Chapter 5

Role of modifier oxide in emission spectra and

kinetics of Er-Ho codoped Na2SO4-MO-P2O5 glasses

The glasses of the composition 19 Na2SO4–20 MO–60P2O5: 1.0 Ho2O3

/1.0 Er2O3 (M= Mg, Ca, Ba ) have been synthesized. Optical absorption and

fluorescence spectra (in the spectral range 350 nm – 2100 nm were studied at

ambient temperature. The spectra were characterized using Judd-Ofelt theory.

From the luminescence spectra, various radiative properties like transition

probability A, branching ratio β and the radiative life time τ for blue (B), green

(G) and red (R) emission levels of these glasses have been evaluated. The energy

transfer between the two rare earth ions (Ho3+

and Er3+

) in co-doped Na2SO4–

MO–P2O5 glass systems in the visible and NIR regions has also been investigated.

Highest intensity, the highest quantum efficiency and maximum energy transfer

with low phonon losses of B, G, and R lines has been observed in BaO mixed

glasses. The reasons for such higher values of these parameters have been

discussed in the light of varying field strengths at the rare earths ion site due to

replacement of one modifier oxide with the other. The enhanced intensity of NIR

emission (at 2.0 µm) has also been discussed in terms of cross relaxation of Er3+

ions from 4I13/2 level to

5I7 of Ho

3+ ions.

Role of modifier oxide in emission spectra and kinetics of

Er-Ho codoped Na2SO4-MO-P2O5 glasses

5.1 Introduction

The prevailing research devoted to alkali sulphate-phosphate glasses are

mainly confined to conductivity properties [1-4]. Despite many welcoming

features of sulphate–phosphate glasses as laser hosts, the studies available on

emission features of rare earth ions in these glasses are rare. Sulphate ions are

expected to dissolve largely in the phosphate glass matrix. However, these ions

and metaphosphate ions interact relatively slightly and give rise to a small

dynamic concentration of dithiophosphate (DTP) units. Such slight and

variable interactions between these two ions is a convivial feature for the

incorporation of rare earth ions to achieve desired high luminescence

efficiencies with minimal non-radiative losses in these glasses.

Er3+and Ho3+ ions are well known due to their effective blue and green

emissions and also due to the near infrared emission at ~2.0 µm. Additionally

they are well recognized due to their upconversion of infrared to visible light

[5-12]. Ho3+ ions possess strong infrared emission transition viz., 5I6→5I8

(~1.20 µm) and (5F4, 5S2)→

5I5 (~1.38 µm). However, in the conventional oxide

host glass matrices these two transitions are disturbed by significant multi-

phonon deexcitation due to the presence of these glass favoring effective losses

174

due to non-radiated transitions. Such losses can be minimized, if the glasses

are co-doped with another rare earth high phonon energy contribution of ion.

In fact in some of earlier investigations it was demonstrated that infrared laser

emission with high efficiency can be achieved by co-doping the glass by

erbium and holmium ions simultaneously. The Er3+ ion strongly absorbs the

visible light and non-radiative relaxations occur feeding the lowest excited

state 4I13/2 from which energy may be transferred to 5I7 level of Ho3+ which acts

as the ground state for the 2 µm infrared laser transition [13-16].

When sodium sulphate-phosphate glasses are mixed with different

alkali-earth oxide (viz., MgO, CaO and BaO) modifiers, there occur some

structural modifications and local electrostatic field variations near the rare

earth ion (due to differences in their ionic radii) dopant in the glass matrix. So

the symmetry and (or) covalency of the glass at the rare earth ion will be

different for different modifiers. Such variations should have strong bearing on

various luminescence transitions and as well on the upconversion.

The objective of the present investigation is to characterize the optical

absorption and the fluorescence spectra of Ho3+ and Er3+ ions as well as

upconversion fluorescence during the energy transfer from Ho3+ to Er3 in

Na2SO4–P2O5 glasses mixed with three interesting alkaline earth modifier

oxides, viz. MgO, CaO and BaO and to clarify the relationship between the

structural modifications in the glass network and luminescence efficiencies.

175

5.2 Brief review of previous work on Ho3+

and Er3+

doped glasses

5.2.1 Ho3+

doped glasses

A large number of studies on spectroscopic properties of holmium ions

in a variety of glass matrices are available. A brief review of few investigations

is reported here. Fusari et al. [17] have reported on the spectroscopic

characterization, continuous wave and continuous wave mode-locked laser

performance of bulk Tm3+:GPNG fluorogermanate and Tm3+-Ho3+:TZN

tellurite glass lasers around 2 µm. Tian et al. [18] have studied Tm3+/Ho3+

codoped fluorophosphate glasses, the higher predicted spontaneous transition

probability (76.54 s-1) along with the larger calculated emission cross-section

(6.15×10-21 cm2) gives evidence of intense 2 µm fluorescence. They found that

the optimum doping concentration is 4 mol% Tm3+/1 mol% Ho3+ for the

strongest 2 µm emission in our prepared samples. Wang et al. [19] investigated

mid-infrared luminescence properties of Tm3+ and Ho3+ codoped germanate-

niobate glasses. They have concluded that the energy transfer efficiency of

Tm3+(3F4) → Ho3+(5I7) ions was enhanced and the infrared fluorescence

intensity at 2.0 µm was increased as Ho3+ was codoped into the glasses. Guhur

and Jackson [20] demonstrated a highly efficient and high power Ho3+-doped

fluoride glass fiber laser that is resonantly pumped with a Tm3+ doped silicate

glass fiber laser operating at 2.051 µm. They have observed strong visible

upconversion fluorescence centered at a variety of wavelengths including 491

176

nm which results from three sequential energy transfer upconversion

processes; the fluorescence to pump energy ratio for this emission is one the

largest reported to date. Wang et al. [21] have achieved the white light close to

the standard white light emission by adjusting the concentration of rare earth

doping. Their result has practical significance for developing high-quality

white LED. Wang et al. [22] have prepared Ho3+-Tm3+ codoped GeS2-Ga 2S3-

CsCl chalcohalide glasses and their upconversion (UC) show that the green UC

luminescence from Ho3+ ions under an 808 nm diode laser excitation is

considerably enhanced by Tm 3+ ions codoping while the red UC luminescence

from Tm3+ ions is weakened. Li et al. [23] have investigated the mid-infrared

luminescence and energy transfer characteristics of Ho3+/Yb3+ codoped

lanthanum-tungsten-tellurite glasses. They have calculated the absorption,

emission cross-sections and gain coefficient of Ho3+: 5I7 → 5I8 and they have

analyzed the energy transfer processes of Yb3+-Yb3+ and Yb3+-Ho3+. Their

results showed that the Yb3+ ions can transfer their energy to Ho3+ ions with

large energy transfer coefficient, and a maximum efficiency of 84%. Tian et al.

[24] have reported 2 µm Emission of Ho3+-doped fluorophosphate glass

sensitized by Yb3+ upon excitation of 980 nm laser diode. Zhu et al. [25] have

investigated mid-infrared emission properties of Ho3+ ion in nanocrystals

embedded chalcohalide glass and obtained better emission at 2.0 µm and 2.9

µm. Seshadri et al. [26] have investigated spectroscopic properties (absorption

177

and emission) of Ho3+ doped alkali, mixed alkali and calcium phosphate

glasses. They have studied the variation of luminescence intensities of the two

transitions, absorption and emission cross-sections with the variation of

alkalies, mixed alkalies and calcium in phosphate glass systems. Kamma and

Reddy [27] studied energy upconversion in holmium doped lead-germano-

tellurite glass. They have observed room temperature upconversion emissions

from Ho3+ at 497 nm under 532 nm laser excitation, and at 557 and 668 nm

under 762 nm laser excitation and the upconversion emission intensity

enhanced in a heat treated glass. Zhu et al. [28] have studied the effect of

Ho3+ion concentration on the fluorescence spectra of chalcogenide glasses

based on Ge-Ga-S-CsI system. They observed that the intensity of the mid-

infrared fluorescence are enhanced with the Ho3+ ion concentration increasing

from 0.5wt% to 1.0wt%. Lin et al. [29] have reported Fluorescence

investigation of Ho3+ in Yb3+ sensitized mixed-alkali bismuth gallate glasses.

Shi et al. [30] recently reported upconversion luminescence in Tm3+/Yb3+- and

Ho3+/Yb3+-codoped Ga2O3–GeO2–Bi2O3–PbO glasses. Li and Zhang [31] have

recently reported the microscopic interaction parameters for Er3+/Ho3+ energy

transfer in tellurite glasses.

Lee et al. [32] have investigated local structure and its effect on the

oscillator strengths and emission properties of Ho3+ in chalcohalide glasses.

Ratnakaram et al. [33] have reported the results of Spectroscopic investigations

178

on Ho3+ doped mixed alkali phosphate glasses. Feng et al. [34] have

investigated optical properties of Ho3+-doped novel oxyfluoride glasses. Zou et

al. [35] have studied the blue and green upconversion fluorescence at about

424, 485 and 545 nm in Ho3+ singly doped fluoride glasses under 647 nm

excitation. Heo et al. [36] have prepared and studied the emission

characteristics of Ge25Ga5S70 (at.%) glasses doped with Tm3+ and Ho3+. Peng et

al. [37] have systematically investigated the optical properties of the rare

elements Tm3+, Ho3+ and Yb3+ in various glasses. Hormadaly et al. [38] have

measured the non-radiative relaxation rates from 5F4, 5S2 to 5F5 of Ho3+ in

tellurite glasses as a function of temperature. The same group have reported

[39] the intensity parameters of holmium in sodium, barium and zinc tellurite

glasses obtained from the absorption spectra. Wang et al. [40] have studied the

physical properties of R2O-ZnO-TeO2 glasses and also studied feasibility of

these glasses for fiber drawing and rare earth doping.

5.2.2 Er3+

doped glasses

Chillcce et al. [41] investigated the effects of ZnF2 concentration on the

optical and physical properties of Er3+-doped oxy-fluoride tellurite glasses

Er2O3-TeO2-ZnF2-ZnO. They found that the Er3+ ion emission cross section

spectrum at around 1550 nm of oxy-fluoride tellurite glass containing 30 mol%

of ZnF2 was very similar to those of Fluoride glasses. Bilir et al. [42] have

investigated Judd-Ofelt analysis and near infrared emission properties of the

179

Er3+ ions in tellurite glasses containing WO3 and CdO. They have determined

the absorption and emission cross-section spectra and the Stark Levels splitting

for the 4I13/2 to 4I15/2 transition of Er3+ centered at 1.5 µm. Fang et al. [43] have

carried out DSC measurements for Na 2O-Li2O-B2O3-Al 2O3-BaO-P2O5 glasses

with different particle size. Two crystallization peaks appear on the DSC

curves for sample sized 90-110µm. They have also studied the effect of mixed

alkali on glass thermal stability and evaluated the surface and bulk

crystallization active energies. Feng et al. [44] investigated the optimization of

doping concentration in Er:tellurite glass based on heat analysis. Their results

show that upconversion is an important heat-generation process and heat

problem is serious especially at high Er3+ doping level. Based on the heat

analysis, the optimized doping concentration range of erbium ions is about 0.5

mol% to 1.0 mol%.

Zhao et al. [45] investigated intense upconversion luminescence of

Er3+/Yb3+ codoped oxyfluoride borosilicate glass containing Ba2GdF7

nanocrystals. Środa et al. [46] have studied optical properties of phosphate,

borate, silicate and lead-silicate glasses. They showed that the integral intensity

of the two main optical absorption transitions monotonically increases with the

order: phosphate < borate < silicate < lead-silicate. Zhang et al. [47] have

synthesized Er3+ and Dy3+ codoped tellurite glasses and observed five emission

bands in the PL spectrum under 325 nm pumping. Their studies revealed that

180

the intensity of Dy3+ characteristic emission was enhanced as Er3+

concentration increased while keeping Dy3+ concentration constant. They

claimed that these glasses with the controllable CIE coordinates might be a

potential candidate for the widely realistic application such as solid-state white

lighting and multicolor display. Rivera et al. [48] showed the annealing effect

on silver and erbium- doped tellurite glasses in the formation of nanoparticles

(NPs) of silver, produced by the reduction of silver (Ag+→Ag0), aiming to an

fluorescence enhancement. Their observations demonstrated that the

photoluminescence enhancement is due to the coupling of dipoles formed by

NPs with the Er3+4I13/2→4I 15/2 transition. Gonzalez-Prez et al. [49] investigated

the control of the local devitrification on oxyfluoride glass doped with Er3+

ions under diode laser irradiation. They have studied green upconversion

emissions around 525 and 545 nm originated from the thermalized 2H11/2 and

4S3/2 levels when the glass structure changes to glass ceramic (under 2300 mW

and temperature around 783 K) of laser power during irradiation with a laser

beam. Ichikawa et al. [50] investigated emission properties of Er3+ in Ga2S 3-

GeS2-Sb2S3 glasses at the mid-infrared region. They identified clear mid-

infrared emissions were observed at 2750 and 4300 nm assigned to the 4I11/2 →

4I 13/2 and 4I9/2 → 4I11/2 transitions, respectively. The lifetime of the initial level

of the 4.3 µm emission, 4I9/2, rapidly decreased with the Er3+ concentration

because of the cross relaxation of this level, which can take place even at

181

considerably low Er3+ concentration. Frej et al. [51] have reported

photoluminescence properties of Er3+ doped Ga10Ge25S65 glasses excited at 980

and 532 nm. They have analyzed the PL temporal behavior through rate

equations for the population densities and using the Inokuti-Hirayama model.

Chen et al. [52] have studied the spectroscopic properties of Er3+ doped

tellurite glasses for 1.5 µm optical amplifier. Huang et al. [53] have

investigated mid-infrared spectral properties of Er3+-doped GeS2-Ga2S3-CsI

glass-ceramics. Their results showed that the density and micro-hardness of the

glass are significantly increases after heat treatment at 440°C for 14 h, while

the infrared transmittance remains the same. The mid-infrared emission

intensity at 2.8 µm corresponding to electronic transitions of 4I11/2→ 4I13/2 in

Er3+ ions increased slightly.

Yang et al. [54] have reported the results of their comparative

investigations on energy transfer mechanisms between Er3+ and Ce3+ in tellurite

glasses. Tanebe et al. [55] have reported the compositional dependence of J-O

parameters in Er3+ ion alkali metal borate glasses. Qian et al [56] have reported

the spectroscopic properties of Er3+-doped Na2O–Sb2O3–B2O3–SiO2 glasses.

Kassab et al. [57] have reported Er3+ laser transition in PbO-PbF2-B2O3 glasses.

Shixun Dai et al. [58] have reported the effect of OH− content on emission

properties in Er3+-doped tellurite glasses. Padlyak et al. [59] have studied the

optical spectroscopic properties of Er3+ doped CaO-Ga2O3-3GeO2 glasses.

182

Haro-Gonzalez et al. [60] have reported optical properties of Er3+-doped

strontium barium niobate nanocrystals obtained by thermal treatment in glass.

Nii et al. [61] have reported the upconversion of Er3+ and Yb3+ doped TeO2

based glasses. Shih [62] reported the thermal, chemical and structural

characteristics of Er3+ doped sodium phosphate glasses. Yang et al. [63]

investigated non-radiative 4I13/2→4I11/2 transitions of Er3+ oxide glasses. Hood et

al. [64] have reported the studies on Er3+ doped borotellurite glasses for 1.5 µm

broadband amplification. Chena et al. [65] have reported the spectroscopic

properties of Er3+ ions in bismuth borate glasses. Jose et al. [66] have fabricated

active wave-guide Ag-Na ion exchange Er3+-Yb3+ doped phosphate glasses.

Ding et al. [67] have studied the spectral properties Er3+ doped lead halo

tellurite glasses for 1.5µm broadband amplification. Marjanovica et al. [68]

have reported the characterization of new Er3+ doped tellurite glass systems.

Courrola and Kassab et al. [69] reported the spectroscopic properties of heavy

metal oxide glasses doped with erbium. Vermelho et al. [70] have reported the

temperature investigation of infrared visible frequency upconversion in Er3+

doped tellurite glasses excited at 1540 nm. Chiodini et al. [71] studied the photo

sensitiveness of Er3+ Tin-Silicate glasses.

5.2.3 Ho3+

-Er3+

co-doped glasses

Li and Zhang [72] investigated the energy transfer between Er3+/Ho3+ in

tellurite glasses and analyzed the main channels of energy transfer between

183

Er3+/Ho3+. Their results showed that the resonant energy transfers

Er3+(2H11/2(4S3/2))→Ho3+(5F4(

5S2)) and Er3+(4F9/2)→Ho3+(5F5) are very efficient

and non-resonant energy transfers Er3+(4I13/2)→Ho3+(5I7) and

Er3+(4I11/2)→Ho3+(5I6), which are a phonon-assisted energy transfer process. Yi

et al. [73] investigated 2.0 µm spectroscopic properties of Er3+/Tm3+/Ho3+

triply-doped fluorophosphate glasses pumped by 808 nm and the energy

transfer mechanisms between the three rare earth ions. Their energy transfer

analysis indicated that the cross-relaxation of Tm3+ was important and the

resonant energy transfer in Er3+→Ho3+, Tm3+→Ho3+, Er3+→Tm3+→Ho3+

process was the main channel. Their studies revealed that the Er3+/Tm3+/Ho3+

triply-doped fluorophosphate glass would be a potential material for 2.0 µm

emission because of the efficient sensitization of Er3+ and Tm3+ to Ho3+. Zhang

et al. [74] investigated the energy transfer and frequency upconversion in

Er3+/Ho3+ co-doped tellurite glasses. They have observed intense upconversion

luminescence emissions at around 525, 548, and 660 nm, which correspond to

Er3+:2H11/2→4I15/2, Er3+:4S3/2→

4I15/2 + Ho3+:5S2(5F4)→

5I8, and Er3+:4F9/2→4I15/2 +

Ho3+:5F5 →5I8 transitions, respectively. They have also estimated and evaluated

possible upconversion mechanisms and energy transfer between Er3+ and Ho3+.

Singh et al. [75] have studied the energy transfer between two similar rare

earth ions (Ho3+↔Er3+) doped in lithium tellurite glass. They have measured

the change in fluorescence emission and the lifetime of the donor and acceptor

184

involved in the energy transfer by varying the concentrations of Ho3+ (donor),

keeping the Er3+ (acceptor) concentration fixed and found that the intensity of

Er3+ bands increases on increasing the Ho3+ concentration. Dai et al. [76] have

investigated high efficiency infrared-to-visible upconversion emission in Er3+,

Yb3+, and Ho3+ co-doped tellurite glasses. They have evaluated the dependence

of upconversion intensities on excitation power and the possible upconversion

mechanisms. Kumar Singh et al. [77] have studied the optical properties and

upconversion in Er3+ and Ho3+ doped in lithium tellurite glass. They have

observed intense upconversion emission in ultraviolet/violet, green and red

regions when these glasses are pumped with NIR radiation. Moine et al. [78]

have investigated optical properties of ZBLA glass and BATY and BIZYT

fluoride glasses single and codoped with Er3+ and Ho3+ ions and tested their

ability to be good solid-state laser materials emitting near 2 µm. Joshi et al.

[79] have investigated the visible green emission of Ho3+ and Er3+ in tellurite

glass.

5.3 Results

The compositions chosen for the present study are

19 Na2SO4–20 MO–60P2O5: 1.0 Ho2O3

19 Na2SO4–20 MO–60P2O5: 1.0 Er2O3

18 Na2SO4–20 MO–60P2O5: 1.0 Ho2O3 + 1.0 Er2O3

(With M = Ba, Ca & Mg) (all in mol %).

185

5.3.1 Physical parameters

Following the measured values of the density and average molecular

weight M for the samples possessing various physical parameters like rare

earth ion concentration Ni, mean rare earth ion separation Ri and molar volume

were computed and they are presented in Table 5.1.

Table 5.1 Physical parameters of Na2SO4- MO- P2O5: Ho2O3/Er2O3 glasses.

Glass d (g/cm3)

Ni(1020

ions/cm3) Ri

(Ao) Rp

(Ao) Fi

(1015 cm-2) nd

BaHo 2.8325 1.164 20.48 8.253 0.440 1.653

CaHo

2.5523 1.209 20.22 8.149 0.451 1.651

MgHo 2.5551

1.241 20.04 8.078 0.459 1.652

BaEr 2.8120 1.155 20.53 8.274 0.438 1.654

CaEr 2.5552 1.21 20.21 8.146 0.452 1.653

MgEr 2.5361 1.231 20.09 8.099 0.457 1.651

5.3.2 Optical absorption spectra

The optical absorption spectra of holmium doped Na2SO4–MO–P2O5

glasses recorded at room temperature in the spectral range 300−2500 nm

exhibited several absorption bands originating from the ground state 5I8 (Fig.

5.1). These levels are assigned to the following electronic transitions [80, 81]:

5I8 →5G2,

3K6, 3H6,

3K7 + 5G4, 5G5 + 3G5,

5G6, 3K8,

5F2 +5F3,

5F4+5S2,

5F5, 5I6.

186

The absorption spectra of erbium doped Na2SO4–MO–P2O5 glasses (Fig. 5.2)

have exhibited the following principal bands within the same spectral range

[82, 83]:

4I15/2 → 2G7/2, 4G11/2,

2G9/2 + 2H9/2, 4F3/2,

4F7/2, 2H11/2,

4S3/2, 4F9/2 ,

4I9/2, 4I11/2,

4I13/2.

In Fig. 5.3, the spectra of Ho3+ and Er3+ co-doped glasses are presented. In

these spectra, the bands caused by transitions of both the ions are retained,

however some of them show some overlap. Such overlapping may be also a

consequence of the some redistribution of the occupied states due to effective

electron-phonon interactions.

The comparison of the spectra indicated the highest intensity for all the

bands for BaO mixed glasses.

The oscillator strengths (OS) of the electric dipole transition between

two states have been calculated using the standard Judd-Ofelt (JO) theory, with

the conventional equation [84, 85] mentioned in Chapter – I.

187

300 350 400 450 500 550 600 650 700

Wavelength, λ (nm)

Abs

orba

nce

(arb

. uni

ts)

5G23K6

3H6 5G5 + 3G5

5G4 + 3K7

5G6

5F2+5F3

3K8

5F4 + 5S2

5F5

BaHo

MgHo

CaHo

1100 1150 1200 1250

Wavelength, λ (nm)

Abs

orba

nce

(arb

. uni

ts)

5I6 BaHo

MgHoCaHo

Fig. 5.1 Optical absorption spectra of Na2SO4−MO−P2O5 glasses doped with Ho3+ recorded at room temperature. Inset shows the absorption spectra of these glasses in NIR region. All transitions are from the ground state 5I8.

188

300 400 500 600 700 800

3+Wavelength, λ (nm)

Abs

orba

nce

(arb

. uni

ts)

2G7/2

4G11/2

2G9/2 + 2H9/2

4F3/2

4F7/2

2H11/2

4S3/2

4F9/2

4I9/2

BaEr

MgErCaEr

900 1100 1300 1500 1700Wavelength, λ (nm)

Abs

orba

nce

(arb

. uni

ts)

BaEr

MgErCaEr4I11/2

4I13/2

Fig. 5.2 Optical absorption spectra of Na2SO4−MO−P2O5 glasses doped with Er3+ recorded at room temperature. Inset shows the optical absorption spectra of these glasses in NIR region. All transitions are from the ground state 4I15/2.

189

300 400 500 600 700 800

Wavelength, λ (nm)

Abs

orba

nce

(arb

. uni

ts)

3+ 3+

5G2

3K6

3H6 + 2G7/2

3K7+4G11/2

2G9/2+4F9/2

5G5+3G5

5G6

3K8

4F7/2

2H11/2+4G11/2

5F4

5

4F9/2

4I9/2

900 1100 1300 1500 1700Wavelength, λ (nm)

Abs

orba

nce

(arb

. uni

ts)

4I11/25I6

4I13/2

CaHoEr

MgHoEr

BaHoEr

CaHoErMgHoEr

BaHoEr

Fig. 5.3 Optical absorption spectra of Na2SO4−MO−P2O5 glasses co-doped with Ho3+ and Er3+ recorded at room temperature. Inset shows the spectra in NIR region.

190

The λU reduced matrix elements have been additionally evaluated,

using recent reference data for the Er3+ and Ho3+ Hamiltonian parameters [89].

The quality of fitting between theoretical and experimental oscillator strengths

is determined by the root mean squared deviation and is presented in Tables

5.2 (a) and 5.2(b). The deviation indicates sufficiently good fitting between

theory and experiment demonstrating the applicability of JO theory. The

evaluated JO parameters Ωλ for Ho3+ and Er3+ doped Na2SO4–MO–P2O5

glasses are presented in Table 5.3.

The values of Ωλ are established to be in the following order for the

Ho3+ doped glasses: Ω4 > Ω2 > Ω6 and for Er3+ the order is Ω2 > Ω4 > Ω6. The

comparison of the data on Ωλ parameters of Ho3+ and Er3+ ions in various other

glass matrices [90-95] indicated the similar trend with few exceptions.

191

Table 5.2 (a) The absorption band energies (cm-1), the experimental (fexp) and calculated (fcal) OSs (x10-6) for the absorption transitions of Ho3+ doped Na2SO4-MO-P2O5 glasses.

Transitions from 5I8

NHoBaP

Energy fexp fcal (cm-1) (x10-6) (x10-6)

NHoCaP

Energy fexp fcal (cm-1) (x10-6) (x10-6)

NHoMgP

Energy fexp fcal (cm-1) (x10-6) (x10-6)

3H6 27248 0.39 0.54 27855 0.43 0.45 27855 0.94 0.36

3K7 26316 0.76 0.44 26385 0.75 0.39 26385 1.62 0.45

5G5 24038 0.77 1.43 24038 0.94 0.28 24096 1.66 4.25

5G6 22127 2.68 3.11 22124 2.76 2.63 22124 3.96 3.41

5F4 18656 4.81 5.76 18553 4.16 3.29 18553 8.69 7.83

5F5 15673 2.41 3.09 15601 2.79 1.44 15601 4.52 6.49

5I6 8711 2.49 2.13 8703 2.49 1.94 8703 4.83 3.02

5I7 5118 3.14 2.86 5120 2.59 2.49 5115 3.84 3.99

r.m.s. deviation

± 0.208 ± 0.272 ± 0.326

192

Table 5.2 (b) The absorption band energies (cm-1), the experimental (fexp), and calculated (fcal) OSs (x10-6) for the absorption transitions of Er3+ doped Na2SO4-MO-P2O5 glasses.

Transitions from 4I15/2

NErBaP

Energy fexp fcal (cm-1) (x10-6) (x10-6)

NErCaP

Energy fexp fcal (cm-1) (x10-6) (x10-6)

NErMgP

Energy fexp fcal (cm-1) (x10-6) (x10-6)

4G11/2 26385 0.35 0.45 26178 0.27 0.12 26385 0.28 0.23

2G9/2 24937 0.38 0.55 24875 0.25 0.41 24875 0.27 0.43

4F3/2 22522 0.23 0.31 22522 0.16 0.24 22522 0.16 0.25

4F7/2 20533 0.31 0.99 20242 0.22 0.71 20533 0.23 0.74

2H11/2 19157 0.11 0.25 19230 0.88 0.69 19157 0.90 0.68

4F9/2 15360 0.24 0.37 15455 0.18 0.43 15360 0.19 0.45

4I11/2 10266 0.13 0.41 10266 0.97 0.37 10266 0.11 0.39

4I13/2 6531 0.46 0.90 6531 0.42 0.71 6527 0.45 0.74

r.m.s. deviation

± 1.082 ± 0.6206 ± 0.6623

193

Table 5.3 The summary of JO parameters Ωλ.

Glass

Ho3+ doped glasses Er3+ doped glasses

Ω2 (x10-20, cm-2)

Ω4 (x10-20,cm-2)

Ω6 (x10-20,cm-2)

Ω2 (x10-20,cm-2)

Ω4 (x10-20,cm-2)

Ω6 (x10-20,cm-2)

NBaP 3.47 7.4 3.41 1.52 1.37 1.08

NCaP 3.38 9.1 2.27 1.39 1.22 0.824

NMgP 5.06 10.32 3.46 1.43 1.29 0.867

194

5.3.3 Photoluminescence spectra

The fluorescent spectra for Ho3+ and Er3+ doped Na2SO4–BaO–P2O5

glasses recorded at room temperature in the visible region are shown in Figs.

5.4 (a) and 5.4 (b). The spectra of individual ion doped glasses exhibited the

following prominent emission bands [80-83]:

Ho3+ glass (λexc = 448 nm): 5F3→5I8,

5S2(5F4)→

5I8, 5F5→

5I8, 5G5→

5I6, 5F4→

5I7,

5G5→5I5,

5F2→5I6, and 5F3 → 5I6.

Er3+ glass (λexc = 379nm): 2H9/2→4I15/2,

4F5/2→4I15/2,

4F7/2→4I15/2,

2H11/2→4I15/2,

4S3/2→4I15/2,

4F9/2→4I15/2 and 2G9/2→

4I11/2.

The spectra of the co-doped glasses (excited at 890 nm) exhibited all

these transitions, however, some of the bands seem to be overlapped (Fig. 5.5).

The comparision of the spectra for different modifier oxide mixed glasses have

indicated the highest intensity of all the luminescence transitions for BaO

mixed glasses. Further it may be worth mentioning here that some rare earth's

ions may be shifted outside the particular crystalline chemical structural

fragments due to the local vacancies which may also contribute to the changes

in the observed emission.

195

480 530 580 630 680 730 780 830

5F3→5I8

5S2(5F4)→

5I8

5G5→5I6

5F4→5I7

5G5→5I5

5F2→5I6

5F3→5I6

5F5→5I8

Inte

nsit

y (a

rb. u

nits

)

Wavelength λ (nm)

BaHo

MgHo

CaHo

Fig. 5.4 (a) Comparison plot of emission spectra of Na2SO4−MO−P2O5: Ho2O3 glasses (λexc=448 nm).

196

350 400 450 500 550 600 650 700 750

2H9/2→4I15/2

4F5/2→4I15/2

4F7/2→4I15/2

2H11/2→4I15/2

4S3/2→4I15/2

2G9/2→4I13/2

4F9/2→4I15/2

2G9/2→4I11/2

Wavelength, λ (nm)Fig. 5.4(b) Comparison plot of emission spectra of Na SO −MO−P O : Er O glasses (λ =379 nm).

Inte

nsity

(ar

b. u

nits

)

BaEr

MgEr

CaEr

Fig. 5.4(b) Comparison plot of emission spectra of Na2SO4−MO−P2O5: Er2O3 glasses (λexc=379 nm).

197

350 400 450 500 550 600 650 700 750

Inte

nsity

(ar

b. u

nits

)

Wavelength, λ (nm)

2H9/2→4I15/2

4F5/2→4I15/

2

4F7/2→4I15/2

2H11/2→4I15/2

2G9/2→4I13/2

& 5S2→

5I8

4S3/2→4I15/2

5F3→5I8

& 5F4→

5I8

4F9/2→4I15/2

&

5F5→5I8

4G11/2→4I11/2

&3K8→

5I7

4F7/2→4I13/2

&5F3→

5I7

5F4→5I7

1400 1650 1900 2150

Inte

nsity

(ar

b. u

nits

)

Wavelength, λ (nm)

5I7→5I8

4I13/2→4I15/2

CaHoEr

MgHoEr

BaHoEr

Fig. 5.5 Photoluminescence spectra of Na2SO4−MO−P2O5 glasses co-doped with Ho3+ and Er3+ recorded at room temperature (λexc= 890 nm).

198

The energy level diagram containing observed absorption and emission

transitions for Ho3+ and Er3+ ions in the case of one of the glasses (glass mixed

with BaO) is shown in Fig. 5.6. Using JO intensity parameters, the radiative

transition probability from the excited state [ ]JLSf N ,,γ to the lower state

[ ] '',',' JLSf N γ were evaluated by the standard Eq. (1.39).

Performing summation of the 'JJA quantities over all possible final states, one

can get the radiative life time τ of excited energy level and branching

ratio 'JJβ are evaluated using the Eqs. (1.40) and (1.41), respectively.

The summary of the emission parameters for both Ho3+ and Er3+ doped glasses

for the three principal emission lines is presented in Tables 5.4.

199

0

5

10

15

20

25

g

c

d

b

a

e

f

5I7

5I6

5F5

5F4

5F3

5F2

3K8

5G6

5F1

5I8

5I5

5I4

5S2

Ho3+Er3+

4F7/2

2H11/2

4S3/2

4F9/2

4I9/2

4I11/2

4I13/2

4I15/2

Ene

rgy

(cm

-1, 1

03 )

c'

d'

b'

a'

e'

f'

Fig. 5.6 Energy level diagram containing important transitions of Ho3+ and Er3+ ions for Na2SO4−BaO−P2O5 glasses.

200

Table 5.4 Transition probabilities and branching ratios of Ho3+ and Er3+ doped glasses.

Blue emission Green emission Red emission

Sample A (s-1) β (%) A (s-1) β (%) A (s-1) β (%)

BaHo CaHo MgHo BaEr CaEr MgEr

(5F3 → 5I8, ν = 19794 (cm-1)) 6820.1 38.78 4526.4 28.81 6899.3 33.62 (4F7/2 → 4I15/2, ν = 21276 (cm-1)) 3153.6 79.53 2493.2 78.03 2616.2 77.99

(5F4(5S2 → 5I8, ν = 18211 (cm-1))

9330.2 67.79 6588.5 57.37 9631.2 62.44

(4S3/2 → 4I15/2, ν = 18248 (cm-1)) 1082.4 65.87 824.1 65.75 863.7 65.71

((5F5 → 5I8, ν = 15372 (cm-1)) 9110.6 77.84 9305.6 77.23 11365 77.45 (4F9/2 → 4I15/2, ν = 15901 (cm-1)) 1478.3 92.39 1237.6 92.27 1301.1 92.18

201

5.4 Discussion

In Na2SO4–MO–P2O5 glass system, the chains or rings of phosphate

groups are subjected to alterations to various extents depending upon the

nature of the modifier ions (its polarizability, sizes etc.) and MO/P2O5 ratio.

Hence truncated chains in which a fraction of the phosphate tetrahedra possess

three unshared oxygen corners produce the structure. These tetrahedra can be

presented as [POO2/2O]- with two bridging oxygens and [POO1/2O2]2- with one

bridging oxygen.

The two bridging oxygen phosphate groups in the presence of the modifier

oxide can be described as:

2 [POO3/2]0 + MO → 2 [POO2/2O]- + M2+

Similarly, the phosphate group with one bridging oxygen in the presence of

modifier oxide can be presented as:

2 [POO2/2O]- + MO → 2 [POO1/2O2]2- + M2+

With the combination of Na2SO4 it will take a form:

[POO2/2O] - + [SO4]2- → [SPO7]

3-

The concentration of such dithiophosphate (SPO7)3- species (which are

expected to disturb the emission probabilities of the rare earth ion) in the glass

matrix depends upon the nature of the modifier (M2+) ion. Following the order

of the ionic size or radius for the three modifier ions (rMg2+ < rCa2+ < rBa2+), one

can understand that the replacement of MgO successively by CaO and BaO,

202

leads to enhancement of disordering degree for the titled glass network

causing substantial structural modifications at the lanthanide ion site in the

glass network.

Out of the three JO parameters, the main contribution to Ω2 is from local

crystalline field electronic charge density due to the differences in the

distortion at the rare earth ion sites. The variations in the sites with non-centro-

symmetric potential may be a consequence of superimposition of long-ranged

dielectric media constants associated with the central rare earth ions bounded

to the ligand atoms.

The comparison of Ω2 parameter for both Er3+ and Ho3+ ions doped glasses

(Table 5.3) shows that the lowest value is obtained for BaO mixed glasses. The

larger the ionic radius of the modifier ions, the larger is the average distance

between S–O–P and P–O–P chains causing the increasing average Er–O and

Ho–O distance to increase. Such increase in the bond lengths produces weaker

local field near Er3+ and Ho3+ ions and lead to lower value of Ω2 .

Branching ratio ‘β’ which is connected with the luminescence efficiency

are evaluated for different transitions of Ho3+ ions and are presented in Table

5.4. Among various transitions originated from 5S2 and 5F5 energy levels, the

values ‘β’ are found to be the highest for green emission (5S2→5I8) and red

emission (5F5→5I8) for all the three glasses. The comparison shows that BaO

mixed glasses exhibit the highest value of β for these two transitions. In

203

general, the 5S2→5I8 green emission is strongly affected by multi-phonon

relaxation, since the energy gap between the 5S2 and the lower-lying 5F5 levels

is about 2670 cm−1 (nearly equal to three phonons energies). The same is true

in case of 4S3/2 and the lower-lying 4F9/2 levels of erbium doped glasses. Such

multi-phonon losses seemed to be low for BaO mixed glasses. The branching

ratios, β values obtained for blue, green and red emission levels of Er3+ ions for

the glasses for the three modifier oxides mixed glasses are furnished in Table

5.4. The comparison of β values for these transitions shows that the largest

value for the BaO mixed glasses may indicate that these glasses exhibit better

lasing action among three Er3+ doped glasses.

In Fig. 5.7, the measured fluorescence decay curves of the one of the

emission lines viz., 5F3 → 5I8 in case of Ho3+ doped glass, 4F7/2 → 4I15/2 in case

of Er3+ doped and also co-doped (Ho3+-Er3+ glasses) mixed with three modifier

oxides are presented. The fluorescence lifetimes τ, evaluated from these graphs

are presented in Table 5.5 (a). These lifetimes are apparently shorter than

calculated life times following the J–O theory (Table 5.5 (b)). Such difference

obviously suggests intense multi-phonon relaxations. The highest life time

either calculated or measured for BaO mixed glasses doped with any rare earth

ion suggests a low phonon losses or higher degree of disorder in these glasses.

The comparison of experimental and theoretical data concerning radiative

204

lifetimes for one of the principal emission lines for Ho3+ and Er3+ doped BaO

mixed glasses is furnished in the (Table 5.5 (b)).

Table 5.5(a) Calculated radiative lifetimes of Ho3+ and Er3+ doped glasses.

Table 5.5(b) Data related to quantum efficiencies of the blue emission lines of BaHo and BaEr glasses.

The mechanism for R, B and G emissions when exited at the

wavelength corresponding to 5I8→5I5 /

4I15/2→4I11/2 transition in Er3+ - Ho3+

co-doped glasses is depicted in the Fig. 5.6. For Ho3+ doped glasses the red

emission (5F5→5I8 ) results from the subsequent excitation of 5I7 to

5F5 , the

τ (µs) Ho3+ emission

τ (µs) Er3+ emission MO

Blue Green Red Blue Green Red

BaO 57.2 73.4 85.3 49.3 82.5 70.4

CaO 64.6 87.1 84.5 61.8 78.2 76.8

MgO 58.3 65.6 68.4 62.7 75.6 71.3

5F3 → 5I8 4F7/2 → 4I15/2

BaHo CaHo MgHo BaEr CaEr MgEr

Measured (τm) 40.2 34.5 29.1 35.9 40.2 34.9

Calculated (τ) 57.2 64.6 58.3 49.3 61.8 62.7

Quantum yield (η%) 70.2 53.4 49.9 72.81 65.1 55.66

205

blue emission (5F3→5I8 ) originates from excitation of 5I5 to5F1 following

relaxation to 5F3 and green emission (5S2→5I8) is possible when 5I6 is excited to

5F4 with subsequent relaxation to 5S2.

Similarly in the case of Er3+ doped glasses when excited at the

wavelength corresponding to 4I15/2→4I11/2 transition, the 4I11/2 level is

subsequently excited to 4F7/2. Then blue emission is possible due to 4F7/2→4I15/2

transition; the relaxation of 4F7/2 level to 4S3/2 and 4F9/2 and consequent

transitions to 4I15/2 gives green and red emission respectively.

Further, energy transfer between 5F1 ↔ 4F7/2, 5S2 ↔ 4S3/2 and

5F5 ↔

4F9/2 energy levels of Ho3+ and Er3+ ions in the co-doped glasses reinforce the

strengths of blue, green and red emissions, respectively as shown in the Fig.

5.6. Based on the rate equations one can arrive at the final intensities of blue,

green and red emissions in the co-doped glasses as [96]:

(5.1)

( )))()(()(

1))()(( 111

2

1111

2

S S 3/24

25 −

′′

−

′′

−

′′

′′′′′′′′′

−

′′

′

−−−++++

+

+−

+++=

ccbbee

gcgbcebge

ee

ee

eebbff

gfegbbfeg

QQQ

NVWV

Q

W

QQQ

NWVVGreenI

τττ

φω

ττττ

φω ℏℏ

(5.2)

( )

+−

+++=

−

′′

′−−−

′′′

+ )(1

))()((Re 1111

2

F F 9/24

55

dd

dd

ddbbff

gfdgbbfgd

Q

W

QQQ

NWVVdI

ττττ

φωℏ

(5.3)

( )))(()(

1))(( 11

2

111

2

FF 27/4

35 −

′′

−

′′

′′′′′′′

−

′′

′

−−+ +++

+−

++=

ccff

gcgfcgf

ff

ff

bbff

ggbbffg

QQ

NVV

Q

W

QQ

NVVBlueI

ττ

φω

τττ

φω ℏℏ

206

10

1000

0 20 40 60 80

10

1000

0 20 40 60 80

10

1000

0 20 40 60 80

Time (µs)

Fluo

resc

ence

inte

nsity

(ar

b. u

nits

)

Fluo

resc

ence

inte

nsit

y (a

rb. u

nits

)

Fluo

resc

ence

inte

nsity

(ar

b. u

nits

)

Time (µs)Time (µs)

Ba Series Ca Series Mg Series

BaHo

BaErBaHoEr

CaHo

CaHoEr

CaEr

MgHo

MgErMgHoEr

Fig. 5.7 Fluorescence decay curves of Na2SO4−MO−P2O5 glasses co-doped with Ho3+ and Er3+ recorded at room temperature corresponding to the transitions 5F3 → 5I8 (of Ho3+ ion), 4F7/2 → 4I15/2 (of Er3+ ion) and 4F7/2 → 4I15/2 in co-doped glasses.

207

In the Eqs. (5.1–5.3) Ni represent the electron occupations populations of the

excited level i, Wij represents transition probabilities between levels i and j of

a rare earth ion; τI indicates the life time of level i of rare earth ion and Qi

represents the energy quenching rates. The absorption cross-section for each

transition is denoted by Vij where as the incident pumping flux is indicated by

φ.

Eqs. (5.1) to (5.3), suggests that the intensities of B, G and R lines are

proportional to radiative life times of the upper levels. The life time τ for any

upper level as mentioned earlier is the largest for the glass BaO mixed glasses.

So the intensities of the B, G and R emission lines are expected to be the

highest for the glasses mixed with BaO as is also observed. The energy

transfer efficiency (ηeff) from Ho3+ to Er3+ can be expressed as

Ho

ErHoeff

τ

τη +−= 1 (5.4)

The value of ηeff for the glass in case of BaO mixed co-doped is found to be

16 % for blue emission and it is 11% and 8.5% for CaO and MgO mixed

glasses respectively. The analysis clearly indicates maximum energy transfer

efficiency for the glass mixed with BaO. The energy transfer efficiency for

green and red emission also exhibited the similar trend.

208

The NIR emission ~ 2.0 µm is due to 5I7 → 5I8 transition. In the co-

doped glasses there are several ways of populating 5I7 level: (i) relaxation of

Ho3+ ions from 5I5 to 5I7 level, (ii) cross relaxation of Er3+ ions from 4I13/2 level.

Once 5I7 state is populated, most of Ho3+ ions relax to 5I8 ground state resulting

strong 2.0 µm emission. Comparing the intensity of 5I7 → 5I8 line for the three

MO mixed glasses, it is established to be slightly higher for BaO mixed glasses.

The energy gap between 5I7 and 5I8 states is found to be slightly lower for BaO

mixed glasses. Hence one can expect the losses due to multi- phonon

relaxation between 5I7 and 5I8 states is minimal for BaO mixed glasses causing

high intensity of this transition.

5.5 Conclusions

We have investigated principal role of modifier oxides on the features of

the photoluminescence spectra of Ho3+, Er3+ ions in Na2SO4–MO–P2O5 glass

system. With the replacement of modifier oxide MgO successively by CaO and

BaO, a slight red shift in the peak positions with considerable increase in the

intensity of various emission bands have been observed. When Ho3+ and Er3+

are present together in the glass matrix, the incident radiation 5I8 → 5I5 (890

nm) excites both the ions and give rise to red, blue and green emissions with

enhanced intensities.

The observed lower values of Ω2 parameter for BaO mixed glasses

indicates low degree of interaction of rare earth ions with the ions of the host

209

materials due to higher distortion or structural change at the vicinity of rare

earth ions owing due to the larger ionic radiations of Ba2+ ions.

The NIR emission ~ 2.0 µm due to 5I7→5I8 transition of Ho3+ is found to

be strengthened in the co-doped glasses due to cross relaxation of Er3+ ions

from 4I13/2 level to 5I7. Comparing the intensity of 5I7→5I8 line for the three MO

mixed glasses, it is established to be slightly higher for BaO mixed glasses.

210

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