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