bhanu p. singh department of physics indian institute of technology bombay, mumbai- 400076
TRANSCRIPT
Bhanu P. SinghBhanu P. SinghDepartment Of PhysicsDepartment Of PhysicsIndian Institute of Technology Bombay, Indian Institute of Technology Bombay, Mumbai- 400076Mumbai- 400076
Nonliear optical Nonliear optical systems, systems,
Nonlinearity & Its Nonlinearity & Its influence on opto - influence on opto - electronic response electronic response in low-d quantum in low-d quantum confined systemsconfined systems
Patterns in naturePatterns in nature
Spatial Spatial pattern in a pattern in a fluid heated fluid heated from belowfrom below
Kerr slice with feedback mirror
Theoretical model
Pattern generation in saturable absorber
where is given by following equation
Threshold intensity is given by
Artificial design of complexity
Nonlinear optical system to simulate 2-component reaction-diffusion system dynamics
System with 2 Kerr slices and bounded feedback loops
Variety of patterns
Some observed patternsSome observed patternsApplication to information processing
Isolated States as memories
Conjugated Conjugated PolymersPolymers
SemiconductorsSemiconductors
Capacity for tailoring Capacity for tailoring the optical properties the optical properties such as such as
(3) (3) E Egg-n-n and and r r -3-3
Property relationship Property relationship with structure, with structure,
interactions and interactions and ensuing processes ensuing processes
Microscopic origin of nonlinearityMicroscopic origin of nonlinearity
B.P.Singh et al,JCP109,685(1998)
B.P.Singh et al,Europhys.lett.45,456(1999)
NIdtdN
NIIdzdI
12
3412
B.P.Singh et al,JNOPM,7,571(1998)
Quantum confined 0-d semiconductors
2
22
2 RE
+
-R
Quantum dot transition probability spatial restriction
3
3
Ra
f
fB
exc
qd
Surface states in semiconductor nanoparticles
Surface states provide highly efficient nonradiative channels and significantly quench the photoluminescence yield
non-radiativetransition
HOMO
PL emission
primary absorption
surface states
LUMO
Nanocomposites of CdS and ZnO
CdS
(molar %)
ZnO
(molar %)
nano CdS:ZnO-1 45 55
nano CdS:ZnO-2 40 60
nano CdS:ZnO-3 33 67
EDAX and TEM - Approximately stoichiometric CdS and ZnO
(Cd:S = 1:1.20 and Zn:O = 1:1.18)
RF magnetron sputtering - Experimental setup
LN2-COOLEDSUBSTRATE
HOLDER
SHUTTER
GAS FLOW
TURBOPUMP
PRESSUREGAUGE
SCRAPER VIEWPORT
MAGNETRONGUN
Linear absorption spectroscopy
250 350 450 550 650 750 8500.0
0.5
1.0
1.5
2.0
(E)
(D)
(C)
(B)
(A)
(A) bulk CdS (d>5nm)(B) nano CdS (d~2nm)(C) nano CdS:ZnO-1(D) nano CdS:ZnO-2(E) nano CdS:ZnO-3
t
wavelength (nm)
Tunable source
DetectorSample
Itr= Iine-t
Comparative study of PL in CdS and CdS:ZnO nanocomposite films
Vasa, Singh and Ayyub (in preparation)
sample
400 450 500 550 600 650 700 7500.0
1.0
2.0
3.0
4.0
5.0
6.0
Filter
(A) bulk CdS (d>5nm)(B) nano CdS (d~2nm)(C) nano CdS:ZnO-1(D) nano CdS:ZnO-2(E) nano CdS:ZnO-3
ex=391nm
(E)
(D)
(C)
(A)X5(B)
PL
int. (
mV
)
wavelength (nm)
exc
onochromator + PMT
0 500 1000 1500 2000 2500 30000
20
40
60
80
100
120
(B)
(A)
(A) bulk CdS (d>5nm)(B) nano CdS:ZnO-2
ex
= 440nm Pulse width=1ps
PL
. in
t. (
arb
. u
nits
)
time (ps)
Decay-time measurement
Faster decay higher PL yield
Coherent PL from nanocomposite thin films
650 600 550 500
wavelength (nm)
energy (eV)1.8 2.0 2.2 2.4 2.6
coun
ts /
s
0
1000
2000
3000
4000
(a)
(b)
(c)
(d)
(e)
X 100
X 2
exc = 458 nm
Multiple beam interference observed in PL spectra
film
excemi
Vasa, Singh and Ayyub (submitted) J. Phys. Cond. Mat
Double slit experiment - Setup
Slit separation = 178 mSlit width = 30 mSample-slit = 6.15 cmSlit-detector = 88.6 cmPMT slit width ~ 1 mm
Ti:SapphireLaser System
100 MHz, 800 nm, 80 fs
Lock-inAmplifier
BBO
400 n
m
SampleDouble slit
121 HzGG475
PMT
Experimental results
-3 -2 -1 1 2 30
10
20
30
40
avg
(emi) = 500 nm
Degree of spatial coherence (j
12) = 0.2
Spatial coherence length ~ 10m
inte
nsi
ty (
V)
distance from the central line (arb. units)
i(fit) i(exp) i(max) i(min)
Vasa, Singh and Ayyub J. Phys. Cond. Mat17,189(2005)
Photocurrent spectroscopy
Vasa, Singh, Taneja, Ayyub et. al, J. Phys. Cond. Mat, 14, 281 (2002)
320 350 380 410 440 4700.0
0.4
0.8
1.2
1.6
nano CdS:ZnO-2
bg = 440nm
Vapp
= 300 V
phot
ocur
rent
(arb
. uni
ts)
wavelength (nm)
Tunablesource
Powersupply
Lockin
sample
IR Photocurrent spectroscopy
720 740 760 780 800 8200
5
10
15
20
25
30
nano CdS:ZnO - 2Incident power = 150mWV
app = 270V
Electrode separation = 1mmImax
= 767nm
phot
ocur
rent
(pA
)
wavelength (nm)
Vasa, Singh and Ayyub (in preparation)
Measurement against dark background Higher sensitivity
ARINS - Experimental setup
50%
PD2Data acquisition
Ti:Sapphire Laser System
774 nm, 68 fs, 100 MHz
ARR
Pockels cell
Variable attenuator
50%
/2polarizer
PD1
R = 0.04
68 fs, 3 Hz774 nm
sample R = 0.04
HR mirror
ARINS - Experimental setup
RtFwrEEcw )(exp21 20
20 )(exp21 2
02
0 tFwrEEccw
effin
out
LIknRE
qtF
zw
rL
zw
wE
2
2
0
222
22
2
2
202
cos
2
12
1
21
2
12expexp
3324224exp
222
2
22 inin
out
ILknILRILI
CdS thin film (thickness = 1.3 m)
Wavelength = 776 nm Pulse width = 82 fs Pulse rep. Rate = 3 Hz
Isample (max) ~ 0.8 GW/cm2
= 48 cm/ GW
input intensity (GW/cm2)
0.0 0.5 1.0 1.5 2.0 2.5
outp
ut in
tens
ity (
KW
/cm
2 )
0
20
40
60
80
100
120
Quadratic fitLinear transmission ( = 0)Experimenatal
(CdS Single crystal) = 6.4 cm/GW at 780 nm
Vasa, Singh and Ayyub (in preparation)
Presence of mid bandgap states Free carrier absorption Significant one photon, photo-current observed in IR
Dispersion of for a CdS:ZnO nano-composite thin film
720 740 760 780 800
40
60
80
100
120
140
160
180
Expt. Linear fit
nano CdS:ZnO-2Film thickness = 1.1m
cm
/GW
wavelength (nm)
720 770 8200
10
20
30
ph
otoc
urre
nt (
pA)
wavelength (nm)
129CdS:ZnO-2
48nano CdS
6.4CdS
(Single X´tl)
776nm
(cm/GW)sample
Quantitative measurement of One photon resonant nonlinearity
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.50.84
0.88
0.92
0.96
1.00
1.04
1.08
T=exp(-t+Iinc
)
sample
z
detector
nano CdS:ZnO-2
ex=391nm
=16,500cm/GW
T(n
orm
aliz
ed
)
z (cm)
expt. fit
Vasa, Singh and Ayyub (in preparation)
detector
samplechopper
Ti:Sapphire+ BBO391nm
100MHz
Ar+oscilloscope
Carrier dynamics by pump-probe spectroscopy - Setup
Pump-probe spectroscopy - Results
0 2 4 6 8 103.5
5.0
6.5
8.0
9.5
11.0
12.5
ex=393nm
probe
=Ar+ wavelengths
chopper waveform
(E)
(D)
(C)
(B)
(A)
pro
be
tra
nsm
issio
n (
arb
. u
nits)
time (ms)
Carrier generation and relaxation time measurement
Origin of photo-darkening
Free carrier absorption Excited state absorption
Photo-induced chemical and/or structural changes
LUMO
PL emissionprimary
absorptionof pump
HOMO
non radiativetransition
PL emission
primary absorptionof pump
secondary absorption of PL or probe
non radiativetransition
LUMO
HOMO
Proposed 4-level model
Vasa, Singh and Ayyub (in preparation)
non-radiativetransition
(~10ps, b)
fast non-radiativetransition (~ps)
N1
N2
secondary absorption of pump/PL/ probe (~ps)
N3
HOMO
LUMO
pl. emission
(~100ps, a)
primary absorptionof pump (~ps, )
N4
slow non-radiative
transition (~2ms, c)
Solutions of rate equations
Nnnn
nγnγntn
nγcnγbtn
nγγntn
ntn
nn
1
ca
ba
ii
ii
32
2311
23
2
313
ω
σI
dd
dd
ω
σI
dd
in population of change of Ratedd
statein Population
tγγγ
γntγ
γ
γn cpc
c
bc
c
b
exp1expI
and exp1I
22
periodLight OFF"" During
exp1expI
1
periodLight ON"" During
exp1I
1
I22
tγγγ
γI
tγγ
γI
ndz
dI
cpcc
btr
cc
btr
Carrier generation and relaxation - data fitting
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01.00
1.10
1.20
1.30
1.40
Experimental Fit
nano CdS:ZnO - 2
ex= 393nm
probe
= Ar+ wavelengths
OFF light periodRelaxation
y = y0" + A"exp(-Bt)
ON light periodGeneration
y = y0' - A'exp(-Bt)
prob
e tr
ansm
issi
on (
arb.
uni
ts)
time (ms)
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
=395nm
sample z
monochromator + PMT
lockin amp.
nano CdS:ZnO-2
ex = 395nm
pl = 519nm
P
L.
int.
(n
orm
aliz
ed
)
z (cm)
PL as a function of intensity - z scan
PL spectra as a function of incident intensity
450 500 550 600 650 7000.0
0.1
0.2
0.3
0.4
0.5
0.6
wavelength (nm)
nano CdS:ZnO-2
ex=395nm
sample at the focus
sample away from the focus
P
L. in
t. (a
rb. u
nits
)
Conclusions :Conclusions : Self-organizing nonlinear optical system and information processing – enormous potential
Organic & inorganic low-d semiconductors – adaptable to property engineering
Constructive interference of one- and two-photon tributaries – must for large nonlinearity in organics by molecular engineering
Nonlinearity originating from exciton-phonon coupling – potential for NLO devices
Geometric ease in tailoring inorganic semiconductor quantum dots but organics have an edge
NLO processes may be detrimental to optoelectronic properties
IITB
Prof. T. Kundu
A.V.V. Nampoothiri
Subal Sahani
Biswajit Pradhan
Binay Bhushan
Rajeev Sinha
AcknowledgementTIFR
Parinda Vasa
Prof. P. Ayyub
Department of Science and Technology