Bhanu P. Singh
Department Of Physics
Indian Institute of Technology Bombay,
Mumbai- 400076
Nonliear optical
systems,
Nonlinearity & Its
influence on opto electronic response
in low-d quantum
confined systems
Patterns in nature
Spatial
pattern in a
fluid heated
from below
Kerr slice with feedback mirror
Theoretical model
Pattern generation in saturable absorber
Threshold intensity is given by
where  is given by following equation
Artificial design ofVariety
complexity
of patterns
Nonlinear optical system to
simulate
2-component
reaction-diffusion
system
dynamics
System with 2 Kerr slices and
bounded feedback loops
Some observed
patterns
Application
to information
processing
Isolated States as memories
Capacity
tailoring
Propertyfor
relationship
the optical
properties
with structure,
suchinteractions
as
and
processes
(3) ensuing
 Eg-n and
  r -3
Conjugated Polymers
 Semiconductors
Microscopic origin of nonlinearity
B.P.Singh et al,JCP109,685(1998)
B.P.Singh et al,Europhys.lett.45,456(1999)
dI
 12 I   34 NI
dz
dN 12 I N


dt


B.P.Singh et al,JNOPM,7,571(1998)
Surface states
in semiconductor
nanoparticles
Quantum
confined 0-d
R
+
-
semiconductors
LUMO
PL emission
 
E

2
surface
2

R
states
2
2
Quantum dot transition probability 
primary
absorption
HOMO spatial restriction
3
f qd
aB
 highly efficient
Surface states provide
3 nonradiative channels
and
quench the photoluminescence
yield
f significantly
R
exc
nonradiative
transition
Nanocomposites of CdS and ZnO
ZnO
(molar %)
nano CdS:ZnO-1
CdS
(molar
%)
45
nano CdS:ZnO-2
40
60
nano CdS:ZnO-3
33
67
55
EDAX and TEM - Approximately stoichiometric
CdS and ZnO
(Cd:S = 1:1.20 and Zn:O = 1:1.18)
RF magnetron sputtering Experimental setup
SHUTTER
PRESSURE
GAUGE
GAS FLOW
LN2-COOLED
SUBSTRATE
HOLDER
MAGNETRON
GUN
TURBO
PUMP
SCRAPER
VIEW
PORT
Linear absorption spectroscopy
Tunable
source
Detector
Sample
Itr= Iine-t
2.0
(C)
1.5
t
(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
1.0
(D)
(A)
0.5
(B)
(E)
0.0
250
350
450
550
650
wavelength (nm)
750
850
Comparative study of PL in CdS and
CdS:ZnO nanocomposite films
6.0
PL int. (mV)
5.0
exc
sample
Monochromator
+ PMT
4.0
Filter
3.0
(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
(D)
2.0
(C)
1.0
0.0
400
(E)
(B)
450
500
550
600
650
(A)X5
700
wavelength (nm)
Vasa, Singh and Ayyub (in preparation)
750
Decay-time measurement
120
(A) bulk CdS (d>5nm)
(B) nano CdS:ZnO-2
PL. int. (arb. units)
100
ex= 440nm
80
Pulse width=1ps
(A)
60
(B)
40
20
0
0
500
1000
1500
2000
2500
time (ps)
Faster decay  higher PL yield
3000
Coherent PL from nanocomposite
thin films
wavelength (nm)
600
650
550
500
exc
counts / s
4000
emi
(a)
3000
film
(b)
2000
(c)
X 100
1000
(d)
(e)
X2
0
1.8
2.0
2.2
2.4
energy (eV)
2.6
Multiple beam interference
observed in PL spectra
exc = 458 nm
Vasa, Singh and Ayyub (submitted) J. Phys. Cond. Mat
Double slit experiment - Setup
Ti:Sapphire
Laser System
BBO
100 MHz, 800 nm, 80 fs
Lock-in
Amplifier
Slit separation = 178 m
Slit width = 30 m
Sample-slit = 6.15 cm
Slit-detector = 88.6 cm
PMT slit width ~ 1 mm
121 Hz
Sample
GG475
PMT
Double slit
Experimental results
40
i(fit)
i(exp)
i(max)
i(min)
intensity (V)
30
20
avg(emi) = 500 nm
10
Degree of spatial coherence
(j12) = 0.2
Spatial coherence length ~ 10m
0
-3
-2
-1
1
2
3
distance from the central line (arb. units)
Vasa, Singh and Ayyub J. Phys. Cond. Mat17,189(2005)
photocurrent (arb. units)
Photocurrent
spectroscopy
1.6
1.2
0.8
0.4
0.0
320
Lockin
Tunable
source
sample
Powers
upply
Vapp = 300 V
nano CdS:ZnO-2
bg = 440nm
350
380
410
440
470
wavelength (nm)
Vasa, Singh, Taneja, Ayyub et. al, J. Phys. Cond. Mat, 14, 281 (2002)
IR Photocurrent spectroscopy
30
photocurrent (pA)
25
20
15
nano CdS:ZnO - 2
Incident power = 150mW
Vapp = 270V
Electrode separation = 1mm
Imax = 767nm
10
5
0
720
740
760
780
800
820
wavelength (nm)
Measurement against dark background  Higher sensitivity
Vasa, Singh and Ayyub (in preparation)
ARINS - Experimental setup
Pockels cell
Ti:Sapphire
Laser System
774 nm, 68 fs, 100 MHz
HR
mirror
ARR
PD1
50%
sample
R = 0.04
PD2
50%
Data acquisition
774 nm
68 fs, 3 Hz
/2
polarizer
R = 0.04
Variable
attenuator
ARINS - Experimental setup


Ecw   1 2   E0 exp  r 2 w02 F (t ) R
E out
2

2
2
 w02
  2r 2  2   1

1 2     2 1 

  2 exp  L  exp  2  F t       
  2  
1  q  
2 

 w z  
 w z 
  2
 E0 R coskn2 I in Leff 
2
I out

Eccw  1 2   E0 exp  r 2 w02 F (t )
2
2


 L2 I in2 

L

I
kn



 
2
in
2
 exp  L RI   4 
    

 
2 2  4   2   3 3 

CdS thin film (thickness = 1.3 m)
output intensity (KW/cm2)
120
 Wavelength = 776 nm
 Pulse width = 82 fs
 Pulse rep. Rate = 3 Hz
 Isample (max) ~ 0.8 GW/cm2
100
80
60
40
Quadratic fit
Linear transmission ( = 0)
Experimenatal
20
0
0.0
0.5
1.0
1.5
2.0
 = 48 cm/ GW
2.5
input intensity (GW/cm2)
 (CdS Single crystal) = 6.4 cm/GW at 780 nm
Dispersion of  for a CdS:ZnO nanocomposite thin film
180
160
 cm/GW
140
120
photocurrent (pA)
30
100
20
776nm
(cm/GW)
CdS
(Single X´tl)
6.4
nano CdS
48
CdS:ZnO-2
129
10
0
720
770
820
wavelength (nm)
Expt.
Linear fit
80
nano CdS:ZnO-2
Film thickness = 1.1m
60
40
720
sample
740
760
780
800
wavelength (nm)
 Presence of mid bandgap states
 Free carrier absorption
 Significant one photon, photo-current observed in IR
Vasa, Singh and Ayyub (in preparation)
Quantitative measurement of
One photon resonant nonlinearity
sample
1.08
detector
T(normalized)
1.04
z
1.00
0.96
0.92
0.88
0.84
-1.5
expt.
fit
nano CdS:ZnO-2
ex=391nm
=16,500cm/GW
-1.0
-0.5
T=exp(-t+Iinc)
0.0
0.5
1.0
z (cm)
Vasa, Singh and Ayyub (in preparation)
1.5
Carrier dynamics by
pump-probe spectroscopy - Setup
Ar+
Ti:Sapphire
+ BBO
391nm
100MHz
oscilloscope
detector
chopper
sample
Pump-probe spectroscopy - Results
probe transmission (arb. units)
12.5
ex=393nm
+
probe=Ar wavelengths
11.0
chopper waveform
9.5
(A)
8.0
(B)
6.5
(C)
(D)
5.0
(E)
3.5
0
2
4
6
8
10
time (ms)
Carrier generation and relaxation time measurement
Origin of photo-darkening
LUMO
LUMO
non
radiative
transition
PL
emission
primary
absorption
of pump
HOMO
Free carrier absorption
non
radiative
transition
PL
emission
secondary
absorption of
PL or probe
primary
absorption
of pump
HOMO
Excited state absorption
Photo-induced chemical and/or structural changes
Solutions
of 4-level
rate equations
Proposed
model
ndI
i  Population
  2 n2 Iin state ni
ddz
ni
 Rate of change of population in nLUMO
i
 I γb
dt
N4
1
1  exp  γc t
I
tr  σI
d n3
N3

γnon-radiative
 fast
n
1  cγ a  γb  n3
secondary
d t  ωtransition (~ps)
During " ON" Light period
non-radiative
absorption of
d n2 γ n γ n
transition
 b 3  c pl.
pump/PL/ probe
2 emission
 I γb
dt
(~10ps, gb)
(~ps)

1

exp

1
exp

t
γ
γ
(~100ps,
g
)
ac  p
c
σI γ 
dIntr1

n1 c γ a n3  γc n2
N2
dt
 ω primary
During " OFF" Light period
n1  n2  n3  Nabsorption
slow non-radiative
HOMO I γ transition (~2ms, g )
of pump
 I γb
b

 ) 1  exp  γc t  and n2 
exp γc  p   1exp c γc t 
n2(~ps,
N

 
γc 


 


γc 
1
Vasa, Singh and Ayyub (in preparation)
Carrier generation and relaxation data fitting
probe transmission (arb. units)
1.40
nano CdS:ZnO - 2
ex= 393nm
+
probe= Ar wavelengths
1.30
1.20
ON light period
Generation
y = y0' - A'exp(-Bt)
Experimental
Fit
OFF light period
Relaxation
y = y0" + A"exp(-Bt)
1.10
1.00
0.0
0.5
1.0
1.5
2.0
time (ms)
2.5
3.0
3.5
4.0
PL as a function of intensity - z scan
1.4
monochromator
+ PMT
PL. int. (normalized)
1.2
lockin
amp.
=395nm
1.0
z
sample
0.8
0.6
nano CdS:ZnO-2
ex = 395nm
pl = 519nm
0.4
0.2
-2.0
-1.5
-1.0
-0.5
0.0
z (cm)
0.5
1.0
1.5
2.0
PL spectra as a function of
incident intensity
0.6
nano CdS:ZnO-2
ex=395nm
PL. int. (arb. units)
0.5
sample away from
the focus
0.4
0.3
sample at the focus
0.2
0.1
0.0
450
500
550
600
wavelength (nm)
650
700
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
Acknowledgement
IITB
TIFR
 Prof. T. Kundu
 Parinda Vasa
 A.V.V. Nampoothiri
 Prof. P. Ayyub
 Subal Sahani
 Biswajit Pradhan
 Binay Bhushan
 Rajeev Sinha
Department of Science and
Technology