receive and

process a large amount of information. For large bandwidths, optical sources

would serve the purpose and extensive development of optoelectronics

(optics + electronics) and photonics is being undertaken. Optical devices

possess the advantage and the important properties of high frequencies,

broad spectral range, ultra-high speed (~few fs), the capability of parallel

processing and high electromagnetic noise resistance.

Third order nonlinear optical properties provide the means to control

light with light, to change the frequency of light, to amplify one source of light,

switch it, or alter its transmission characteristics through a medium. The

intensity dependant refractive index provides the mechanism for control in

most of the third order nonlinear optical materials based devices. The

discovery of the phenomena of optical bistability gives a boost to the idea of

exploiting the large bandwidth (~ 1012 Hz). For the realization of photonics

era, the search for novel materials is gained the most importance. The

studies of optical materials with large third order nonlinearity and ultrafast

response times is received increasing attention of the researchers over the

last two decades for their potential applications in the area of optical

communications, optical data processing, optoelectronics and many other

related fields.

The progress in the realizing the potential of the materials depends on

i) Obtaining a fundamental understanding of the basic physics of

nonlinear processes

27

ii) Based on the understanding, optimization at molecular level

and later at the bulk level

iii) Using these materials to engineer viable integrated devices.

These steps include a unique blend of theoretical modeling, chemical

synthesis, materials processing, complete characterization and investigation

of device application [29].

2.5 NONLINEAR OPTICAL CRYSTALS

The nonlinear optical (NLO) crystals are the important material for the

development of laser science and technology because there is almost a kind

of materials that have functions to change frequency of laser beam and

modulate it in amplitude and phase. It might be said that lasers could not be

used so widely in modern science and technology today world, without

nonlinear optical (NLO) crystals. Development of nonlinear optical crystals

with better linear optical (LO) and nonlinear optical (NLO) properties, wider

spectral transmission, and phase-matching range in particular is obviously

essential for further widening the application field of lasers, the deep-UV, far

IR, and even THz spectral regions. That is why many scientists working in

the field today are still putting in great effort to search for new NLO crystals,

even more than four decades after the invention of the laser. Advances in the

development of nonlinear (NLO) optical crystals could be divided into two

different areas:

Synthesis and growth of new NLO crystals

Improving the properties of NLO crystals

In the beginning, studies were concentrated on inorganic materials

such as quartz, potassium di hydrogen phosphate (KDP), lithium niobate

(LiNbO3) and semiconductors such as cadmium sulfide, selenium, and

tellurium. At the end of 1968, the Kurtz and Perry powder second harmonic

generation (SHG) method was designed in order to find the SHG efficiency of

nonlinear optical materials. In this method, a powdered sample is irradiated

with a laser beam and scattered light is collected and analyzed for its

28

harmonic content with the use of suitable filters. For the first time, rapid,

qualitative screening for second order NLO effect was possible. The stage

was set for a rapid introduction of new materials, both inorganic and organic.

The second order nonlinear optical materials are used in optical

switching (modulation), frequency conversion (SHG, wave mixing) and

electro-optic applications, especially in electro-optic modulators. Inorganic

materials are much more matured in their application to second order NLO

materials than organics. Most commercial materials are inorganic especially,

for high power use. However, organic materials are perceived as being

structurally more diverse and therefore, are believed to have more long- term

promise than in organics. The recent studies indicate the new crystals

superior nonlinear optical properties and opened an area in the field of

research.

For optical applications, a nonlinear material should have the following

characteristics [4]:

A wide optical transparency domain

Large nonlinear figure of merit for frequency conversion

High laser damage threshold

Be readily available in large single crystals

Wide phase matches able angle

Ability to process into crystals, thin films, etc.

Ease of fabrication

Non toxicity and good environmental stability

High mechanical strength and thermal stability

Fast optical response time.

29

2.6 THE CLASSIFICATION OF NONLINEAR OPTICAL CRYSTALS

The physio-chemical properties of a material are extensively analyzed

as the material is to be grown in the form of single crystal. The new materials

are always investigated and the list of applications for crystals is considerably

increased. After the discovery of lasers, the importances of nonlinear optical

(NLO) crystals are realized. The growth of optical quality crystals is important

due to the rapid requirement for the crystals. The materials required for the

design of nonlinear optical (NLO) devices must fulfill certain basic conditions

such as large nonlinear, appropriate transparency range, high resistance to

good environmental stability. A typical second harmonic generation (SHG)

active molecule must be non-centrosymmetry in nature. This symmetry

requirement eliminates many materials from being SHG active and hence, at

the early stage of designing and synthesizing new materials, one has to

consider ways of introducing absence of centrosymmetry in the molecular

structures. The classifications of nonlinear optical crystals are following:

Inorganic crystal

Organic crystal

Semi organic crystal

2.6.1 Inorganic crystal

The nonlinear optical properties such as second harmonic generation

(SHG), frequency up and down conversions, optical parametric amplification

(OPA), optical parametric oscillation (OPO) optical emission and ferroelectric

properties such as piezoelectric and pyro electric were demonstrated in

several inorganic crystals. The several inorganic crystals possess both

nonlinear optical and ferroelectric properties. Lithium Niobate (LiNbO3) is one

of the most interesting inorganic materials for a wide range of applications. It

possesses both nonlinear optical and ferroelectric properties. Lithium Niobate

crystals are one of the most investigated materials for a wide spread and

promising applications in nonlinear optical properties, parametric

amplification and second-harmonic generation [30]. LiNbO3 are widely used

in optical devices such as photorefractive, holographic data storage, optical

30

information processing, phase conjugation and wavelength filters [31-34].

During the last decade, many new borate based inorganic NLO crystals are

discovered which is greatly expanded the range of laser wavelength from the

near infrared (IR) through the visible to the ultra violet (UV), deep- and

vacuum-UV spectral regions. The growth of new NLO borate single crystals

for UV light generation is reported by analyzed the effect of etching on the

various planes for several borate crystals [35,36].

The second order nonlinear optical (NLO) properties were found in

quartz crystal. Many efficient NLO inorganic crystal like KDP, Potassium

pentaborate (KB5), Ammonium di hydrogen phosphate (ADP), Beta barium

borate (BBO), Potassium Niobate (KNbO3), Potassium Titanyl Phosphate

(KTP), lithium tri borate (LBO) and lithium iodate (LiIO3) were developed in

the past decades for NLO applications [37,38].Yoshida et al investigated the

bulk laser induced damage threshold in KDP crystals [39]. During last few

years, newly developed NLO crystals GdCa4O(BO3)3 (GdCOB) and

YCa4O(BO3)3 (YCOB) have attracted much attention due their promising

optical properties [40]. Generally the inorganic NLO crystals possess very

good thermal and mechanical stability.

2.6.2 Organic crystals

The organic NLO materials play an important role in second harmonic

generation, frequency mixing, electro-optic modulation, optical parametric

oscillation, optical bistability, etc. [41]. Organic crystals have been extensively

studied due to their nonlinear optical (NLO) coefficients being often larger

than those of inorganic materials. The search for new frequency conversion

materials over the past decade is concentrated primarily on organic

compounds. Many new organic crystals are found, based on the predictive

molecular engineering approach and are shown to have potential

applications in nonlinear optics. In recent years, a considerable research is

made in exploring novel organic materials for potential use in a variety of

devices. The materials, which can produce green/blue laser light and could

withstand high-energy light radiation, are of vital importance for their use in

devices.

31

The organic materials are known for their applications in

semiconductors superconductors and nonlinear optical (NLO) devices [42-

44]. A large number of organic compounds with non-localized electron

systems and a large dipole moment are synthesized to realize the nonlinear

susceptibilities larger than the inorganic optical materials. However, their

potential applications are limited by, poor chemical stability and red cut off

wavelength caused by large birefringence, which results from the layer

stacking of the structure and other factors [45]. The basic structure of organic

nonlinear optical materials is based on bonding systems. Due to the

overlap of bonding electrons, delocalization of the charge distribution takes

place, which leads to a high mobility of the electron density. The extensive

research shows the organic crystals exhibit nonlinear optical (NLO)

efficiencies two orders of magnitude higher than those of inorganic materials

[46, 47]. Hence, organic crystals are rated high as compared to inorganics in

view of their large electro-optic coefficients with low-frequency dispersion,

high damage resistance to optical radiation, low cost, fast and large nonlinear

response over a broad frequency range, inherent synthetic flexibility and

intrinsic tailor ability [48, 49]. Similarly, nowadays, Organic ferroelectrics are

demanded for its diverse, useful and valuable applications such as

piezoelectric devices, actuators, nonlinear optical devices, in addition to non-

volatile memory, high permittivity materials and others for

electronicapplications.4-dimethylamino-N-methyl-4-stilbazoliumtosylate

(DAST) is one of the potential nonlinear optical crystals among the organic

materials. Preparation of 4-N,N-dimethylamino-4-N-methylstilbazolium

tosylate (DAST) crystal was first reported by Marder et al, in 1989 and is still

being recognized as the state of the art organic nonlinear optical (NLO)

crystal and nonlinear optical coefficients that are among the highest of all

known materials. [50-52]. In this sequence, tartaric acid belongs to an

important class of materials because of their interesting physical properties

such as ferroelectricity, piezoelectricity and nonlinear optical properties [53-

55].

32

2.6.3 Semi organic crystal

Presently, inorganic and organic materials are being replaced by semi-

organics. They share the properties of both organic and inorganic materials.

Recent interest is concentrated on metal complexes of organic compounds

owing to their large nonlinearity [56]. Semi organic crystals are less

hygroscopic compared to inorganic crystals and could be easily grown as

single crystals compared to organics. metal-organic crystals form the new

class of materials under semi organics. Compared to organic molecules,

metal complexes offer a large variety of structures, the possibility of high

environmental stability, and a diversity of electronics properties by virtue of

the coordinator mental centre [57].Further amino acids could be used as a

basis for synthesizing organic-inorganic compounds like L-arginine

phosphate and its derivatives. L-arginine phosphate monohydrate (LAP) is a

potential nonlinear optical (NLO) material first introduced by Chinese in 1983

[58]. LAP crystals are usually grown from aqueous solution by the

temperature lowering technique. L-arginine phosphate (LAP) crystal

possesses high nonlinearity, wide transmission range (220−1950 nm), high

conversion efficiency (38.9%) and high damage threshold [59]. Yokotani et al

is reported the synthesis and growth of deuterated L-arginine phosphate

(DLAP) crystal with a higher harmonic generation [60].

2.7 REVIEW OF Z-SCANSTUDY

The Z-scan is a study which could measure the sign and magnitude of

the materials related to grown crystals. This is simple and famous method to

measure third-order nonlinear optical properties. The following observations

were made by various scientists in the nonlinear optical research.

Sheik-Bahae reported a simple and highly sensitive single-beam

experimental technique for the determination of both sign and magnitude of

nonlinear refractive index 2( )n [61]. Said et al studied the real and imaginary

parts of the third order susceptibility in CdTe, GaAs, and ZnTe using Z-scan

technique [62].The Z-scan technique has been a very important technique to

measure the nonlinearity because of its high sensitivity and simplicity. The Z-

33

scan method has been employed to measure the nonlinear absorption and

refraction in a large number of direct band gap semiconductors [63-65].

Nalda de et al measured the nonlinear refractive index and absorption

coefficient of the Cu:Al2O3 films on glass substrates by Z-scan technique

[66]. Krauss et al studied the nonlinear refraction of CdS, ZnSe, and ZnS

using Z-scan technique [67]. Clementi et al [68] studied the nonlinear self-

focusing and absorption effects in nano structure and absorption effects. P.V.

Dhanaraj reported the third order nonlinear properties of 2-Aminopyridinium

trichloroacetate, (2APTC) a novel organic optical material has been

synthesized and crystals were grown from aqueous solution employing the

technique of controlled evaporation [69]. N.P.Rajesh reported the an organic

material, nicotinium tri fluoroacetate (NTF) crystal The third order nonlinear

refractive index and nonlinear absorption coefficient of (NTF) were

determined by Z-scan technique [70]. P.Kalaiselvi et al reported the linear

and nonlinear optical properties of L-alanine cadmium chloride (LACC) and

determine the values of nonlinear optical parameter such as refractive index

2( )n ,nonlinear absorption coefficient absorption coefficient ( ) and nonlinear

susceptibility 3 in LACC single crystal [71]. I.P. Bincy et al reported

thenegativethirdordernonlinearopticalparameterslikerefractiveindex 2( )n

absorption coefficient ( ) and susceptibility 3 were estimated by

2-Aminopyridinium p-Toluenesulfonate (2APPTS) organic single crystal using

the Z-scan technique [72].

34

3. CHARACTERIZATION TECHNIQUES

3.1 INTRODUCTION

The characterization and techniques of crystals are examined in this

chapter. The complete description of the physical and chemical properties of

a material of interest is analyzed as characterization of material and it

consists of determination of chemical composition, structure, defects,

thermal, mechanical, optical properties etc [1]. Standard techniques of

etching, chemical analysis, scanning electron microscopy etc., could be used

for defect characterization. The measurement of optical properties includes

the study of optical transmission and absorption of the crystal, second

harmonic generation (SHG) conversion efficiency, nonlinear optical (NLO)

coefficients, electro-optical coefficients and structural dependence of these

properties. The characterization of nonlinear optical (NLO) crystals could

generally be divided into three types:

i) Crystal structural analysis

ii) Investigation of growth defects

iii) Measurement of optical properties

In this chapter, describes the brief theory of various characterization

techniques related to single crystal X-ray diffraction analysis, powder X-ray

diffraction, Fourier transform infrared (FT-IR) spectroscopy, optical

absorption studies, thermal analysis, mechanical, etching, and nonlinear

optical (NLO) test are adopted for the present investigation. The crystals that

are taken up for the study, are pure, potassium doped meta-Nitroaniline,

potassium dichromate (KDC), L-alanine (LA); They are subjected to the

above experimental techniques, that are involved in the characterization of

the grown crystals. The single crystals chosen for the present investigation

were subjected to the following studies:

35

Collection of XRD data by single crystal X-ray diffraction

analysis and Powder X- ray diffraction analysis to confirm the

structure of the grown crystals.

FT-IR spectral analysis to confirm the various functional groups

present in the compound.

UV-Vis-NIR analysis for the detection of the transparency

region and cutoff wavelength.

Thermal analysis to study the thermal stability of the grown

crystals.

Hardness studies to estimate the hardness value of the grown

crystals.

SHG test to check the nonlinear response of the crystal to the

incident coherent light.

Z-scan studies to determine the nonlinear parameter values of

grown crystals.

Chemical etching studies were carried out to study the growth

mechanism and asses the perfection of the grown crystals.

3.2 X-RAY DIFFRACTION ANALYSIS

X-ray diffraction is an important analytical technique for investigating

crystalline solids, which include ceramics, metals, and polymers. The

discovery of diffraction of X-rays by crystals led to the exploration of the

internal arrangement of atoms in a crystal. Max von Laue, in 1912,

discovered the crystalline substances act as three-dimensional diffraction

gratings for X-ray wavelengths similar to the spacing of planes in a crystal

lattice. X-ray diffraction is now a common technique for the study of crystal

structures and atomic spacing.

36

3.2.1 X-ray diffraction and Bragg’s Law

W. H. Bragg and W.L. Bragg first suggested the use of X-rays as a

tool for investigating the structure of crystals. Bragg considered X-ray

diffraction from a crystal as a problem of reflection from atomic planes.

(Figure 3.1).The diffraction is the consequence of constructive interference of

these reflected rays. The interaction of the incident rays with the sample

produces constructive interference when conditions satisfy Bragg‟s law.

2 sind n (3.1)

X-ray diffraction is based on constructive interference of

monochromatic X-rays and a crystalline sample. These X-rays are generated

by a cathode ray tube, filtered to produce monochromatic radiation,

collimated to concentrate, and directed toward the sample. The interaction of

the incident rays with the sample produces constructive interference (and a

diffracted ray) when conditions satisfy Bragg's Law .

Figure. 3.1: Diffraction of X-rays from the planes of atoms in a

crystalline solid.

The Bragg‟s law relates the wavelength of electromagnetic radiation to

the diffraction angle and the lattice spacing in a crystalline sample. These

diffracted X-rays are detected, processed and counted. By changing the

geometry of the incident rays, the orientation of the centered crystal and the

detector, all possible diffraction directions of the lattice should be attained.

All diffraction methods are based on generation of X-rays in an X-ray tube.

These X-rays are directed at the sample, and the diffracted rays are

37

collected. A key component of all diffraction is the angle between the incident

and diffracted rays. The typical mineral structures contain several thousand

unique reflections, whose spatial arrangement is referred to as a diffraction

pattern. Indices (hkl) might be assigned to each reflection, indicating its

position within the diffraction pattern. This pattern has a reciprocal Fourier

transform relationship to the crystalline lattice and the unit cell in real space.

This step is referred to the solution of the crystal structure. After the structure

is solved, it is further refined using least-squares techniques.

3.2.2 Single crystal X-ray diffraction (SXRD) analysis

The single crystal X-ray diffraction (SXRD) is a non-destructive

analytical techniques which provides detailed information about the internal

lattice of crystalline substances, including unit cell dimensions, bond-lengths,

bond-angles and details of site-ordering. It is directly related to single-crystal

refinement, where the data generated from the X-ray analysis is interpreted

and refined to obtain the crystal structure. The single crystal X-ray diffraction

is most commonly used for precise determination of a unit cell, including cell

dimensions and positions of atoms within the lattice. The bond-lengths and

angles are directly related to the atomic positions. The crystal structure of a

mineral is the characteristic property which is the basis for understanding

many of the properties of each mineral. Specific applications of single-crystal

diffraction include:

New mineral identification, crystal solution and refinement

Determination of unit cell, bond-lengths, bond-angles and site-

ordering

Characterization of cation-anion coordination

Variations in crystal lattice with chemistry

With specialized chambers, structures of high pressure and/or

temperature phases could be determined

Determination of crystal-chemical vs. environmental control on

mineral chemistry

38

Powder patterns could also be derived from single-crystals by use

of specialized cameras.

The single crystal X-ray diffraction analysis for the grown crystals was

recorded using Bruker AXS diffractometer are shown in the Figure3.2. In

this thesis, all the four samples mNA, mNAK, KDC, LA is using Bruker AXS

diffractometer to find out the lattice parameter and space groups.

Figure. 3.2: Bruker AXS diffractometer.

39

3.2.3 Powder X-ray diffraction (PXRD) analysis

The crystallinity of the chosen material could easily be determined

from powder X-ray diffraction analysis. This powder method is applicable only

to finely divided crystalline powder or to a fine granite polycrystalline

specimen. This method is also called as Debye, Scherer and Hull method.

This method is more convenient, single crystals are not required and it is

used for accurate determination of lattice parameters in crystal of know

structure and for accurate determination of lattice parameters in crystal of

known structure and for the identification of lattice parameters in crystal of

known structure and for the identification of elements and compounds. The

powdered specimen was kept inside a small capillary tube, which does not

undergo diffraction. A narrow pencil of monochromatic X-rays is diffracted

from the powder and recorded by the photographic film as a series of lines of

varying curvature. The full opening angle of the diffraction cone 4θ is

determined by measuring the distance „S‟ between the two corresponding

arcs on the powder photograph symmetrically displaced about the exit point

of the direct beam. The distance „S‟ on the film between two diffraction lines

corresponding to a particular plane is related to the Bragg angle by the

equation.

4

S

R radians (3.2)

1804

S

R

degree (3.3)

where R is the distance between the specimen and film. A list of values

could be obtained from the measured values of S. since wavelength is

known, substitution of in Bragg‟s equation (3.1) gives a list of spacing d.

each space is the distance of neighboring planes (h k l) [73]. The powdered

XRD pattern is a plot of peak intensity versus diffraction angle (2 ) . The

crystalline materials shows sharp intense peaks whereas amorphous salts

shows only under fined broad peaks of relatively low intensity. The powder

XRD pattern exhibits one peak for all repeating planes with equal d-spacing

40

irrespective of their orientation. Powder XRD patterns are unique for a given

crystal structure and are used to detect changes in the hydrate due to

hydration, dehydration or polymorphic transitions. This is accomplished by

peak matching, which might be done either manually or by specially designed

software.

3.3 FOURIER TRANSFORM INFRARED SPECTRAL (FT-IR) ANALYSIS

Infrared spectroscopy is a workhorse technique for material analysis in

the laboratory for over seventy years. An infrared spectrum represents a

fingerprint of a sample with absorption peaks which correspond to the

frequencies of vibrations between the bonds of the atoms making up the

material. It is effectively used to determine the molecular structure and the

identification of functional groups in the synthesized samples. It is one of the

most powerful analytical techniques which propose the possibility of chemical

identification. Each different material is a unique combination of atoms; more

two compounds produce the exact same infrared spectrum. Therefore,

infrared spectroscopy could result in a positive identification of every different

kinds of material. In addition, the size of the peaks in the spectrum is a direct

indication of the amount of material present. With modern software

algorithms, infrared is an excellent tool for quantitative analysis. The original

infrared instruments were of the dispersive type. These instruments

separated the individual frequencies of energy emitted from the infrared

source. This was accomplished by the use of a prism or grating. An infrared

prism works exactly the same as a visible prism which separates visible light

into its colors (frequencies). A grating is a more modern dispersive element

which better separates the frequencies of infrared energy. The detector

measures the amount of energy at each frequency that has passed through

the sample. This results in a spectrum which is a plot of intensity vs.

frequency. Fourier transform spectroscopy was first developed by

astronomers in the early 1950s to study the infrared spectra of distant stars.

The Fourier techniques could isolate the weak signals from environmental

noise and since then it is developed into a powerful experimental technique.

The conventional spectroscopy is the frequency domain spectroscopy, in

which the radiant power is recorded as a function of frequency. The Fourier

41

transform spectroscopy is time domain spectroscopy, in which the radiant

power changes with time. The Michelson interferometer is used form the

modulation of the high frequency signal. The basic components of a Fourier

transform interferometer are the moving mirror, fixed mirror and a beam

splitter. Radiation from the infrared source is collimated by mirror and the

resultant beam is divided at the beam splitter, half the beam passes to the

fixed mirror and half is reflected to the moving mirrors, which are at right

angles to each other. After reflection, the two beams interfere constructively

or destructively depending on the path difference. When the movable mirror

is moved with a constant velocity the intensity of the emerging radiation is

modulated in a regular sinusoidal manner. The modulated frequency after

passing through the sample compartment is focused on to the detector. The

detector signal is sampled at precise intervals during mirror scan. The

transform is carried out by a computer and the spectrum is plotted in a paper.

Fourier Transform Infrared Spectroscopy (FT-IR) is a powerful tool for

identifying types of chemical bonds in a molecule by producing an infrared

absorption spectrum which is like a molecular "fingerprint”. FT-IR is preferred

over dispersive or filter methods of infrared spectral analysis for several

reasons:

It provides a precise measurement method which requires no

external calibration.

It is a non-destructive technique.

It could increase sensitivity- one second scan could be co-added

together to ratio out random noise.

It is mechanically simple with only one moving part.

It could increase speed, collecting a scan every second.

It has greater optical throughput.

The FT-IR spectrum was taken for the synthesized samples by

palletizing them with KBr (Merck). The FT-IR spectral measurements were

42

carried out using Perkin Elmer FT-IR instrument shown in Figure 3.3. The

sample were scanned between the wave numbers ranging 4000 – 400 cm -1.

Figure. 3.3: Perkin Elmer FT-IR instrument.

3.4 OPTICAL STUDIES

Ultraviolet and visible spectrophotometers were in general use for the

last 35 years since it is an important analytical instrument in the laboratory. In

many applications other techniques could be employed, but none rival

UV–Visible spectrometry for its simplicity, versatility, speed, accuracy and

cost-effectiveness. This description outlines the basic principles of

UV–Visible spectrometry. It is intended purely as a brief introduction to this

technique and it is Thermo spectronic's policy to frequently add to this range

of documentation for further details, as they become available. The

UV-vis-NIR spectrometer as shown in Figure 3.4.

43

Radiation source

Wavelength selectors

Sample containers

Radiation detectors and

Signal processors and read out devices.

The hydrogen or deuterium discharge lamp operating at a low

pressure of 2 or 3 torr with quartz produces a continuous UV spectrum in

region 180–370 nm. Quartz or fused silica is used for the construction of cells

that hold the sample and solvent as these are transparent in UV region.

Diffraction grating or prism could be used as mono chromators. To avoid

serious energy loss due to mono chromators, filter photometers are used.

Photovoltaic cells and photomultiplier tubes are used initially as detectors

and nowadays photodiodes are used. The modern spectrometers are

computerized and the resulting detector output is digitalized and stored.

Samples are scanned, the absorbance is calculated and the spectrum is

displayed. The optical absorption spectrum for the grown crystals was

recorded in the wavelength region ranging from 200 nm to 2000 nm using

Perkin-Elmer lamda 25 spectrophotometer. For optical fabrications, the

crystal should be highly transparent in the considerable region of wavelength

[74].The optical absorption coefficient was calculated using the following

relation

(3.4)

where T is the transmittance and d is the thickness of the crystal. As a direct

band gap, the crystal under study is an absorption coefficient obeying the

following relation for high photon energies ( )h :

(3.5)

)1

log(1

Td

h

EhA g

2/1)(

44

whereg

E is optical band gap of the crystal and A is a constant. The plot of

the variation of 2( )h vs h yields the band gap energy of the grown

crystals.

Figure. 3.4: UV-vis- NIR spectrometer.

3.5 THERMAL ANALYSIS

Thermal analysis is a branch of materials science where the properties

of materials are studied as they change with temperature. Here we are

interested in analyzing two main techniques:

45

Thermo gravimetric analysis (TG)

Differential thermal analysis (DTA)

3.5.1 Thermo gravimetric analysis (TGA)

Thermo gravimetric analysis (TG) is a type of testing that is performed

on samples to determine changes in weight in relation to change in

temperature. The analysis relies on a high degree of precision in three

measurements: weight, temperature, and temperature change. As many

weight loss curves look similar, the weight loss curve require transformation

before results might be interpreted. A derivative weight loss curve could be

used to tell the point at which weight loss is most apparent. Again,

interpretation is limited without further modifications and de convolution of the

overlapping peaks might be required. TG is commonly employed in research

and testing to determine degradation temperatures, absorbed moisture

content of materials, the level of inorganic and organic components in

materials, decomposition points of explosives, and solvent residues. It is also

often used to estimate the corrosion kinetics in high-temperature oxidation.

3.5.2 Differential thermal analysis (DTA)

Differential thermal analysis (DTA) is a thermo analytic technique,

similar to differential scanning calorimeter. In DTA, the material under study

and an inert reference are heated (or cooled) under identical conditions,

while recording any temperature difference between sample and reference

[75].This differential temperature is plotted against time, or against

temperature (DTA curve or thermogram). The changes in the sample, either

exothermic or endothermic, could be detected relative to the inert reference.

The DTA curve provides data on the transformations which is occurred, such

as glass transitions, crystallization, melting and sublimation. The

experimental arrangement of modern commercial thermo gravitometric

analysis instrument is shown in Figure 3.5.The thermal studies are taken at

Department of Nano- Technology, B.S. Abdur Rahman University , Chennai-

600 048, India, and SAIF, IIT Madras, Chennai 600 048, India.

46

Figure. 3.5: TGA-DTA analyser.

3.6 MECHANICAL STUDY

The hardness is a physicochemical property that characterizes the

state of the material under test and gives information on some specific

features of the material, such as the character of the chemical bonding. It is

the resistance which the material offers to indentation by a much harder body

and might be termed as a measure of the resistance against lattice

destruction or permanent formation or damage. As the hardness properties

are basically related to the crystal structure of the material and the bond

strength, microhardness studies are applied to understand the plasticity of

the crystals. Hardness tests are commonly carried out to determine the

mechanical strength of materials and it correlates with other mechanical

properties like elastic constants and yield stress [76]. The hardness

measurements could be defined as macro, micro and nano according to the

forces applied and displacement obtained [77]. The various methods using

which hardness measurement could be carried out are classified as follows:

47

Static indentation test

Dynamic indentation test

Scratch test

Rebound test and

Abrasion test

The most popular and simplest test is the static indentation test, where

in an indenter of specific geometry is pressed into the surface of a test

specimen under a known load. The indenter might be a ball or diamond cone

or diamond pyramid. A permanent impression is retained in the specimen

after removal of the indenter. The hardness is calculated from the area or the

depth of indentation produced. The variables are the type of indenter or load.

The indenter is made up of a very hard material to prevent its deformation by

the test piece, so that it could cover materials over a wide range of hardness.

For this reason, either a hardness steel sphere or a diamond pyramid or cone

is employed. A pyramid indenter is preferred as geometrically similar

impressions are obtained at different loads. In this static indentation test, the

indenter is pressed perpendicularly to the surface of the sample by means of

an applied load. By measuring the cross sectional area or depth of the

indentation and knowing the applied load, empirical hardness number might

be calculated. This method is followed by Brinell, Meyer, Vickers, Knoop and

Rockwell tests [78].

The Vickers hardness number vH defined as

2

2 sin( / 2)v

pH

d

(3.6)

where „ ‟ is the apex angle of the indenter ( =136 ). The Vickers hardness

number vH is thus calculated using the relation,

2

2

1.8544/v

pH kg mm

d (3.7)

where „p‟ is the applied load in „kg‟ and „d‟ is the diagonal length of the

indentation mark in „mm‟. The hardness values are always measured from

48

the observed size of the impression remaining after a loaded indenter has

penetrated and is removed from the surface. Thus, the observed hardness

behavior is the summation of a number of effects involved in the materials

response to the indentation pressure during loading, in the final

measurement of the residual impression.

The importance of micro hardness study lies in the possibilities of

making an indirect estimate of mechanical characteristics of materials, the

strength and toughness having a specific correlation with the hardness. The

hardness measurement depends upon the orientation of the indented

crystals. To study the hardness anisotropy present in crystals, the crystals

are initially mounted on the stage of microscope properly and indented. The

initial position is 0 degree. The stage of the microscope was then rotated

keeping the indenter fixed and „ vH ‟ was measured at every 30 degrees

interval till the original position. No distortion in shape of indenter would be

observed with crystal orientation when the variation of „ vH ‟ with angular

displacement is periodic, then it brings the anisotropic nature of crystals.

The mechanical studies are taken at Nuclear Physics Department,

University of Madras, Chennai 600 025, India, and Department of Physics

St.Joseph‟s Engineering college, Thiruchirappalli, India.

3.7 SECOND HARMONIC GENERATION (SHG) MEASUREMENT

TECHNIQUE

Nonlinear optics is the study of the interaction of intense

electromagnetic field with materials to produce modified fields that are

different from the input field in phase, frequency or amplitude [79]. Second

harmonic generation (SHG) is a nonlinear optical process that results in the

conversion of an input signal in to output of twice the input frequency. This

process occurs within a nonlinear medium, usually in crystals. The nonlinear

optical (NLO) process has applications, such as frequency conversion and

optical switching, optical data storage, optical signal processing,

optoelectronic technologies, including electro-optical devices,

telecommunications, and optical information storage devices etc [80]. Kurtz

49

and Perry method could be employed for the qualitative analysis of the

second harmonic generation (SHG) of the samples. A material should posses

the following requirements to have the SHG property:

Polarizable material (the electrons need to be greatly perturbed

from their equilibrium positions)

Asymmetric charge distribution (incorporation of donor-acceptor

molecules)

A pathway of -conjugated electrons

Excited states close to the ground state

A large difference between ground state and excited state dipole

moments.

This technique was the first used in the study of organo-metallic

materials. A laser is directed onto a powdered sample and the emitted light at

the second harmonic frequency is collected and compared to that of a

reference sample, such as quartz, potassium di hydrogen phosphate (KDP),

or urea, to obtain a measure of the second harmonic generation efficiency.

Since all the grown crystals are noncentrosymmetric, it is desirable to

check the SHG property of the crystals. The SHG efficiencies were tested

using a modified setup of Kurtz and Perry. The experimental setup is shown

in Figure 3.6. A Q-switched Nd: YAG laser (λ = 1064 nm; Quanta Ray) with

pulse duration of 6 ns and repetition rate of 10Hz was employed to excite the

powdered sample. The second harmonic signal generated was confirmed

from the emission of green radiation 532nm from the sample. The

intensity of the green light was measured using a photomultiplier tube. The

same particle size of KDP sample was used as reference material. NLO tests

of the sample (crystal) were also taken at department physics, B .S. Abdur

Rahman University, Vandalur, Chennai-600 048, India.

50

Figure.3.6: Functional block diagram for SHG determination.

3.8 Z-SCAN TECHNIQUE

The single beam Z-scan technique is based on the principle of spatial

beam distortion arising from an optically induced nonlinear refractive index.

Using technique to determine the sign and magnitude of the nonlinear

refractive index 2n could be easily extracted from experimental data with a

minimum of analysis [81]. Z-scan studies give information about the nonlinear

characteristics of a material, and excitation of this study yields important

information regarding the response time and the dynamics of the transient

process contributing to the nonlinear refractive index. In cases where

nonlinear refraction is accompanied by nonlinear absorption, it is possible to

separately evaluate the nonlinear refraction as well as the nonlinear

absorption, by performing two Z-scans, one with aperture another without

aperture. Basically in this method nonlinear sample through the focal plane of

a tightly focused Gaussian beam and monitoring the changes in the field

intensity pattern. For a purely refractive nonlinearity the light field induces an

intensity dependent nonlinear phase and as consequence of the transverse

Gaussian intensity profile the sample presents a lens-like behavior. The

induced self-phase modulation has the tendency of defocusing or re-

collimating the incident beam, depending on its Z- position with respect to the

focal plane. By monitoring the transmittance change through a small circular

aperture placed at the far field position, one is able to determine the

nonlinear refractive index. Any nonlinear absorption present in the sample

Sample holder

= 1064 nm

Monochromator M

PMT Recorder

Nd: YAG Laser

51

could be found in a measurement where the aperture is replaced by a lens

(open aperture Z-scan).

Development of high power laser sources has motivated an extensive

research in the study of nonlinear optical properties and optical limiting

behavior of materials [82, 83]. For the usage of the crystal to be efficient, it is

necessary to have quantitative information about their nonlinear optical

properties. Such nonlinear optical phenomena as nonlinear refraction and

absorption could seriously affect the operation of laser devices. In part,

intensity dependent refraction index of laser material causes the changes in

the spatial distribution of laser field and could lead to self-focusing of the

radiation and breakdown of device. It is the main factor which restricts the

light intensities used and output laser power [84]. A laser beam propagating

through a nonlinear medium will experience both amplitude and phase

variations. If transmitted light is measured through an aperture placed in the

far field with respect to focal region, the technique is called closed aperture

Z-scan. In this case, the transmitted light is sensitive to both nonlinear

absorption and nonlinear refraction. On the other hand without an aperture,

the mode of measurement is referred to as open aperture Z-scan.

Several techniques are known for the measurements of nonlinear

refraction (NLA) in materials. Nonlinear interferometer [85, 86]. Degenerate

four wave mixing [87]. Nearly degenerate three-wave mixing [88], rotation

and beam distortion measurement [89] these techniques are frequently

reported. The first three methods, interferometer and wave mixing are

potentially sensitive techniques requires a complex experimental apparatus.

Beam-distortion measurements, on the other hand, require precise beam

scans followed by detailed wave propagation analysis. Based on these

principles single beam techniques are used for measuring the sign and

magnitude of refractive nonlinearity that offers a very high sensitivity. The

technique is based on the transformation of phase distortion to amplitude

distortion during beam propagation. Z-scan methods yield the real and

imaginary parts of nonlinear susceptibility (3) respectively. Usually closed

aperture Z-scan data are divided by open aperture data to cancel the effect

52

of nonlinear absorption contained in the closed aperture measurements. The

new graph called divided Z-scan contains information on nonlinear refraction

alone.

3.8.1 Z-scan technique experimental setup

The Z-scan experiments were performed using a 532-nm diode-

pumped Nd: YAG laser beam (Coherent CompassTM215M-50), which was

focused by a 3.5cm focal length lens. The laser beam waist 0 at the focus is

measured to be 15.84µm and the Rayleigh length is 1.48 mm. The schematic

of the experimental setup used is shown in Figure 3.7.

Figure. 3.7: Experimental set up for Z-scan technique.

A 1mm wide optical cell containing the sample in solvent is translated

across the focal region along the axial direction that is the direction of the

propagation laser beam. The transmission of the beam through an aperture

placed in the far field is measured using photo detector fed to the digital

power meter (Field master Gs- coherent). For an open aperture Z-scan, a

lens to collect the entire laser beam transmitted through the sample replaced

aperture.

53

3.8.2 Closed aperture Z-scan

The Z-scan trace is expected to have minimum transmittance (valley)

to maximum transmittance (peak), which corresponds to a positive

nonlinearity i.e. self-focusing. An inverse Z-scan curve peak followed by

valley characterizes a negative nonlinearity i.e. self-defocusing. Using closed

aperture Z-scan method to determine third order nonlinear refractive index (

2n ) of the grown crystals.

3.8.3 Open aperture Z-scan

The Z-scan with a fully open aperture (S=1) is insensitive to nonlinear

refraction. Such Z-scan traces with no aperture are expected to be symmetric

with respect to the focus (Z=0) where they have a minimum transmittance

(e.g. two photon or reverse saturation absorption) or maximum transmittance

(e.g. saturation of absorption). In fact the coefficients of nonlinear absorption

could be easily calculated from such transmittance curves. Investigations are

carried out to study the dependence of nonlinear absorption coefficient ( )

on the concentration of the crystals.

3.8.4 Nonlinear absorption

The nonlinear absorption coefficient could be estimated from the

open aperture Z-scan data. The normalized transmittance for the open

aperture condition is given by [81].

0

3/20

[ ( )]( , 1) ,

( 1)

m

m

q zT z S

m

(3.8)

For 0 (0) 1,q where q0 (z) = βI0Leff/(1+z2/zR2), zR= kω0

2/2 is the diffraction

length of the beam and ω0 is the beam waist radius at the focal point and

2k

is the wave vector.

A useful feature of the Z-scan trace is that the Z-distance between

peak and valley pvZ is a direct measure of the diffraction length of the

54

incident beam for a given nonlinear response. In a standard Z-scan (i.e.

using a Gaussian beam and a far-field aperture), this relation for a third-order

nonlinearity is given by

01.7pv

Z Z

(3.9)

An easily measurable quantity p v

T could be defined as the difference

between the normalized peak and valley transmittances, p v

T T . The

variation of this quantity as a function of 0 is given by [81].

where 0 is the on-axis phase shift at the focus, 2

02

0

1 expr

S

is the aperture linear transmittance with 0r denoting the aperture radius and

0 denoting the beam radius at the aperture in the linear regime. The

nonlinear refractive index is then given by [81].

02

02 eff

nI L

(3.10)

where is the laser wavelength, 0I is the intensity of the laser beam at focus

0,Z [1 exp( )] /eff

L L is the effective thickness of the sample, a is the

linear absorption coefficient and L is the thickness of the sample.

3.8.5 Nonlinear optical susceptibility 3

Experimentally determined nonlinear refractive index 2n and nonlinear

absorption coefficient could be used in finding the real and imaginary parts

of the third-order nonlinear optical susceptibility 3 according to the following

relations [90].

2 2 2(3) 4 0 0

2Re ( ) 10c n cm

esu nW

(3.11)

55

and

2 2 2(3) 2 0 0

2Im ( ) 10

4

c n cmesu

W

(3.12)

where 0 is the vacuum permittivity, and c the light velocity in vacuum:

The absolute value of the third-order nonlinear optical susceptibility

3 is given by the relation

The absolute value of 3 is calculated from

(3) (3) 2 (3) 2 1/2[(Re( )) (Im( )) ] (3.13)

The Z-scan experimental studies were taken at Anna University,

Chennai-600 025, India.

3.9 ENERGY-DISPERSIVE X-RAY (EDX) ANALYSIS

Energy-dispersive X-ray spectroscopy (EDX) is an analytical

technique. It is used to analysis the presence of chemical elements on

prepared sample. The higher energy X-ray impacts on the sample and

studied about the presence of chemical elements on prepared sample. The

incident X-ray would t excite an electron from inner shell, an excited electron

will create a hole on before placed that electron. The electron of the outer

shell will occupy the empty space of the electron. When an electron is

removed from the inner shell, it causes X-rays and this allows the elemental

composition of the specimen to be measured [91,92]. The EDX analyze was

taken at Nuclear Physics Department, University of Madras, Chennai 600

025, India.

3.10 ETCHING STUDIES

Etching studies were carried out to understand the growth mechanism

and to assess the perfection of the grown crystals. The chemical etching is a

simple and very powerful tool for identification of the defects present in the

growing crystal surfaces. This is able to develop some of the features such

56

as growth hillocks, etch pits, grain boundaries on the crystal surface and is

essential to study the micro structural imperfection of the growth surface

[93].Etching studies are carried out to understand the growth mechanism and

to assess the perfection of the grown crystals. Chemical etching is the oldest,

easiest and most widely used method to evaluate the quality of the crystal.

Etching is selective dissolution of the crystal which is used to reveal the

crystal symmetry and lattice defects. The establishment of the relation

between etch pits and the dislocation array present in imperfect crystals by

employing the etching technique has turned out to be a powerful tool for the

preparation and characterization of single crystals for electronic and optical

devices [94,95].

The rate of reaction of a solution with a solid surface could depend

distinctly on the crystallographic orientation on the reacting surfaces. This

rate dependence is the basis of etching and brought about intentionally by

specific chemical attack on the crystal surface. When a surface is etched,

well defined etch patterns of negative volume are produced at the dislocation

sites according to Thomas and Renshaw [96]. Etching might be considered

as the reversal of crystal growth; therefore, a surface step, for example, a

screw dislocation, dissolves more easily than a flat surface as reported by

Burton et al., [97]. The relative rates of removal of atoms along different

directions determine the geometry of etch pits. The shape of the etch pits

might be changed by varying the solvent. Moreover, factors such as

temperature of etching, stirring and adsorption of impurities or reaction

products, which alter the absolute value of these rates, also lead to the

change in the geometry of etch pits [98]. Since the geometrical variation of

etch pits strongly depends on the arrangement of ions or atoms which

comprises the crystal faces, the shapes of the etch pits also differ for various

crystal faces. The shape of the etch pits might be changed by varying the

amount of the solvent. In the present work, etching studies were carried out

using methanol and water as etchant. The optical microscopy (REICHERT

POLYVAR 2 MET Microscope) was used to analyze dislocation of etch pits

and domain patterns in crystals. The measurements was taken at Nuclear

Physics Department, University of Madras, Chennai- 600 025, India.

57

4. GROWTH, THERMAL, MECHANICAL, LINEAR AND NONLINEAR

OPTICAL PROPERTIES OF PURE, POTASSIUM DOPED META-

NITROANILINE SINGLE CRYSTALS

4.1 INTRODUCTION

In modern day technological society, nonlinear optical (NLO) materials

are most useful in the area of optical data storage, lasers, optical signal

processing, second harmonic generation (SHG) etc., For the past few years,

third order nonlinear optical materials play a vital role due to their potential

application in optical signal processing, optical limiting, optical logic gates,

laser radiation protection, etc. [99,100]. In meta-Nitroaniline (mNA) with

chemical formula C6H6N2O2, is a well-known organic nonlinear optical

material which has been examined by several researchers in the recent past

on its crystal growth and properties [101-105]. The meta-Nitroaniline (mNA)

is one of the simple donar-acceptor molecules with a noncentrosymmetric

structure and its large optical nonlinearities [106]. It possesses strong

nonlinear optical effect, compared to conventional inorganic materials, as

well as strong piezoelectric, pyroelectric and electro-optic effects [107-109]. A

number of organic materials have been identified and synthesized, showing

considerable NLO effects. However, only a few of them could be crystallized

and investigated for second order NLO applications. The growing organic

crystals of large size are still a big challenge. The Z-scan method has gained

rapid acceptance by the nonlinear optics community as a standard technique

for separately determining the nonlinear changes in index and changes in

absorption. This acceptance is primarily due to the simplicity of the technique

as well as the simplicity of the interpretation. In this experiment, the index

change ,n and absorption change , could be determined directly from the

data without resorting to computer fitting [110]. However, it must always be

recognized that this method is sensitive to all nonlinear optical mechanisms

that give rise to a change of the refractive index and absorption coefficient,

so that determining the underlying physical processes present from a Z-scan

is not in general possible. A series of Z-scan at varying pulse widths,

frequencies, focal geometries etc. along with a variety of other experiments

58

are often needed to unambiguously determine the relevant mechanisms. The

Z-scan technique is a method which could rapidly measure both nonlinear

absorption (NLA) and nonlinear reflectance (NLR) in solids, liquids and liquid

solutions [61, 81].

In the present study, an attempt was made to grow good-quality single

crystal of pure and potassium doped meta-Nitroaniline by slow evaporation

method. The grown crystals were subjected to various characterization

studies, such as EDX, XRD, thermal, mechanical, optical studies of pure and

potassium doped meta-Nitroaniline single crystals.

4.2 CRYSTAL GROWTH

The AR grade potassium hydroxide (10%) and mNA were dissolved in

methanol. The solution was stirred continuously until homogenization for 9

hours at room temperature. The saturated solution was filtered using

Whatmann filter paper and collected in a beaker. The beaker was closed with

a perforated cover and kept in dust-free environment. Transparent single

crystals of mNAK with dimensions 15×6×3 mm3 were obtained after 25 days

(Figure 4.1b). Simultaneously, a pure mNA crystal (Figure 4.1a) was grown

using methanol as a solvent, grown mNA crystal with dimension 10×7×3

mm3. Comparing Figure1a and 1b, mNAK shows a better morphology than

mNA, mainly due to the doping effect. Potassium selectively inhibited the

crystal faces at favorable adsorption sites, as visible in the crystal

morphology.

Figure. 4.1: As grown crystals of (a) pure mNA (b) mNAK.

59

4.3 RESULTS AND DISCUSSIONS

4.3.1 Energy-dispersive X-ray (EDX) spectroscopy

Energy-dispersive X-ray diffraction (EDX) analysis under a scanning

electron microscope JEOL model SU-66600; it was carried out for the study

of the presence of potassium. The EDX spectrum of mNAK crystal is shown

in Figure 4.2, confirming the presence of potassium. The weight% and

atomic% of elements present in the mNAK crystal are given in Table 4.1.

Table 4.1 Elemental compositions present in mNAK crystal

Element Weight % Atomic %

C 57.27 66.69

N 0.70 0.70

O 32.47 28.78

K 10.56 3.83

Total 100 100

Figure. 4.2: EDX spectrum of mNAK crystal.

60

4.3.2 Single and powder X-ray diffraction studies

Good quality pure and potassium doped meta-Nitroaniline single

crystals were selected and subjected to single crystal X-ray diffraction studies

using BRUKER AXS KAPPA APEX (II) CCD diffractometer with MoKα

( = 0.710693 Å) radiation. The lattice parameter and the cell volume of

mNA and doped mNAK crystals are presented in table 4.2. It is observed

from the X-ray diffraction data that both mNA and mNAK crystals belong to

orthorhombic crystal system with space group Pbc21. The observed values

are in good agreement with reported value [111]. Observed that there is a

slight change in the lattice parameters obtained from single crystal X-ray

diffraction in the case of pure, and potassium doped mNA crystals. This slight

variation in the lattice parameters is due to the incorporation of potassium

molecule in the crystal lattice.

The crystalline quality of the chosen material could easily be

determined by powder X-ray diffraction analysis. X-ray powder diffraction

(XRD) was performed on the grown crystals, to study the effect of doping on

mNA crystals with potassium. The powder XRD pattern was recorded using

powder Rich Seifert diffractometer with CuKα radiation ( = 1.54060Å). The

sample was scanned from 10°-60° at a scan rate of 2˚min-1. The XRD

patterns of the mNA and mNAK were indexed with PROSZKI ver. 2 software

program. The position of the peaks in the doped crystals was found to be in

agreement with that of pure mNA crystals. The indexed XDR patterns of the

pure and doped crystals are presented in Figure 4.3.The cell parameters

obtained from powder X-ray diffraction are in good agreement with the single

crystal X-ray diffraction data. The experimental and calculated „d‟ values

along with the „hkl‟ indices of the corresponding reflecting planes and 2

between the observed and calculated values for the mNA and mNAK crystals

are tabulated in table 4.3 and 4.4.

61

Table 4.2 Structural data for mNA and mNAK single crystals

Lattice para

meters

Single crystal XRD values mNA crystal

Powder XRD values mNA

crystal

Single crystal XRD values

mNAK crystal

Powder XRD values mNAK

crystal

a (Å) 5.08 5.08 5.01 5.11

b (Å) 6.50 6.48 6.43 6.46

c (Å) 19.21 19.30 19.10 19.29

90° 90° 90° 90°

90° 90° 90° 90°

90° 90° 90° 90°

Crystal System

Orthorhombic Orthorhombic Orthorhombic Orthorhombic

Volume 636.88 (Å 3) 636.88 (Å 3) 640.48 (Å 3) 654.48(Å 3)

Figure. 4.3: Indexed powder XRD patterns of mNA and mNAK crystals.

62

Table 4.3 Indexed powder X-ray diffraction data of mNA crystal

h k l d (obs.)

Å d(cal.)

Å 2θ(obs.) degree

2θ(cal.) degree

I (rel) I (abs)

1 0 0 6.86247 6.47472 12.89995 0.77611 15.7 56

1 1 0 6.17214 6.13842 14.34995 0.07926 23 82

0 4 0 4.84785 4.82436 18.29993 0.08984 41.6 149

1 3 0 4.60595 4.56331 19.26993 0.18179 25.1 90

1 1 1 3.94632 3.91977 22.52992 0.15463 17.4 62

1 2 1 3.72466 3.6976 23.88992 0.1774 100 357

0 4 1 3.52979 3.50261 25.22991 0.19908 10.1 36

1 3 1 3.42568 3.39878 26.00991 0.20957 50.5 180

2 0 0 3.23414 3.23736 27.57991 0.02797 8.2 29

1 4 1 3.10937 3.08072 28.70989 0.27279 22.8 82

1 5 1 2.80705 2.7785 31.87989 0.33651 17 61

0 7 0 2.74504 2.74504 32.61989 0.1427 11.2 40

2 4 0 2.66027 2.6882 33.68988 -0.36036 8.9 32

0 0 2 2.54673 2.54673 35.23989 0.0281 7.1 26

1 8 0 2.25776 2.26041 39.92988 -0.04884 6.3 22

63

Table 4.4 Indexed powder X-ray diffraction data of mNAK crystal

h k l d (obs.)

Å

d(cal.)

Å

2θ(obs.) degree

2θ(cal.) degree

I (rel) I (abs)

1 0 0 7.48696 6.485 11.81995 1.83433 11.9 87

1 0 0 6.92124 6.485 12.78995 0.86433 9.5 69

1 1 0 6.17214 6.14745 14.34996 0.05794 17.6 129

0 4 0 4.85837 4.8265 18.25995 0.12161 78.2 572

1 3 0 4.60358 4.56793 19.27995 0.15194 27.2 199

1 1 1 3.94978 3.91917 22.50994 0.17816 12.5 91

1 2 1 3.7262 3.69728 23.8799 0.18959 100 732

0 4 1 3.52842 3.50132 25.2399 0.21973 9.9 72

1 3 1 3.42698 3.39876 25.99991 0.21973 52.6 385

2 0 0 3.24106 3.2425 27.51987 0.01246 7.7 57

1 4 1 3.11149 3.08095 28.6899 0.2906 32.3 236

1 5 1 2.8062 2.77889 31.88984 0.32184 27.3 199

2 0 1 2.74423 2.73428 32.62985 0.12211 7.7 57

2 4 0 2.6618 2.69151 33.66994 0.38261 7.4 54

0 0 2 2.54324 2.5435 35.28986 0.00373 6 44

0 8 0 2.40199 2.41325 37.43983 0.18114 3.9 28

2 6 1 2.07721 2.08359 43.56976 0.14019 5.4 39

1 10 0 1.92481 1.9306 47.21976 0.02815 3.7 27

3 6 0 1.79527 1.79434 50.85976 0.02815 2.8 20

64

4.3.3 FT- IR analysis

To qualitatively analyze the presence of functional groups in mNAK

and mNA crystals, their respective FT-IR spectra were recorded with Perkin-

Elmer FT-IR spectrometer using potassium bromide pellet technique in the

range of 400–4000 cm-1and are shown in Figure 4.4. The sharp peak at

3413.85 cm-1 is due to N-H stretching [112]. The peak at 2889.61 cm-1 is due

to very weakly bonded N-H stretching. Aromatic C-H stretching band is found

at 3084.14 cm-1[113], N-H plane bending mode at 1615.78 cm-1, NO2

aromatic stretching mode at 1504.04, 1328.17, and 870.69 cm-1 and C-N

stretching amino group at 1080.34 cm-1 [114], are also found 644.59 cm-1

showed the presence of benzene ring [115].

Figure. 4.4: FT-IR spectrum of mNA and mNAK crystals.

65

4.3.4 Thermal analysis

Thermo gravimetric (TG) and differential thermal analysis (DTA) of

mNAK crystal was carried out using TG-DTA 6200 (SII EXSTAR 6000) with

alumina as reference. A sample weighing 2.84 mg was taken in an alumina

crucible and heated at a rate of 20ºC/ min in the nitrogen atmosphere.

Thermo grams recorded in the temperature range of 30–500ºC are shown in

Figure.4.5. DTA curve shows an endothermic peak at 114.4ºC, which

corresponds to the melting point of mNAK. The melting point of pure mNA is

112.7°C [116]. The coexistence of a second endothermic peak in DTA with

weight loss trace of TG could be attributed to the release of volatile material

from the mNAK melt. The sharpness of this endothermic peak shows good,

natural crystalline formation [117].

Figure. 4.5: TG-DTA curve of mNAK crystal.

Temp Cel500.0450.0400.0350.0300.0250.0200.0150.0100.050.0

2.00

0.00

-2.00

-4.00

-6.00

-8.00

-10.00

-12.00

TG %

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

0.0

114.4Cel

-11.84uV

220.9Cel

-7.80uV

94.1%

66

4.3.5 Mechanical study

The mechanical strength of a crystal determines its potential use in

device fabrication [118]. Microhardness study (applied load 25–100 g) was

carried out on the crystals using Vicker‟s microhardness tester with an optical

microscope (Shimadzu Scientific Instruments). Vicker‟s microhardness

number, vH was calculated using the following expression:

2

2

1.8554/V

PH kg mm

d

(4.1)

where P is the applied load (in kg) and d is the mean diagonal length in

( mm). For the mNAK crystal, vH is 107.5 kg/mm2 for an applied load of 100g

and mNA vH is 101.2 kg/mm2 for an applied load 100 gas shown in Figure

4.6. The plot of log P against log d is shown in Figure. 4.7; the straight line

indicates good agreement with Meyer‟s law. The slope of the graph denotes

the work hardening coefficient n, which was determined at 2.75 for mNAK

and 3.25 for mNA. According to Onitsch [119], n should lie between 1 and 1.6

for a hard material and >1.6 for soft ones. Thus, mNAK could be classified as

soft material. However, potassium doped meta-Nitroaniline has superior

hardness over pure meta-Nitroaniline.

67

Figure. 4.6: Microhardness value vs Load for mNA and mNAK crystals.

Figure. 4.7: log (P) vs log (d) of mNA and mNAK crystal.

68

4.4 LINEAR AND NONLINEAR OPTICAL STUDIES

4.4.1 UV-vis-NIR transmittance study

In order to reveal optical properties of the pure and potassium doped

meta-Nitroaniline single crystals, UV-vis-NIR transmittance studies were

carried out for the grown mNA and mNAK crystals between the wavelength

range of 200 nm and 1100 nm using Perkin-Elmer lamda 35 UV-

Spectrophotometer in the crystals of thickness 2 mm. Figure 4.8 shows the

transmittance spectrum of mNA and mNAK crystals. The lower cut-off

wavelength of mNA and mNAK crystal in the range of 345 nm and 352 nm

respectively. It is evident that there is no remarkable absorption observed

beyond 345 nm to the entire region of the spectrum. The percentage of

transmission about 90 % is attributed to the better optical quality of the grown

mNA and mNAK crystals. The very low absorption in the entire visible region

confirms the suitability of crystal for the fabrication of optical devices.

Figure. 4.8: Transmittance spectrum of mNA and mNAK crystals.

69

4.4.2 Determination of optical band gap

Spectrophotometer investigates the absorption of light by different

substances between the wavelengths from UV to NIR through the visible

region. In this wavelength range, the absorption of the electromagnet

radiation is caused by the excitation of the bonding and nonbonding electrons

of the molecules. To determine the nature of the absorption study was

performed for the crystals. The optical absorption coefficient (α) was

calculated from optical transmittance data using the following relation [120].

2.303log (1/ )T

t

(4.2)

where T is transmittance and t is the thickness of the crystal. The optical

band gap (Eg) was calculated from transmission spectrum using the relation

[121].

1

2

gh A h E (4.3)

where A is a constant, h is Planck‟s constant and ν is the frequency of

incident photon. A plot (Tauc‟s graph; figure 4.9) between the square of

product of absorption coefficient and incident photon energy, i.e. (h)2, and

the photon energy h at room temperature showed a linear behavior, which

might be considered an evidence of direct transition. The intercept obtained

by the extrapolation of the plot to the energy axis represents the band gap

energy of mNAK and mNA, which are found as 3.76 and 3.91 eV,

respectively.

70

Figure. 4.9: The plot of h vs (h)2 (eV).

4.4.3 Determination of optical constants

The optical constants were estimated by Kramers-Kronig

transformation of transmittance and the reflectance data or from direct

calculation when reflectance and transmittance are both measured [122]. In

fact, the optical constants, such as refractive index (n) and extinction

coefficient (k) are determined from the transmittance data.

The extinction coefficient (k) is the fraction of light lost due to

scattering and absorption per unit distance. In other words, k denotes the

amount of absorption loss as electromagnetic waves propagated through the

material. To compute k, the following equation was used [123]:

4k

(4.4)

Where α is the absorption coefficient.

The transmittance (T) is given by the following relation [124];

71

2

2

1 exp( )

1 exp( 2 t)

R tT

R

(4.5)

The reflectance (R) in terms of the absorption could be obtained from

the equation [125];

2exp exp exp 3 expexp 2

exp exp 2

t t T t T t TR

T t T

(4.6)

Refractive index (n) was calculated directly from the following well

known following equation [126];

1 2

1

R R

Rn

(4.7)

Figure. 4.10: Extinction coefficient and Refractive index vs wavelength

of mNAK crystal.

From the Figure 4.10, it is clear that refractive index and extinction

coefficient strongly depend on wavelength in UV region. That is, refractive

index would decrease as wavelength increases. The variation of n and k with

wavelength shall signify some interaction between photons and electrons.

Thus, refractive index for mNAK has been estimated at 1.51 for the visible

region. For pure mNA, refractive index is 1.78 [127].

72

The optical conductivity (σ) of the crystal was calculated using the

following relation [128];

Im4

(4.8)

where Im( ) is given by

2

2Im( ) ( )

( )r

ck

(4.9)

where r is relative permeability, which for most crystalline materials shall be

very close to 1 at optical frequencies. The relation between refractive index

(n) and extinction coefficient (k) is given by,

ckn

(4.10)

Comparing equation (4.8) with equation (4.9),

4

nc

(4.11)

From the plot of optical conductivity verses absorption coefficient (Figure.

4.11), we could note that optical conductance increased with increase in

absorption coefficient.

Figure. 4.11: Plot of optical conductivity against absorption coefficient

for mNAK crystal.

73

4.5 NONLINEAR OPTICAL STUDIES

The nonlinear optics, a frontier in science and technology, plays a

major role in the emerging era of photonics which involves photons for

applications. Nonlinear optical (NLO) process has application, such as

frequency conversion and optical switching, optical data storage; optical

signal progressing etc., power SHG test offers the possibility of assessing the

nonlinearity in materials. Kurtz and Perry proposed a powder SHG method

for comprehensive analysis of the second order nonlinear optical effects

[129]. This is a useful method for characterizing the materials before

fabrication. The limitation of this method is that the strength of the second

harmonic signal is affected by the size and shape of the particle. Using this

technique, Kurtz analyzed a very number of compounds for potential

applications.

4.5.1 Second harmonic generation (SHG) analysis

The second harmonic generation (SHG) efficiency of the grown pure,

and potassium doped meta-Nitroaniline crystals in the present work was

studied by Kurtz powder test. A Q-switched Nd:YAG laser 1064nm ;

Quanta Ray, USA with pulse duration of 6 ns and repetition rate of 10 Hz was

used to excite the powdered sample. Firstly, single crystals of pure mNA,

potassium doped meta-Nitroaniline (mNAK) were grinded into fine powder of

uniform particle size to be filled into a micro-capillary tube. An emission of

bright green radiation ( 532 )nm from the sample shall confirm the

generation of second harmonics. A dual-channel energy meter (Coherent

Molectron, USA) showed a second harmonic signal of 5.0 mJ for an input

energy of 0.68 joule. For potassium di hydrogen phosphate (KDP) crystal, a

second harmonic signal of 8.8 mJ was produced for the same input energy.

SHG efficiency of the mNA and mNAK are 1.27, 1.988 times than that of the

standard KDP crystal. The SHG efficiency of the mNA and mNAK crystals

are compared with other known compounds and is shown in table 4.5.

74

Table 4.5 Comparisons of different NLO crystals compound (SHG)

efficiency

S.No NLO crystals

Relative SHG

efficiency with

KDP

References

1 Potassium dihydrogen phosphate

(KDP)

1 130

2 Potassium titanyl phosphate (KTP) 1.5 130

3 L-valine cadmium acetate 0.5 131

4 Guanidinium phenyl arsonate

(GPA)

0.785 132

5 Pure L-valine 0.82 133

6 meta-Nitroaniline (mNA)* 1.290

7 Potassium doped meta-Nitroaniline

(mNAK)*

1.950

* Present work

4.5.2 Z- Scan study

Using the Z-scan technique, the sign and magnitude of the nonlinear

refractive index 2n could be easily extracted. The Z-scan experiments, a

532-nm, diode-pumped Nd:YAG laser (Coherent Compass 215M-50), using

a lens of focal length 3.5 cm, was employed. Laser beam waist at the focus

0 , was measured at 15.84 μm, and Rayleigh length at 1.48 mm.

A 1-mm wide optical cell, containing the sample in solid, was translated from

75

the focal region along the axial direction (direction of propagation of laser

beam). Transmission of the beam through an aperture placed in the far field

was measured using a photo detector attached to a digital power meter (Field

Master GS Coherent). For an open aperture Z-scan, a lens was replaced for

the aperture to collect the entire laser beam transmitted through the sample.

The development of high power laser sources has motivated an extensive

research in the study of nonlinear optical properties and optical limiting

behavior of materials. Z-scan technique [61,81] based on the spatial

distortion of a laser beam, passed through a nonlinear optical material, is

widely used in material characterization because of its simplicity, high

sensitivity and well-elaborated theory.

The measurable quantity p v

T could be defined as the difference

between the normalized peak and valley transmittances, p v

T T . The

variation of this quantity as a function of an axis phase shift 0 is given by

0.25

00.406(1 ) ,p v

T S (4.12)

where2

21 exp a

a

rS

is the aperture linear transmittance (0.01), 0 is

the on-axis phase shift. The on-axis phase shift is related to the third-order

nonlinear refractive index by

0 2 0effkn L I , (4.13)

where 2 / , [1 exp( )] /eff

k L L , is the effective thickness of the

sample, α is the linear absorption coefficient, L is the thickness of the

sample, I0 is the on-axis irradiance at focus and n2 is the third order nonlinear

refractive index. Figures 4.12 (a) and 4.12 (b) show the close and open

aperture Z-scan curves for mNA crystal. In the closed aperture Z-scan curve,

the pre-focal transmittance peak is followed by valley, which is the signature

of negative nonlinearity [134]. The Figures 4.12 (d) and 4.12 (e) show the

close and open aperture Z-scan curves for mNAK crystal. Both mNA and

76

mNAK crystals have the negative refractive index; it indicates the defocusing

nature of the material, which is an essential property for the application in the

production of optical sensors, such as night vision devices. From a graph of

normalized transmittance versus sample position, for both closed and open

aperture, nonlinear refractive index 2( )n and saturation absorption coefficient

( ) could be determined. Here, because the closed aperture transmittance

was affected by NLR and absorption, the determination of 2( )n was less

straight forward using the closed aperture scans. Closed and open aperture.

NLA coefficient ( ) was estimated from the open aperture Z-scan data. For

the open aperture condition, normalized transmittance is given by it is

necessary to separate the effect of nonlinear refraction from that of the

nonlinear absorption.

A simple and approximate method to obtain purely effective 2n is to

divide the closed aperture transmittance by the corresponding open aperture

scans. Represents such plots obtained for the samples, i.e., the ratio curve of

mNA and mNAK as shown in the figure.4.12 (c), 4.12(f). The data obtained in

this way reflects purely the effects of nonlinear refraction.

The nonlinear absorption coefficient could be estimated from the

open aperture Z-scan data. The normalized transmittance for the open

aperture condition is given by

0

3/20

[ ( )]( , 1) ,

( 1)

m

m

q zT z S

m

(4.14)

For 0 (0) 1,q where q0 (z) = βI0Leff/(1+z2/zR2), zR= kω0

2/2 is the diffraction

length of the beam and ω0 is the beam waist radius at the focal point and

2k

is the wave vector.

77

Figure. 4.12: (a) Closed aperture curves (s = 0.035) of mNA crystal, (b)

Open aperture curve (s = 1) of mNA crystal,(c) Ratio curve

for mNA crystal.

78

The experimental measurements of 2n and allow one to determine

the real and imaginary parts of the third-order nonlinear optical susceptibility

(3) according to the following relations

2 2 2

(3) 4 0 02Re ( ) 10

c n cmesu n

W

(4.15)

where 0 is the vacuum permittivity, and c the light velocity in vacuum:

2 2 2

(3) 2 0 0

2Im ( ) 10

4

c n cmesu

W

(4.16)

The absolute value of (3) is calculated from

(3) (3) 2 (3) 2 1/2[(Re( )) (Im( )) ] (4.17)

The nonlinear parameters, such as nonlinear refractive index 2n ,

nonlinear absorption coefficient , and nonlinear susceptibility (3) , has been

evaluated mNA and mNAK values tabulated in table 4.6.

79

Figuer. 4.12: (d) Closed aperture curves (s = 0.035) of mNAK crystal, (e)

Open aperture curve (s = 1) of mNAK crystal, (f) Ratio

curve for mNAK crystal.

80

Table 4.6 The nonlinear parameters values of mNA and mNAK crystals

S.No Compound 2n × 10-8

(cm2 / W)

× 10-4

(cm / W)

(3) × 10-6esu

1 mNA 1.43 2.82 1.53

2 mNAK 3.03 4.24 1.732

4.6 CONCLUSION

Single crystals of pure and potassium doped meta-Nitroaniline crystals

were grown by slow evaporation method using methanol as a solvent. The

presence of potassium in mNAK was confirmed by using EDX analysis.

Single-crystal and powder X-ray diffraction studies of mNA and mNAK

revealed an orthorhombic crystal system with space group Pbc21. Vibration

frequencies from FT-IR spectral analysis confirmed the presence of all

functional groups. The DTA curves show an endothermic peak at 114ºC,

which corresponds to the melting point of mNAK. By Vicker‟s microhardness

technique, the crystal was identified as a soft material. Optical transmittance

study, the UV cut-off wavelength of the mNA and mNAK crystals was

measured to be 345 nm and 352 nm. The crystals showed good absorbance

in the entire UV region and transmittance in the entire visible and near-

infrared regions. Refractive index and optical band gap of mNAK crystal were

1.51 eV and 3.76eV. The relative SHG activity of the crystal was confirmed

by Kurtz and Perry method using high intensity Nd:YAG laser. From Z-scan

measurements, we further confirmed that mNAK exhibited large third-order

nonlinear optical properties. Closed aperture Z-scan study revealed the

negative nonlinearity of the crystal, and open aperture Z-scan study

ascertained the saturation absorption. The negative sign of the nonlinear

refractive index suggests that mNA and mNAK exhibited self-defocusing

optical nonlinearity.

81

5. GROWTH AND CHARACTERIZATION OF POTASSIUM DICHROMATE

(KDC) SINGLE CRYSTAL

5.1 INTRODUCTION

In recent years, the nonlinear optical materials are attracted by

researchers owing to their application to high-speed all-optical switching

devices [135]. The search for large third order optical nonlinearities and fast

response is essential for technological development in nonlinear optical

materials [72].The nonlinear optical processes provide the significant

functions of frequency of the system. The applications depend upon the

various properties of the materials, such as transparency, birefringence, laser

damage threshold, refractive index, dielectric constant, second order

nonlinearity, large third order susceptibilities, etc., Nonlinear optical materials

with large third order nonlinear susceptibilities are essential for all-optical

switching, modulating and computing devices because the magnitude of this

quantity dominates the device performance [136-139]. The introduction of the

Z-scan technique [81] for the determination of third order optical

nonlinearities brought a flood of activity to the field of third order nonlinear

optical (NLO) materials. The achievement of the technique and the analysis

of the results being rather simple, the Z-scan were used in numerous NLO

investigations [119].V. Natarajan et al has reported third order nonlinear

optical properties in Centro-symmetry nature potassium aluminium sulphate

single crystal using Z-scan technique[140].The third order nonlinear optical

(NLO) materials with weak nonlinear absorption (NLA) but strong nonlinear

refraction (NLR) has attracted considerable attention because of their

potential uses in the optical signal processing devices [141,142]. The Z-scan

technique is a popular method to measure the optical nonlinearity of a given

material. It has the advantage of high sensitivity and simplicity. [67,143]. One

could simultaneously measure the magnitude and sign of the nonlinear

refraction and nonlinear absorption, which are associated with the real part

Re (3) and imaginary part Im (3) of the third order nonlinear susceptibilities.

82

5.2 CRYSTAL GROWTH

The potassium dichromate (KDC) crystals were grown by solution

growth technique. AR grade potassium dichromate was dissolved in double

distilled water and stirred well for 6 hours at room temperature. The purity of

the salt was increased by re crystallization process. The saturated solution

was filtered through Whatmann filter paper then it was closed with a

perforated cover and kept in a dust free atmosphere. The seed crystals were

harvested in a time period of 22 days. Optically transparent and defect-free

crystal were having dimensions 15 × 8 × 5 mm3 was grown and the

photograph of the grown crystal is shown in Figure.5.1.

Figure. 5.1: As grown single crystal of KDC.

5. 3 RESULTS AND DISCUSSION

5.3.1 Single crystal and powder X-ray diffraction studies

Well shaped, transparent and quality single crystal of potassium

dichromate crystal was selected and it was subjected to single crystal XRD

analysis at room temperature. It was carried out using a Bruker AXS

diffractometer with MoKα ( = 0.7170 Å) radiation to identify the lattice

parameters. The intensity data were collected up and accurate unit cell

parameters were obtained based on all reflections. From the single crystal X-

ray diffraction study, it is revealed that the KDC crystal belongs to the triclinic

83

system with space group 1P . The lattice parameters obtained as,a = 7.38 Å,

b = 7.46 Å, c = 13.38 Å, = 95.19° = 98.06°, = 90.93° and the volume

of the unit cell is found to be = 724.0 Å3.

The powder XRD study was carried out using a Rich–Seifert

diffractometer with CuKα ( =1.5406 Å) radiation. The indexed powder XRD

pattern of the grown KDC crystal as shown in(Figure 5.2). The powdered

KDC sample was scanned over the range 10- 40° at the rate of 1° per min.

Sharp and strong peaks confirmed the good crystallinity of the grown crystal.

From the powder X-ray data, the various planes of reflections were indexed

using XRDA 3.1 program. The lattice parameters are evaluated as:

a = 7.42Å, b = 7.40 Å, c = 13.40 Å, = 96.17° = 98°, = 90.83°, and

volume of the unit cell is = 733.82 Å3. The obtained lattice parameters from

the powder XRD analysis are in good agreement with the literature reported

values and single crystal XRD analysis [144]. The structural data for KDC

single crystal and powder X-ray data are listed in table 5.1.

Figure. 5.2: Powder X-ray diffraction spectrum of KDC crystal.

84

Table 5.1 Structural data for KDC single crystal

Lattice parameters

Single crystal XRD value

Powder XRD value

Reported value [144]

a (Å) 7.38 7.42 7.37

b (Å) 7.46 7.40 7.44

c (Å) 13.38 13.40 13.36

95.19 96.17 96.21

98.06 98.0 97.96

90.93 90.83 90.75

Volume 724.0 733.82 722.3

Crystal system Triclinic Triclinic Triclinic

Space group 1P 1P 1P

5.3.2 Optical absorption study

The optical absorption spectrum of the grown crystal was recorded in

the wavelength range between 200 and 800 nm using Perkin-Elmer lamda 25

UV-Spectrometer and the resultant spectrum is shown in Figure 5.3. The

crystal is transparent in the entire visible region and the UV cut-off

wavelength is found as 240 nm. The very low absorption in the entire visible

region confirms its suitability for the fabrication of nonlinear optical devices

[145].

85

Figure. 5.3: UV-vis-NIR spectrum of KDC crystal.

5.3.3 Thermal analysis

TGA and DTA analysis of KDC crystal were carried out with the help

of TG/DTA 6200 SII EXSTAR 6000 (Figure 5. 4) using alumina as reference.

A sample of weight 3.84 mg was taken in a crucible. The sample was heated

at a rate of 20ºC/min in the nitrogen atmosphere. Both the TGA and DTA

curves show that the melting point of KDC is 400 °C. The TGA curve shows

that the material has high thermal stability; it is stable up to 397.1°C. The

TGA curve shows that the major decomposition takes place after 700°C. The

DTA line shows two endothermic peaks 277.5 °C and 400 °C. Around 99.4%

of the material decomposes at 800°C.

86

Figure. 5.4: TG-DTA curves for KDC crystal.

5.3.4 Mechanical study

Vicker‟s microhardness study was carried out using REICHERT

POLYVAR 2 hardness test attached–with Micro-Duromat4000E.The

hardness of the material depends on different parameters, such as lattice

energy, Debye temperature heat of formation and inter atomic distance [146,

147]. According to Gong [148], during an indentation process, the external

work applied by the indenter is converted to a strain energy component to the

volume of the resultant impression and the surface energy component

proportional to the area of the resultant impression. The applied loads were

10 g to 60 g and hardness numbers calculated using the relation [149].

2

2

1.8554/V

PH kg mm

d (5.1)

vH -Vicker‟s micro hardness number, P: applied load (g), d: diagonal length

(mm) of the indentation. The microhardness value was taken as the average

of the several impressions made. Crack initiation and materials chipping

become significant beyond 58 g of the applied load (Figure 5.5).

87

Figure. 5.5: Vicker’s hardness number (HV) vs Load (P) of KDC.

5.3.5 Etching analysis

Chemical etching study was carried out using high magnification

REICHERT POLYVAR 2 MET microscope. Etching studies were carried out

to study the growth mechanism and asses the perfection of the growth

crystal. Etching is the selective dislocation of the crystal surface, which

reveals the growth mechanism and surface features. In the present work,

etching study was carried out on KDC crystal using deionized water as an

etchant. The reactants which are absorbed more strongly produce better

contrasting pits. Etching was carried out for 10 s and 30 s. The etched

surface was dried by pressing gently between filter papers. Optical

microscope pictures were taken which are shown in the Figure 5.6 (a) and

(b).Well defined pyramidal etch pits were formed on the surface of the grown

crystal. This is an indicator of two-dimensional layer growth mechanisms due

to the presence of growth hillocks in association with striations.

88

Figure. 5.6: Photograph of the etching pattern for (a) 10s, (b) 30s.

5.4 NONLINEAR OPTICAL (NLO) STUDIES

5.4.1 Second Harmonic Generation (SHG) Study

Powder SHG study for KDC single crystal was carried out in

accordance with the classical powder method developed by Kurtz and Perry

method [129]. A Q-switched Nd: YAG laser ( =1064 nm, Quanta Ray, USA)

beam of wavelength 1064 nm and pulse with of 6 ns with a repetition rate of

10 Hz was used. The potassium dichromate powder placed in a micro

capillary was exposed to laser radiation. The second harmonic signal was

absent for this sample and it confirms the Centro-symmetric nature of the

crystal.

5.4.2 Z-scan study

Z-scan technique [61,81] based on the spatial distortion of a laser

beam, passed through a nonlinear optical material, is widely used in material

characterization because of their simplicity, high sensitivity and well-

elaborated theory. The saturation absorption for the sample in solvent at a

transmission of about 50% .The closed aperture Z-scan curve of potassium

dichromate (KDC) crystal as shown in Figure.5.7.(a).The peak followed by a

valley-normalized transmittance curve obtained from the closed aperture

Z-scan data, indicates that the sign of the refraction nonlinearity is negative

i.e. self-defocusing nature [150]. As the material has a negative refractive

89

index it results in defocusing nature of the material, which is an essential

property for the application in the protection of night vision devices [151]. The

open aperture Z-scan curve is shown in Figure 5.7 (b).Therefore, it is

necessary to separate the effect of nonlinear refraction from that of the

nonlinear absorption. Represents such plots obtained for the samples, i.e.,

the ratio of closed aperture and the open aperture Z-scan as shown in Figure

5.7 (c).The data obtained in this way reflects purely the effects of nonlinear

refraction.

The nonlinear parameters, such as nonlinear refractive index 2 ,n

nonlinear absorption coefficient , and nonlinear susceptibility , values of

potassium dichromate (KDC) crystal were calculated similarly to the

discussed in chapter 4 and the values are presented in table 5.2.

Table 5.2 Nonlinear optical parameters of KDC single crystal

Compound 2n × 10-8 (cm2 / W) × 10-4 (cm/W) (3) × 10-6 esu

KDC 9.41 0.57 4.68

90

Figure.5.7: (a) Closed aperture Z-scan curve for KDC crystal (b)

Openaperture curve for KDC crystal, (c) Ratio curve of KDC

crystal.

91

5.5 CONCLUSION

The single crystal of potassium dichromate (KDC) was successfully

grown from an aqueous solution using slow evaporation technique. The

single crystal X- ray and Powder X-ray diffraction studies confirm the triclinic

structure with space group 1P . The UV cut off wavelength is found occur at

240 nm. TG-DTA studies revealed that the crystal is thermally stable up to

397.1°C .The mechanical and etching studies of the grown crystal was

carried out. The absence of second harmonic generation material confirms

the centro symmetric nature of the grown crystal. Closed aperture Z-scan

study reveals the negative nonlinearity in the crystal and open aperture Z-

scan study confirms the saturation absorption. The nonlinear parameters

values were evaluated. The third order nonlinear properties confirm its

suitability of KDC crystals for nonlinear optical devices, such as optical

limiting and optical switching.

92

6. EFFECT OF KDP ON THE GROWTH, THERMAL AND OPTICAL

PROPERTIES OF L-ALANINE SINGLE CRYSTAL

6.1 INTRODUCTION

Nonlinear optics (NLO) is at the forefront of current research because

of its importance in providing the important functions of frequency shifting,

optical modulation, optical switching, optical logic and optical memory for the

emerging technologies in areas, signal processing, telecommunications and

optical interconnections [152-153]. Organic materials show a good second

harmonic generation (SHG) efficiency compared to inorganic materials [154].

The amino acid family crystals were grown and studied by several

researchers for their excellent nonlinear optical (NLO) properties [155-158].

Amino acids are interesting materials for nonlinear optical applications which

contain a proton donor carboxyl acid (COO−) group and a proton acceptor

amino (NH2+) group in them [159]. The L- alanine could be considered as the

fundamental building block of more complex amino acids which shows strong

nonlinear behaviour and abnormal phonon connection and is a system

exhibiting vibrational solutions [160,161].Most recent works have

demonstrated that organic crystals could have very large nonlinear

susceptibilities compared with that of inorganic crystals, but their use is

impeded by poor mechanical properties and the inability to produce large

crystals. Purely inorganic NLO materials typically have excellent mechanical

and thermal properties with relatively modest optical nonlinearity [162].

Monaco et al identified several salts of L-arginine derivatives with better

nonlinear optical (NLO) properties than potassium di hydrogen phosphate

(KDP) and verified the linear and nonlinear optical properties [58]. Fuchs et al

[163] reported the thermal and mechanical properties of L-arginine

phosphate (LAP) crystals. It is reported that the surface compliances of L-

arginine phosphate (LAP) are two to three times more than those of KDP and

the volume compressibility of LAP is about twice as large as that of KDP.

Mohan Kumar et al [164] investigated the effect of L-lysine in TGS and TGSP

Single crystals. Recently some researchers have investigated of an effect of

amino acid mixing on the properties of various NLO crystals. [165,166].

93

In the present work, single crystal of L-alanine has been grown from

the mixture of alanine and potassium di hydrogen phosphate in the aqueous

solution by slow evaporation techniques. The grown crystals were subjected

to various characterization studies such as single crystal X-ray diffraction,

FT-IR analysis, UV-visible spectral study, thermal studies and NLO test.

Third order nonlinear studies were carried out using Z-scan technique using

continuous wave Nd: YAG Laser.

6.2 EXPERIMENTAL PROCEDURE

The title compound L-alanine was synthesized by taking AR grade

L-alanine and potassium di hydrogen phosphate in the molar ratio of 1:1 and

dissolved in deionized water. All starting materials were of AR grade and the

growth process was carried out in aqueous solution. The solution was stirred

well for 5 hrs, to obtain a homogeneous solution. The resulting solution was

filtered using Whatmann filter paper.The solution was taken in a beaker and

closed with a perforated cover and kept in a dust free atmosphere. The

crystals were harvested when they attained an optimal size and shape within

20 days. Optically transparent and defect- free crystal having dimensions of

about 13 × 8 × 3 mm3as shown in Figure 6.1.

Figure. 6.1: As grown L-alanine single crystal.

94

6.3 RESULTS AND DISCUSSION

6.3.1 Single crystal X-ray diffraction

Single crystal X-ray diffraction analysis was carried out using a Bruker

AXS diffractometer with MoKα ( =0.7170Å) radiation to identify the lattice

parameters. The single crystal X-ray diffraction study confirms the

orthorhombic structure with a non-centro symmetric space group of P212121.

The lattice parameters of L-alanine are; a = 5.773 Å, b= 6.008 Å, c = 12.346

Å, with volume V= 428.1 Å3.The crystal parameters and cell volume found to

be in agreement with the reported values [167].

6.3.2 FT-IR spectral analysis

The FT-IR spectrum of grown L-alanine crystal was recorded in the

solid state using KBr pellet techniques and the recorder spectrum dispersion

using Perkin as shown in Figure 6.2.The absorption peaks at 3085.89,

1620.09, 1513.37 cm−1 are the indication of the presence of NH3+ amine

group and OH group of the water molecule in the grown crystal [168].The

spectrum exhibited a characteristic N-H stretching absorption band in the

high frequency range between 2782 and 3088 cm–1. The peak at 2782 cm–1is

assigned to C-H stretching mode vibration [169]. The peak at 2111.0cm-1, in

the overtone region, is assigned to combinational and asymmetrical bending

vibration of NH3+.Very strong band occurring at 539 cm-1 is contributed of

HO-P-OH bending vibration. The presence of phosphate assigned to the

functional group might be due to the mixed growth solution of water and

potassium di hydrogen phosphate (KDP) [170]. The bending modes of CH3

are positioned at 1316 and 1455 cm-1, respectively. The peaks at 1113 cm-1

are due to C-O stretch and O-H bend of the CO-OH group is observed at

1236, 1306 and1361cm-1[171].

95

Figure. 6.2: FT-IR Spectrum of L-alanine crystal.

6.3.3 Transmittance spectral study

The UV-vis transmission spectrum was recorded at room temperature

using Shimadzu 1601 UV-vis-NIR spectrophotometer in the range of 190–

1200 nm. The resultant spectrum as shown in Figure 6.3.The crystal is highly

transparent in the entire visible region, whereas it has a UV cut-off

wavelength 256 nm. The transmission is uniformly high (73%) for light in the

visible region of the electromagnetic spectrum, which is useful for nonlinear

device application. The optical band gap is obtained by plotting the graph of

h vs 2( )h as shown in (Figure.6.4). From the graph, the optical energy

gap of L-alanine is determined as 4.9 eV.

96

Figure. 6.3: Transmittance spectrum of L-alanine crystal.

Figure. 6.4: Plot of h vs 2( )h for L-alanine crystal.

97

6.3.4 Thermal analysis

The thermo gravimetric analysis of L-alanine crystal was carried out

for the sample of initial weight of 8.23 mg between 50 and 800˚C at a heating

rate of 20 K min-1 in nitrogen atmosphere using NETZSCH STA 409 C/CD

thermal analyser. The resultant spectrum is shown in Figure 6.5. The TGA

curve shows that there is no weight loss up to 230 °C. The weight loss of

96.239% is observed in the temperature range 230–305 °C. The sample is

thermally stable up to 230 °C and suitable for device applications. From DTA

curve, it is observed that there is one endothermic peak at 288.7 °C which

represents the decomposition of L-alanine. The sharpness of this

endothermic peak shows the good degree of crystallinity of the sample [172].

Figure. 6.5: TG-DTA curve for L-alanine crystal.

6.4 NONLINEAR OPTICAL STUDIES

6.4.1 Second harmonic generation (SHG) study

The SHG efficiency of L-alanine was measured by Kurtz and Perry

powder technique [129]. A Q-switched solid state Nd:YAG laser ( = 1064

nm) with the pulse duration of 6 ns and frequency repetition rate of 10 Hz

was used. The grown single crystal of L-alanine was ground into fine powder

with uniform particle size and then filled into the micro-capillary tube. The

98

emission of bright green radiation ( = 532 nm) from the crystalline powder

confirms the generation of second harmonics. The second harmonic signal of

9.0 mJ was obtained for an input energy of 0.68 joule. The SHG value of

potassium di hydrogen phosphate (KDP) crystal gives a signal of

8.8mJ/pulse for the same input energy. Thus it is observed that the SHG

efficiency of the L-alanine is 1.022 times than that of the standard KDP

crystal. For pure L-alanine, it is found to be 0.33 times that of standard KDP

crystal [173]. The enhancement of second harmonic generation efficiency

(SHG) in the present work on L-alanine might be due to the presence of

phosphate in the grown crystalline sample.

6.4.2 Third harmonic optical studies

To study the third order nonlinear optical properties, a single beam

Z-scan technique was employed, which readily provides the magnitude and

sign of nonlinearity [174].The transmission of the beam through an aperture

placed in the far field is measured using photo detector fed to the digital

power meter (Field master Gs- coherent). For an open aperture Z-scan, a

lens to collect the entire laser beam transmitted through the sample replaced

the aperture development of high power laser sources. It has motivated an

extensive research in the study of nonlinear optical properties and optical

limiting behaviour of materials [58,175]. The Z-scan technique developed by

Sheik-Bahae et al, is used to characterize the nonlinear optical properties of

the materials [61,81]. This method is a simple and effective tool for

determining the nonlinear properties. For measuring the refractive nonlinear

property, an aperture was placed in front of the detector and the

transmittance was recorded as a function of the sample position on the

Z-axis (close aperture Z-scan). For measuring the nonlinear absorption the

sample transmittance was measured without aperture as a function of

sample position (open aperture Z-scan).It has been used widely in material

magnitudes of the real and imaginary parts of the nonlinear susceptibility, but

also the sign of the real part. Figure.6.6 (a) and (b) show the closed and open

aperture Z-scan curves for L-alanine crystal. In the closed aperture Z-scan

curve, the pre-focal transmittance peak is followed by the post focal valley,

99

which is the signature of negative nonlinearity [176].i.e., self -defocusing. The

self-defocusing effect is due to the local variation of the refractive index with

temperature. A simple and approximate method to obtain purely effective n2

is to divide the closed aperture transmittance by the corresponding open

aperture scans represents such plots obtained for the samples, i.e., the ratio

of closed aperture and the open aperture Z-scan as shown in Figure 6.6 (c).

The data obtained in this way reflects purely the effects of nonlinear

refraction.

The third order nonlinear optical parameters, such as nonlinear

refractive index 2n , nonlinear absorption coefficient , and nonlinear

susceptibility , values were examined similar to the L-alanine crystal as

discussed in chapter 4 and the values are presented in table 6.1.

Table 6.1 Nonlinear optical parameters of L-alanine single crystal

Compound 2n × 10-8 (cm2 / W) × 10-4

(cm/W)

3 × 10-6 esu

L-alanine 8.82 0.15 4.31

100

Figure. 6.6: (a)Closed aperture curve (s=0.035) of L-alanine crystal (b)

Open aperture curve(s=1) of L-alanine (c) Ratio curve for

L-alanine crystal.

101

6.5 CONCLUSION

A single crystal of L-alanine has been grown from an aqueous solution

of potassium di hydrogen phosphate (KDP) by slow evaporation method. The

single crystal X- ray diffraction studies confirm that title crystal belongs to the

orthorhombic structure with the space group P212121. The presence of

functional groups was determined using FT-IR analysis. UV-vis-NIR

Spectrum of L-alanine shows 73% transparency in visible region and its band

gap energy is found to be 4.9 eV. The grown crystals are thermally stable up

to 288.7˚C .The SHG efficiency of the L-alanine is 1.022 times than that of

the standard KDP crystal.The Z-scan measurements further confirm that the

material exhibit large third order nonlinear optical properties. The third order

nonlinear parameters are highly encouraging for the crystal. All these studies

indicate that the grown crystal is the potential material for second harmonic

as well as third harmonic generation applications.

102

7. SUMMARY AND SUGGESTIONSFOR FUTURE WORK

7.1 SUMMARY

In the past there was a considerable interest in the study of new

materials excellent optical nonlinearities and potential use in applications

including telecommunication, optical computing, optical data storage and

optical devices. The nonlinear optical (NLO) materials for second harmonic

generation (SHG), optical parametric oscillation (OPO) or optical parametric

amplification (OPA) have resulted in the development of numerous organic,

inorganic and semi organic nonlinear optical crystals.

The chapter I dealt with an elaborate introduction of crystals growth.

The various methods of crystal growth are explained in this chapter, with an

emphasis to solution growth techniques. The scope of the thesis is presented

in detail. In chapter II literature survey is made crystals are developed grown

from inorganic, organic and semi organic materials. The theoretical concept

of nonlinear optics is explained thoroughly.

In the chapter III an overview of various principles and instrumentation

techniques, which are used to characterize the grown crystals are described.

The principles, theory and instrumentation which involves in various

characterization techniques, such as single XRD, powder XRD, FT-IR, UV-

Vis-NIR spectra, thermal, hardness, SHG efficiency and Z-scan studies are

presented in detail.

Chapter IV discussed the growth, thermal, mechanical, linear and

nonlinear optical properties of pure and potassium doped meta-Nitroaniline

single crystals. Good quality single crystals of pure and potassium-doped

meta-Nitroaniline crystals were grown by slow evaporation method using

methanol as solvent. Transparent single crystals of mNAK with dimensions

15×6×3 mm3 were obtained after 25 days. Simultaneously, a pure mNA

crystal was grown using the same procedure. The presence of potassium in

mNAK was confirmed by EDX analysis. The lattice parameters were found

by single crystal XRD study and are in good agreement with the reported

103

values. From the powder XRD studies, the various planes of reflection of the

crystals were identified and indexed. The pure and doped crystals belong to

orthorhombic system having space group of Pbc21. Vibration frequencies

from FT-IR spectral analysis confirmed the presence of all functional groups.

The DTA curves show an endothermic peak at 114ºC, which corresponds to

the melting point of mNAK. By Vicker‟s microhardness technique, the crystal

was identified as grown crystals which are soft material categories. From

optical transmittance study, the UV cut-off wavelength of the mNA and mNAK

crystals was measured to be 345 nm and 352 nm. The crystals show good

absorbance in the entire UV region and transmittance in the entire visible and

near-infrared regions. Refractive index and optical band gap of mNAK were

1.51 and 3.76 eV,. SHG efficiency of the mNA and mNAK crystalline sample

is 1.27, 1.988 times than that of the standard KDP crystal.The nonlinear

optical parameter value of mNA and mNAK crystals are nonlinear refractive

index 2n =1.43 x10-8 (cm2/W), nonlinear absorption = 2.82 x 10-4 (cm / W),

nonlinear susceptibility (3) = 1.53×10-6esu nonlinear refractive index 2n =3.03

x10-8 (cm2/W), nonlinear absorption = 4.24 x 10-4 (cm / W), nonlinear

susceptibility (3) = 1.73×10-6esu.

Chapter V dealt with the growth and characterization of potassium

dichromate (KDC) single crystals .The single crystal of potassium dichromate

(KDC) was successfully grown from an aqueous solution using slow

evaporation technique. The grown appreciable size of the order of 15 × 8 × 5

mm3 was reported in a period of 22 days. The single crystal X- ray and

Powder X-ray diffraction studies confirm the triclinic structure of KDC with

space group 1P . The lattice parameters obtained as, a = 7.38 Å, b = 7.46

Å,c= 13.38 Å, = 95.19° = 98.06°, = 90.93° and the volume of the unit

cell is found to be = 724.0 Å3.The UV cut off wavelength of KDC is found to

be at 240 nm. TG-DTA studies revealed that the crystal is thermally stable up

to 397.1°C .The mechanical and etching studies of the grown crystal was

carried out. Absence of second harmonic generation in this material confirms

the Centro-symmetric nature of the grown crystal. Closed aperture Z-scan

study reveals the negative nonlinearity in the crystals and open aperture

104

Z-scan the saturation absorption. The nonlinear optical parameter value of

nonlinear refractive index 2n =9.41 x10-8 (cm2/W), nonlinear absorption =

0.57x 10-4 (cm / W), nonlinear susceptibility (3) = 4.68×10-6esu.The third

order nonlinear properties confirm its suitability for nonlinear optical devices,

such as optical limiting and optical switching.

The chapter VI is devoted to study the effect of KDP on the growth,

thermal and optical properties of L-alanine single crystal single crystal of L-

alanine has been grown from aqueous solution of potassium di hydrogen

phosphate (KDP) by slow evaporation method. The single crystal X- ray

diffraction study confirms that the title crystal belongs to the orthorhombic

structure with the space group P212121. The presence of functional groups

was determined using FT-IR analysis. The presence of phosphate observed

very strong band occurring at 539 cm-1 in the FT-IR spectrum. UV-visible-NIR

Spectrum of L-alanine shows 73% transparency in visible region and its band

gap energy is found to be 4.9 eV. The grown crystals are thermally stable up

to 288.7˚C. The SHG efficiency of the L-alanine is 1.022 times than that of

the standard KDP crystal. The presence of phosphate in enhancement of

SHG and variation in thermal behavior for the L-alanine crystal grown in

present study impact to the pure L-alanine are due to the presence of KDP in

the growth solution. The Z-scan measurements further confirm that the

material exhibit large third order nonlinear optical properties. The nonlinear

optical parameter values such as, nonlinear refractive index 2n = 8.82 x10-8

(cm2/W), nonlinear absorption = 0.15x 10-4 (cm/W), nonlinear susceptibility

(3) =4.31 × 10-6esu.The third order nonlinear parameters are highly

encouraging for this crystal. All these studies indicate that the grown crystal

is the potential material for second harmonic as well as third harmonic

generation applications.

105

7.2 SUGGESTIONS FOR FUTURE WORK

The single crystals are found application in the development of

technologies such as Laser, semiconductor, high and low energy particles.

The nonlinear optical materials play important role in the field optical

communication, frequency conversion, optical switching devices etc.,

It is possible to grow bulk size crystals of mNA, mNAK, KDC, L-

alanine crystals with improved optical quality by carefully

adopting either the slow cooling method or by some innovative

techniques with modified apparatus. Attempts could be made in

future to investigate the nucleation parameters such as

metastable zone width, induction period, interfacial tension etc.,

The dopants in the crystal lattice play a significant role in

improving morphology and second harmonic generation (SHG)

efficiency of the crystal. Hence, by adding rare earth

compounds as the dopant in different concentration morphology

and SHG efficiency can be studied.

The laser damage threshold studies could be carried out on the

grown crystals with improved optical quality and the variation of

laser damage threshold with the beam energy could be

correlated.

Further, phase matching and other higher harmonic generation

studies can be made on the grown crystals.

106

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126

LIST OF PUBLICATIONS

Publications in Refereed Journals

1. Thilak T., Basheer Ahamed M, Vinitha G, “Third order nonlinear

optical properties of potassium dichromate single crystals by Z-

scantechnique”, Optik., Vol.124, pp.4716– 4720, 2013.

2. Thilak T., Basheer Ahamed M,MurugakoothanP “Physical and optical

study of potassium-doped meta-Nitroaniline crystal (mNAK)”, Journal

of Nonlinear optical physics and materials., Vol. 23 No.1, pp.

1450012-28, 2014.

3. Thilak T., Basheer Ahamed M, Marudhu G , Vinitha G, “Effect of KDP

on the growth, thermal and optical properties of L-alanine single

crystals”,Arabian Journal of

Chemistry.,DOI:10.1016/j.arabjc.2013.04.022 (Article in press-2013)

http://www.sciencedirect.com/ science/article/pii/S1878535213001111.

4. Thilak T., Basheer Ahamed M,Murugakoothan P “Growth, linear, non

linear optical studies and third order nonlinear optical properties

studies of m-Nitroaniline (m-NA) single crystal”, Journal of

Optoelectronicsand Advanced Materials-Rapid

Communications.,Vol.16, pp.9-10,2014.

5. G. Marudhu, S. Krishnan, T. Thilak, P. Samuel, G. Vinitha, G.

Pasupathi “Optical, Thermal and Mechanical Studies on Nonlinear

Optical Material Diglycine Barium Chloride Monohydrate (DGBCM)

Single Crystal”, Journal of Nonlinear Optical Physics and

Materials.,Vol.22,pp.1350043-56, 2013.

127

Conference attended

1. Thilak T., Marudhu G, Samuel P , Vinitha G, “Optical and Non Linear

studies On L-Alanine Acetate (LAAc) Nonlinear Optical Single crystal”,

International Conference on Recent Trends in Advanced Materials

(ICRAM), 20-22ndFeb-2012 VIT University- Vellore-632013.

2. Thilak T.,Vinitha G., “Studies on L-alanine doped Tri glycine sulphate

(TGS) crystal” 14 th National Seminor on Crystal Growth (NSCG-XIV),

March 10-12, 2010- VIT University- Vellore-632013.

3. Thilak T., Basheer Ahamed M, Vinitha G, “Third order nonlinear

optical properties of pottasium di chromate doped L-Alanine single

crystal”, National conference on Recent Advances in Materials

(NCRAM-2013)”, April 9-10, 2013-B.S.Abdur Rahman university,

Chennai-48.

4. Thilak T., Basheer Ahamed M,Murugakoothan P, “Studie on third

order nonlinear optical properties of cerium doped m-Nitroaniline

(mNACe) single crystal”, Second National Conference on Recent

Advances in Materials (NCRAM-14) September 3-4, 2014- B.S.Abdur

Rahman University, Chennai-600 048.

128

TECHNICAL BIOGRAPHY

Mr.Thilak.T (RRN.0990201) was born on 6th November 1982, at

Porayar, Tamil Nadu. He did schooling in Walter Scudder Higher Secondary

School, Tindivanam. He received B.Sc, degree in the branch of Physics from

Pachiyappa‟s College, Chennai-600 030, affliated to University of Madras in

the year 2003. He did M.Sc degree in branch of Physics from T.B.M.L

College, Porayar, Bharathidasan University, Thiruchirappalli in the year 2006.

He received the M.Phill degree from T.B.M.L College, Porayar,

Bharathidasan University, Thiruchirappalli in the year 2008. He has seven

years of teaching experience in Engineering Colleges as Assistant Professor

of Physics. He is currently pursuing Ph.D in crystal growth in the Department

of Physics, B.S. Abdur Rahman University, Chennai-600 048. He has

published four research papers in the peer re-viewed international journals.

He has also presented one paper at the International Conference and three

papers in national conferences. The e-mail id is: thilakhh5@gmail.com and

the contact mobile number is: +91 7667814620.