Spectral and nonlinear optical characteristics of nanocomposites of ZnO
Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
super
Active In SP
**

Posts: 43
Joined: Jun 2009
#1
14-06-2009, 08:52 AM


Spectral and nonlinear optical characteristics of nanocomposites
of ZnO“CdS
Litty Irimpan,
a
V. P. N. Nampoori, and P. Radhakrishnan
International School of Photonics, Cochin University of Science and Technology, Cochin,
Kerala 682022, India
Received 21 December 2007; accepted 6 March 2008; published online 8 May 2008
In this article, we present the spectral and nonlinear optical properties of ZnO“CdS nanocomposites
prepared by colloidal chemical synthesis. The optical band gap E
g
of the material is tunable
between 2.62 and 3.84 eV. The emission peaks of ZnO“CdS nanocomposites change from 385 to
520 nm almost in proportion to changes in E
g
. It is possible to obtain a desired luminescence color
from UV to green by simply adjusting the composition. The nonlinear optical response of these
samples is studied by using nanosecond laser pulses from a tunable laser at the excitonic resonance
and off-resonance wavelengths. The nonlinear response is wavelength dependent, and switching
from saturable absorption SA to reverse SA RSA has been observed for samples as the excitation
wavelength changes from the excitonic resonance to off-resonance wavelengths. Such a changeover
in the sign of the nonlinearity of ZnO“CdS nanocomposites is related to the interplay of exciton
bleach and optical limiting mechanisms. The ZnO“CdS nanocomposites show self-defocusing
nonlinearity and good nonlinear absorption behavior at off-resonant wavelengths. The nonlinear
refractive index and the nonlinear absorption increase with increasing CdS volume fraction at 532
nm. The observed nonlinear absorption is attributed to two photon absorption followed by weak free
carrier absorption. The enhancement of the third-order nonlinearity in the composites can be
attributed to the concentration of exciton oscillator strength. This study is important in identifying
the spectral range and composition over which the nonlinear material acts as a RSA based optical
limiter. ZnO“CdS is a potential nanocomposite material for the tunable light emission and for the
development of nonlinear optical devices with a relatively small limiting threshold. © 2008
American Institute of Physics. DOI: 10.1063/1.2919109
I. INTRODUCTION
Semiconductor nanoparticles have been under continu-
ous scientific interest because of their unique quantum na-
ture, which changes the material solid-state properties. The
linear and nonlinear optical properties of the semiconductors
are the subject of much current theoretical and experimental
interest.
1
Among the various nonlinear optical NLO mate-
rials investigated, the direct band gap semiconductors, espe-
cially zinc oxide ZnO and cadmium sulfide Cds , have
attractive nonlinear properties that make them ideal candi-
dates for NLO based devices. ZnO is a wide and direct band
gap II“VI semiconductor with a band gap of 3.37 eV and a
high exciton binding energy of 60 meV having many appli-
cations, such as a transparent conductive contact, thin-film
gas sensor, varistor, solar cell, luminescent material, surface
electroacoustic wave device, heterojunction laser diode, UV
laser, and others. Nanosized ZnO in the form of quantum
dots, nanowires, nanobelts, etc, are referred to as the material
of the 21st century.
2
The optical properties of this material
are currently the subject of tremendous investigations, in re-
sponse to the industrial demand for optoelectronic devices
that could operate at short wavelengths. There is a significant
demand for high nonlinear optical materials, which can be
integrated into an optoelectronic device.
In recent years, interest in the synthesis, characterization,
and application of colloidal quantum dot semiconductor
materials has markedly grown.
3
Nanocrystals of cadmium
sulfide are by far the most studied system among all the
semiconducting nanocrystals.
4
The bulk CdS has a direct
band gap of 2.4 eV at 300 K, and the typical Bohr exciton
diameter of CdS is around 5.8 nm; consequently, CdS nano-
crystals in the size range of 1“6 nm show sizable quantum
confinement effects with remarkably different optical prop-
erties. The size dependent, unusual optical, and electronic
properties of these nanocrystals have been studied in detail
by using a wide variety of experimental and theoretical
techniques.
5
The possibility of tailoring the bulk material properties
by varying the size, structure, and composition of constitut-
ing nanoscale particles makes them candidates for various
important applications in the field of material research. The
field of nanocomposite materials has been widely recognized
as one of the most promising and rapidly emerging research
areas. Promising applications are expected or have already
been realized in many fields of technology such as optical
and electronic materials, solid electrolytes, coating technol-
ogy, sensorics, catalysis, and separation science. Significant
investigations have been done in the photophysical and pho-
tochemical behaviors of single and multicomponent metals
and semiconductor nanoclusters.
6
Such composite materials
are especially of interest in developing efficient light-energy
conversion systems, optical devices, and microelectronics.
a
Electronic mail: littyirimpan@yahoo.co.in.
JOURNAL OF APPLIED PHYSICS 103, 094914 2008
0021-8979/2008/103 9 /094914/8/$23.00
© 2008 American Institute of Physics
103, 094914-1
Author complimentary copy. Redistribution subject to AIP license or copyright, see jap.aipjap/copyright.jspPage 2

For example, photoinduced deposition of noble metals such
as Pt or Au on semiconductor nanoclusters has often been
employed to enhance their photocatalytic activity.
7
With many advantages such as low cost, nontoxicity, and
stability, ZnO is becoming a very promising n-type oxide
semiconductor. Most of the work has been devoted to the
electrical and fluorescent properties of ion-doped zinc oxide
materials,
8
while only a few reports can be found by using
ZnO as the matrix for nanoparticle composite films.
9
These
nanocomposites may lead to optically functional properties.
Extensive investigations of the photoluminescence and the
third-order optical nonlinearities of nanometer-sized semi-
conductor materials have demonstrated interesting physical
properties and potential applications. The absorption and lu-
minescent properties of CdS and PbS particles can be easily
tuned by selecting appropriate matrix materials.
10
Recently, a
microemulsion technique has been developed to prepare
semiconductor nanocomposites such as ZnS/CdSe, ZnSe/
CdSe, or ZnS/CdS in a core-shell structure.
11
Chemically
synthesized semiconductor nanocomposites offer necessary
and basic materials promising color-tunable, flexible, all-
purpose chromophore systems, in which the strong quantum
confinement effect of the carriers leads to unique, size de-
pendent linear and NLO properties.
12
In this study, therefore,
the nanocomposite techniques are applied to improve the
spectral and optical properties of ZnO. In our continued ef-
forts to explore the optical properties of various nanocom-
posites, we have now elucidated the spectral and nonlinear
responses of ZnO“CdS nanocomposites.
Generally, photoluminescence spectrum of a single crys-
tal ZnO mainly consists of two bands.
13
The one in the UV
region corresponding to the near band edge emission at about
380 nm is mainly attributed to near band edge emission, and
the other in the visible region is due to structural defects and
impurities. Soon after the reporting of stimulated UV emis-
sion of ZnO at room temperature, it attracted the attention of
the researchers as a UV laser material.
14
Thereafter, more
and more investigators aimed at applications of ZnO emit-
ting at the short wavelength. Several reviews elaborated the
recent development of photoelectron applications of ZnO in
a short wavelength.
15
Tsukazaki et al. reported the violet
electroluminescence from homostructural ZnO p-i-n junc-
tions at room temperature.
16
However, the improvement of
UV emission and the simplification of growth techniques are
still very important.
We report the wavelength dependent nonlinear absorp-
tion of ZnO“CdS nanocomposites with varying CdS content
under nanosecond excitation. The studies of nonlinear pro-
cesses in photonic materials are significant in the context of
their technological applications, especially in areas such as
passive optical power limiting, optical switching, and the
design of logic gates. Optical limiting occurs when the abso-
lute transmittance of a material decreases with an increase in
input fluence. One mechanism for optical limiting is pro-
vided by reverse saturable absorption RSA , in which the
excited state absorption cross section is higher than the
ground state absorption cross section. It is also known that
doping significantly improve the limiting performance of
ZnO.
II. EXPERIMENT
Colloids of ZnO are synthesized by a modified polyol
precipitation method.
17
The monodisperse ZnO colloidal
spheres are produced by a two-stage reaction process. The
method of preparation involves the hydrolysis of zinc acetate
dihydrate ZnAc in diethylene glycol DEG medium. Among
the different polyols, the DEG is chosen because it is re-
ported to give particles with uniform shape and size distri-
butions. The size of the particles and hence the stability of
this colloidal suspension depend on the concentration of zinc
acetate as well as on its rate of heating. The molar concen-
tration of precursor solution is 0.025M and a heating rate of
4 °C/min is employed for the formation of ZnO at a tem-
perature of 120 °C. The product from the primary reaction is
placed in a centrifuge and the supernatant DEG, dissolved
reaction products, and unreacted ZnAc and water is de-
canted off and saved. A secondary reaction is then performed
to produce the monodisperse ZnO spheres. Prior to reaching
the working temperature, typically at 115 °C, some volume
of the primary reaction supernatant is added to the solution.
After reaching 120 °C, it is stirred for 1 h to get a monodis-
perse stable colloid.
The CdS nanocolloids are prepared by chemical
method.
18
Cd NO
3 2
4H
2
O Merck, India and NH
2
CSNH
2
Merck, India are used as the precursors for the incorpora-
tion of Cd and S, respectively. These precursors are dis-
solved in 2-propanol and distilled water under stirring. The
solution is kept on stirring for 1 h to get a monodisperse
stable colloid. The molar concentration of the precursor so-
lution is 0.025M.
The ZnO“CdS nanocomposites are prepared by colloidal
chemical synthesis by mixing a certain amount of CdS col-
loid to ZnO colloid and stirring for 1 h. The volume fraction
of CdS is changed keeping the volume of ZnO constant. The
samples having ZnO“xCdS composition with x= 0.1%,
0.5%, 1%, 1.5% 2%, and 5% are named as ZnO“0.1CdS,
ZnO“0.5CdS, ZnO“1Cds, ZnO“1.5CdS, ZnO“2CdS, and
ZnO“5CdS, respectively.
The ZnO“CdS nanocomposites are characterized by op-
tical absorption measurements recorded by using a spectro-
photometer JascoV-570 UV/VIS/IR , and the fluorescence
emission measurements are recorded by using a Cary Eclipse
fluorescence spectrophotometer VARIAN . In the present
investigation, we have employed the single beam z scan
technique with nanosecond laser pulses to measure the NLO
absorptive and refractive properties of ZnO“CdS nanocom-
posites. Z scan technique developed by Bahae et al.
19,20
is a
single beam method for measuring the sign and magnitude of
nonlinear refractive index n
2
and has a sensitivity compa-
rable to interferometric methods. A Q-switched neodymium
doped yttrium aluminum garnet laser Spectra Physics LAB-
1760, 532 nm, 7 ns, 10 Hz is used as the light source, and
the wavelength dependences of the samples are studied by
using a tunable laser Quanta Ray MOPO, 5 ns, 10 Hz . The
sample is moved in the direction of light incidence near the
focal spot of the lens with a focal length of 200 mm. The
radius of the beam waist
0
is calculated to be 35.4
m. The
Rayleigh length z
0
= w
0
2
/ is estimated to be 7.4 mm, much
094914-2
Irimpan, Nampoori, and Radhakrishnan
J. Appl. Phys. 103, 094914 2008
Author complimentary copy. Redistribution subject to AIP license or copyright, see jap.aipjap/copyright.jspPage 3

greater than the thickness of the sample cuvette 1 mm ,
which is an essential prerequisite for z scan experiments. The
transmitted beam energy, reference beam energy, and their
ratio are simultaneously measured by an energy ratiometer
Rj7620, Laser Probe Corp. having two identical pyroelec-
tric detector heads Rjp735 . The linear transmittance of the
far field aperture S, defined as the ratio of the pulse energy
passing the aperture to the total energy, is measured to be
approximately 0.21. The z scan system is calibrated by using
CS
2
as the standard. The effect of fluctuations of laser power
is eliminated by dividing the transmitted power by the power
obtained at the reference detector. The data are analyzed by
using the procedure described by Bahae et al., and the non-
linear coefficients are obtained by fitting the experimental z
scan plot with the theoretical plots.
III. RESULTS AND DISCUSSION
Optical absorption measurement is an initial step to ob-
serve the single colloid and nanocomposite behavior. Figure
1 gives the room temperature absorption spectra of the ZnO“
CdS nanocomposites. The excitonic peaks of ZnO and that of
CdS colloids are found to be blueshifted with respect to their
bulk which could be attributed to the confinement effects.
21
The pronounced dependence of the absorption band gap on
the size of semiconductor nanocrystals is used to determine
the particle size. An order of magnitude estimate of the grain
size is possible from the absorption spectra. From the shift of
absorption edge, the size of the dots is calculated. The sizes
of ZnO and Cds nanocolloids are in the range of 10“12 nm.
The presence of excitonic peak itself indicates that the com-
posites are of nanometer size. It is seen that the absorption
edge corresponding to the nanocomposites gets redshifted as
a function of the CdS content. The size evolution of nano-
composites may also have some relation with optical charac-
teristics in addition to the composition, and the study is in
progress.
The direct band gap of ZnO“CdS nanocomposites is es-
timated from the graph of h versus
h
2
for the absorp-
tion coefficient
that is related to the band gap E
g
as
h
2
=k h -E
g
, where h is the incident light energy and
k is a constant. The optical band gap E
g
is found to be
dependent on the composition, and there is a decrease in the
band gap of the semiconductor with an increase in volume
fraction of CdS in the nanocomposites, as shown in Fig. 2.
E
g
changes from 3.84 eV for ZnO to 2.62 eV for CdS almost
in proportion to the composition of CdS. The total change in
the band gap of the material is contributed by the shifts of
the valence band as well as that of the conduction band edges
away from each other. In general, the shift of the top of the
valence band TVB is not the same as that of the bottom of
the conduction band BCB . Moreover, there are recent stud-
ies, although few in number that reported the individual
shifts in TVB and BCB employing various forms of high-
energy spectroscopies, such as the photoemission and the
x-ray absorption spectroscopies.
22
The shifts of the band
edges smoothly decrease to zero for large sized nanocrystals
and the shift in the BCB is, in general, much larger compared
to the shift in the TVB. Within the range of compositions
FIG. 1. Color online Absorption spectra of ZnO“CdS nanocomposites.
FIG. 2. Optical band gap of ZnO“CdS nanocomposites.
094914-3
Irimpan, Nampoori, and Radhakrishnan
J. Appl. Phys. 103, 094914 2008
Author complimentary copy. Redistribution subject to AIP license or copyright, see jap.aipjap/copyright.jspPage 4

studied, the optical band gap is tunable from 2.62 to 3.84 eV.
The band gap engineering in ZnS“CdS is reported to be from
2.58 to 3.91 eV.
23
Photoluminescence spectra of all samples measured at
room temperature are shown in Fig. 3. The 385 nm emission
is the near band edge emission of ZnO and the 520 nm emis-
sion is the near band edge emission peak of CdS. Emission
peaks of ZnO“CdS nanocomposites change from 385 to 520
nm almost in proportion to changes in E
g
. It is possible to
obtain a desired luminescence color from UV to green by
simply adjusting the composition. The tuning of lumines-
cence in ZnS“CdS is reported to be from blue to red in
proportion to change in E
g
.
23
Figure 4 shows the nonlinear absorption of ZnO“CdS
nanocomposites at a typical fluence of 300 MW/cm
2
for an
irradiation wavelength of 532 nm. The open-aperture curve
exhibits a normalized transmittance valley, indicating the
presence of reverse saturable absorption in the colloids. The
obtained nonlinearity is found to be of the third order, as it
fits to a two photon absorption TPA process. The corre-
sponding net transmission is given in Ref. 19,
T z =
C
q
0
-
ln 1 + q
0
e
-t
2
dt,
1
where
q
0
z,r,t = I
0
t L
eff
.
Here, L
eff
=1-e
- l
/ is the effective thickness with lin-
ear absorption coefficient , nonlinear absorption coefficient
, and I
0
is the irradiance at focus. The solid curves in Fig. 4
are the theoretical fit to the experimental data. The obtained
values of nonlinear absorption coefficient at an intensity of
300 MW/cm
2
are shown in Table I.
Interestingly, ZnO and CdS colloids show a minimum
nonlinearity, while the ZnO“CdS nanocomposites clearly ex-
hibit a larger induced absorption behavior. The calculated
nonlinear coefficients given in Table I show fairly high val-
ues of nonlinearity. The nonlinear absorption coefficient sub-
stantially increases in the nanocomposites, as compared to
pure ZnO and CdS colloids. The enhancement of the third-
order nonlinearity can be attributed to the concentration of
exciton oscillator strength.
3
Different processes, such as TPA, free carrier absorption,
transient absorption, interband absorption, photoejection of
electrons, and nonlinear scattering, are reported to be opera-
FIG. 3. Color online Fluorescence spectra of ZnO“
CdS nanocomposites.
FIG. 4. Open-aperture z scan traces of ZnO“CdS nanocomposites at an
intensity of 300 MW/cm
2
for an irradiation wavelength of 532 nm.
TABLE I. Measured values of nonlinear absorption coefficient, saturation
intensity, and refractive index of ZnO“CdS nanocomposites at an intensity
of 300 MW/cm
2
for different irradiation wavelengths.
Nonlinear absorption
coefficient
Nonlinear refractive
index
450 nm
532 nm
532 nm
ZnO“CdS
nanocomposites
cm/GW
I
s
GW/cm
2
cm/GW
n
2
10
-17
m
2
/W
ZnO
51.8
20.7
1.5
CdS
0.20
51.8
4.4
ZnO“0.5CdS
131.3
62.2
5.9
ZnO“1CdS
34.6
155.5
6.9
ZnO“2CdS
17.3
207.4
8.9
ZnO“5CdS
0.04
242.0
11.0
094914-4
Irimpan, Nampoori, and Radhakrishnan
J. Appl. Phys. 103, 094914 2008
Author complimentary copy. Redistribution subject to AIP license or copyright, see jap.aipjap/copyright.jspPage 5

tive in nanoclusters. In general, induced absorption can occur
due to a variety of processes. The theory of TPA process
fitted well with the experimental curve establishes that TPA
is the basic mechanism. There is a possibility of higher order
nonlinear processes such as free carrier absorption contribut-
ing to induced absorption. The free carrier lifetime of ZnO is
reported to be 2.8 ns.
24
Hence, the 7 ns pulses used in the
present study can excite the accumulated free carriers gener-
ated by TPA by the rising edge of the pulse. The larger non-
linear absorption in semiconductors such as ZnSe, ZnO, and
ZnS is reported to be due to two photon induced free carrier
absorption along with TPA.
24
The enhancement of nonlinear
absorption in Ag
2
S“CdS nanocomposites in comparison to
the CdS nanoparticles is reported to be due to free-carrier
absorption, and the free-carrier lifetime of ZnO is determined
to be a few nanoseconds.
11
Thus, we propose that the ob-
served nonlinearity is caused by TPA followed by weak free-
carrier absorption in the nanocomposites.
Figure 5 shows the nonlinearity observed at 450 nm at a
fluence of 300 MW/cm
2
. An absorption saturation behavior
is found in CdS colloid and ZnO“5CdS nanocomposites.
However, all the other ZnO colloids and ZnO“CdS nano-
composites exhibit induced absorption at this wavelength.
Such a changeover in the sign of the nonlinearity is related to
the interplay of exciton band bleach and optical limiting
mechanisms, as found from earlier studies of semiconductor
nanoparticles.
25
Such a behavior can generally be modeled
by defining a nonlinear absorption coefficient
I , which is
a sum of independent positive and negative transmission
coefficients,
26
I =
1 + I/I
S
+ I,
2
where I
s
is the saturation intensity. The obtained values of
nonlinear absorption coefficient
and saturation intensity I
s
at an intensity of 300 MW/cm
2
are shown in Table I.
The excitonic peak is sensitive to laser excitation. As the
particle size is reduced, a series of nearby transitions occur-
ring at slightly different energies in the bulk are compressed
by quantum confinement into a single, intense transition in a
quantum dot. Therefore, the oscillator strength of the nano-
particle is concentrated into just a few transitions and the
strong exciton bleaching can be expected. Exciton bleach
effects are seen when the CdS nanocolloids are excited at
excitonic resonance with nanosecond laser pulses of 450 nm.
So, increased transmission behavior is observed for CdS
nanocolloids, which fits to a saturable absorption mecha-
nism. On the other hand, ZnO colloids exhibit induced ab-
sorption at this wavelength. For a small volume fraction of
CdS, the nonlinear absorption coefficient substantially in-
creases in the nanocomposites, as compared to pure ZnO.
ZnO“0.5CdS exhibits maximum nonlinear absorption at 450
nm. The enhancement of the third-order nonlinearity can be
attributed to the concentration of exciton oscillator strength.
3
When the volume fraction of CdS increases beyond 0.5%,
the nonlinear absorption coefficient decreases with the in-
crease in the volume fraction of CdS and it becomes a satu-
rable absorber at and above 5% CdS due to the interplay of
exciton bleach and optical limiting mechanisms. The exci-
tonic bleaching of the CdS nanocolloids originates from the
transition between the ground state 1S e and the lowest ex-
cited state 1S3/2 h since the excitation at 450 nm corre-
sponds to the absorption close to the resonance of the
1S e -1S
3/2
h excitonic transition.
25
Thus, the nonlinearity
of the ZnO“CdS nanocomposites is related to the interplay of
exciton bleach and optical limiting mechanisms at 450 nm.
The great potential of using the ZnO“CdS nanocomposite
lies in the fact that the composition of the constituent ele-
ments can readily be altered to optimize the desired NLO
properties either as a saturable absorber or as a reverse satu-
rable absorber.
Figure 6 gives the closed-aperture z scan traces of ZnO“
CdS nanocomposites at a fluence of 300 MW/cm
2
for an
irradiation wavelength of 532 nm. The closed-aperture curve
exhibits a peak-valley shape, indicating a negative value of
the nonlinear refractive index n
2
. For samples with sizable
refractive and absorptive nonlinearities, closed-aperture mea-
surements contain contributions from both the intensity-
dependent changes in the transmission and in the refractive
index.
19
By dividing the normalized closed-aperture trans-
mittance by the corresponding normalized open-aperture
data, we can retrieve the phase distortion created due to the
change in the refractive index.
It is observed that the peak valley of closed-aperture z
scan satisfied the condition z 1.7z
0
, thus confirming the
presence of pure electronic third-order nonlinearity.
19
The
value of the difference between the normalized peak and
valley transmittance T
p-v
can be obtained by the best theo-
retical fit from the results of a divided z scan curve. The
nonlinear refractive index n
2
is calculated from T
p-v
in a
closed-aperture z scan by using Eq. 3 and is tabulated in
Table I,
T
p-v
= 0.406 1 - S
0.25
0
,
3
where
FIG. 5. Open-aperture z scan traces of ZnO“CdS nanocomposites at an
intensity of 300 MW/cm
2
for an irradiation wavelength of 450 nm.
094914-5
Irimpan, Nampoori, and Radhakrishnan
J. Appl. Phys. 103, 094914 2008
Author complimentary copy. Redistribution subject to AIP license or copyright, see jap.aipjap/copyright.jspPage 6

0
=
2
n
2
I
0
L
eff
.
The peak-valley trace in a closed-aperture z scan shows
that these samples have self-defocusing negative, n
2
0
nonlinearity, although earlier reports have shown positive
nonlinearity for individual CdS nanoclusters prepared by la-
ser ablation.
27
The nonlinear refractive index substantially
increases in the nanocomposites, as compared to pure ZnO
and CdS colloids. The enhancement of the third-order non-
linearity can be attributed to the concentration of exciton
oscillator strength.
3
Since n
2
increases with absorption, ther-
mal nonlinearity is also taken into account. It is reported that
if the thermal contributions are to dominate, then, there will
be an increase in n
2
with increase in absorption.
28
The figure
of merit for the third-order nonlinearity measured by using z
scan technique shows an increase in the particle size of ZnO
as well as CdS.
25
We therefore attribute the steady increase
in the n
2
values to the increase in third-order susceptibility as
a function of particle size from ZnO“0.1CdS to ZnO“5CdS.
The significant optical nonlinearities of the pure semi-
conductor nanocolloid at 532 nm are reported to have the
nonlinear refractive index of the order of 10
-16
“10
-17
m
2
/W. The third-order nonlinear absorption coefficients of
CdS nanocrystals are reported to be of the order of
10
-10
m/W.
29
It is worth noting that certain representative
third-order NLO materials, such as CuO chain compounds,
Ag
2
S“CdS nanocomposites, organic coated quantum dots,
metal clusters, etc., yielded values of the order of
10
-9
“10
-14
m/W for nonlinear absorption coefficient at a
wavelength of 532 nm.
30,31
These values are comparable to
the value of
obtained for nanocomposites in the present
investigation. Thus, the nonlinear absorption coefficient and
nonlinear refractive index measured by the z scan technique
reveal that the ZnO“CdS nanocomposites investigated in the
present study have good NLO response and could be chosen
as ideal candidates with potential applications in nonlinear
optics either as saturable absorber or as reverse saturable
absorber.
Recently, nanomaterials have drawn significant attention
as optical limiters for eyes or for sensor protection from laser
terror in homeland or agile laser threats on the battlefield.
32
Also, the NLO properties of nanomaterials are of great inter-
est for optical switching, pulse power shaping of optical
parametric oscillator/optical parametric generator, and other
FIG. 6. Closed-aperture z scan traces of ZnO“CdS nanocomposites at an intensity of 300 MW/cm
2
for an irradiation wavelength of 532 nm.
094914-6
Irimpan, Nampoori, and Radhakrishnan
J. Appl. Phys. 103, 094914 2008
Author complimentary copy. Redistribution subject to AIP license or copyright, see jap.aipjap/copyright.jspPage 7

NLO applications. Optical power limiting is operated
through the NLO processes of nanomaterials. However, the
great potentials of nanomaterials as optical power limiters
have just begun to be recognized.
To examine the viability of ZnO“CdS nanocomposites as
optical limiters, the nonlinear transmission of the colloid is
studied as a function of input fluence. An important term in
the optical limiting measurement is the limiting threshold. It
is obvious that the lower the optical limiting threshold, the
better the optical limiting material. Optical limiters are de-
vices that transmit light at low input fluences or intensities,
but become opaque at high inputs. The optical limiting prop-
erty occurs mostly due to absorptive nonlinearity which cor-
responds to the imaginary part of the third-order
susceptibility.
33
From the value of fluence at focus, the flu-
ence values at other positions could be calculated by using
the standard equations for Gaussian beam waist. Such plots
represent a better comparison of the nonlinear absorptions or
transmissions in these samples and are generated from z scan
traces. Figure 7 illustrates the influence of volume fraction of
CdS in ZnO“CdS nanocomposites on the optical limiting
response.
The fluence value corresponding to the onset of optical
limiting optical limiting threshold is found to be high in the
case of ZnO colloids 55 MW/cm
2
in comparison to the
CdS colloids 20 MW/cm
2
. These values are of the order
of the reported optical limiting threshold for CdS
nanocolloids.
34
ZnO“CdS nanocomposites are found to be
good optical limiters compared to ZnO and CdS, and the
optical limiting threshold of ZnO“5CdS nanocomposites is
observed to be 7 MW/cm
2
. The arrow in the figure indicates
the approximate fluence at which the normalized transmis-
sion begins to deviate from linearity. Nanocomposites have a
significant effect on the limiting performance, and increasing
the volume fraction of CdS reduces the limiting threshold
and enhances the optical limiting performance.
IV. CONCLUSION
The spectral and NLO properties of ZnO“CdS nanocom-
posites prepared by a colloidal chemical synthesis are inves-
tigated. The optical band gap is tunable between 2.62 and
3.84 eV. The emission peaks of ZnO“CdS nanocomposites
change from 385 to 520 nm almost in proportion to changes
in E
g
. It is possible to obtain a desired luminescence color
from UV to green by simply adjusting the composition. NLO
response of these samples is studied by using nanosecond
laser pulses from a tunable laser at excitonic resonance and
off-resonance wavelengths. The nonlinear response is wave-
length dependent, and switching from saturable absorption to
reverse saturable absorption has been observed for samples
as the excitation wavelength changes from the excitonic
resonance to off-resonance wavelengths. Such a changeover
in the sign of the nonlinearity of ZnO“CdS nanocomposites
is related to the interplay of exciton bleach and optical lim-
iting mechanisms. The ZnO“CdS nanocomposites show self-
defocusing nonlinearity and good nonlinear absorption be-
havior at off-resonant wavelengths. The nonlinear refractive
index and the nonlinear absorption increase with increasing
CdS volume fraction at 532 nm. The observed nonlinear ab-
sorption is explained through TPA followed by weak free
carrier absorption. The enhancement of the third-order non-
linearity in the composites can be attributed to the concen-
tration of exciton oscillator strength. This study is important
in identifying the spectral range and composition over which
the nonlinear material acts as a RSA based optical limiter. It
is shown that ZnO“CdS is a potential nanocomposite mate-
rial for tunable light emission and for the development of
NLO devices with a relatively small limiting threshold.
ACKNOWLEDGMENTS
L.I. acknowledges UGC for research fellowship.
1
Y. Kayanuma, Phys. Rev. B 38, 9797 1988 .
2
Z. L. Wang, Mater. Today 7, 26 2004 .
3
A. P. Alivisatos, J. Phys. Chem. 100, 13226 1996 .
4
L. E. Brus, J. Chem. Phys. 79, 5566 1983 .
5
M. V. Rama Krishna and R. A. Friesner, Phys. Rev. Lett. 67, 629 1991 .
6
U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters
Springer, Berlin, 1995 .
7
B. Kraeutler and A. J. Bard, J. Am. Chem. Soc. 100, 4317 1978 .
8
A. E. Hichou, A. Bougrine, J. L. Bubendorff, J. Ebothe, M. Addou, and M.
Troyon, Semicond. Sci. Technol. 17, 607 2002 .
9
X. H. Wang, J. L. Shi, S. G. Dai, and Y. Yang, Thin Solid Films 429, 102
2003 .
10
P. Yang, C. F. Song, M. K. Lu, X. Yin, G. J. Zhou, D. Xu, and D. R. Yuan,
Chem. Phys. Lett. 345, 429 2001 .
11
M. Y. Han, W. Huang, C. H. Chew, L. M. Gan, X. J. Zhang, and W. Ji, J.
Phys. Chem. B 102, 1884 1998 .
12
A. Nakamura, Y. L. Lee, T. Kataoka, and T. Tokizaki, J. Lumin. 60“61,
376 1994 .
13
L. Irimpan, A. Deepthy, B. Krishnan, V. P. N. Nampoori, and P.
Radhakrishnan, J. Appl. Phys. 102, 063524 2007 .
14
D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T.
Goto, Appl. Phys. Lett. 70, 2230 1997 .
15
Ã. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V.
Avrutin, S.-J. Cho, and H. Morkoç, J. Appl. Phys. 98, 041301 2005 .
16
A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya,
K. Ohtani, S. F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, and
M. Kawasaki, Nat. Mater. 4, 42 2005 .
17
L. Irimpan, A. Deepthy, B. Krishnan, V. P. N. Nampoori, and P.
Radhakrishnan, Appl. Phys. B: Lasers Opt. 90, 547 2008 .
18
S. K. Panda, S. Chakrabarti, B. Satpati, P. V. Satyam, and S. Chaudhuri, J.
Phys. D 37, 628 2004 .
19
M. S. Bahae, A. A. Said, and E. W. van Stryland, Opt. Lett. 14, 955
1989 .
FIG. 7. Optical limiting response of ZnO“CdS nanocomposites at 532 nm.
094914-7
Irimpan, Nampoori, and Radhakrishnan
J. Appl. Phys. 103, 094914 2008
Author complimentary copy. Redistribution subject to AIP license or copyright, see jap.aipjap/copyright.jspPage 8

20
L. Irimpan, B. Krishnan, A. Deepthy, V. P. N. Nampoori, and P.
Radhakrishnan, J. Appl. Phys. 103, 033105 2008 .
21
D. Luna-Moreno, E. De la Rosa-Cruz, F. J. Cuevas, L. E. Regalado, P.
Salas, R. Rodríguez, and V. M. Castano, Opt. Mater. 19, 275 2002 .
22
V. L. Colvin, A. P. Alivisatos, and J. G. Tobin, Phys. Rev. Lett. 66, 2786
1991 .
23
S. Shionoya and W. M. Yen, Phosphor Handbook CRC, New York,
1999 .
24
X. Zhang, H. Fang, S. Tang, and W. Ji, Appl. Phys. B: Lasers Opt. 65, 549
1997 .
25
J. He, W. Ji, G. H. Ma, S. H. Tang, H. I. Elim, W. X. Sun, Z. H. Zhang,
and W. S. Chin, J. Appl. Phys. 95, 6381 2004 .
26
Y. Gao, X. Zhang, Y. Li, H. Liu, Y. Wang, Q. Chang, W. Jiao, and Y. Song,
Opt. Commun. 251, 429 2005 .
27
R. A. Ganeev, A. I. Ryasnyansky, R. I. Tugushev, and T. Usmanov, J. Opt.
A, Pure Appl. Opt. 5, 409 2003 .
28
P. Prem Kiran, G. De, and D. Narayana Rao, IEE Proc.: Circuits Devices
Syst. 150, 559 2003 .
29
N. Venkatram, R. Sai Santosh Kumar, and D. Narayana Rao, J. Appl.
Phys. 100, 074309 2006 .
30
M. Y. Han, W. Huang, C. H. Chew, L. M. Gan, X. J. Zhang, and W. Ji, J.
Phys. Chem. B 102, 1884 1998 .
31
S. Shi, W. Ji, and S. H. Tang, J. Am. Chem. Soc. 116, 3615 1994 .
32
Y. Sun, J. E. Riggs, K. B. Henbest, and R. B. Martin, J. Nonlinear Opt.
Phys. Mater. 9, 481 2000 .
33
F. M. Quereshi, S. J. Martin, X. Long, D. D. C. Bradley, F. Z. Heneri, W.
J. Balu, E. C. Smith, C. H. Wang, A. K. Kar, and H. L. Anderson, Chem.
Phys. 231, 87 1998 .
34
W. Jia, E. P. Douglas, F. Guo, and W. Suna, Appl. Phys. Lett. 85, 6326
200
Reply

Important Note..!

If you are not satisfied with above reply ,..Please

ASK HERE

So that we will collect data for you and will made reply to the request....OR try below "QUICK REPLY" box to add a reply to this page

Quick Reply
Message
Type your reply to this message here.


Image Verification
Please enter the text contained within the image into the text box below it. This process is used to prevent automated spam bots.
Image Verification
(case insensitive)

Possibly Related Threads...
Thread Author Replies Views Last Post
  One-step synthesis of graphene/SnO2 nanocomposites and its application project girl 0 386 17-11-2012, 04:03 PM
Last Post: project girl
  Bottom and Charm Masses and Lifetimes at the Tevatron; and a Pentaquark Search seminar flower 0 284 27-10-2012, 12:05 PM
Last Post: seminar flower
  Development of High Spectral Resolution Technique for Registration Quasielastic Light seminar flower 0 284 27-10-2012, 11:43 AM
Last Post: seminar flower
  Discovery of Protons and Characteristics of Anode Rays seminar flower 0 894 01-10-2012, 05:53 PM
Last Post: seminar flower
  Recent Development in Optical Fiber Biosensors seminar flower 0 572 01-10-2012, 04:52 PM
Last Post: seminar flower
  NONLINEAR SECOND ORDER HARMONICS seminar girl 0 853 16-08-2012, 04:21 PM
Last Post: seminar girl
  THE ELECTRICAL, OPTICAL AND STRUCTURAL STUDIES ON PHTHALOCYANINE THIN FILMS seminar flower 0 446 28-07-2012, 03:40 PM
Last Post: seminar flower
  SYNTHESIS, GROWTH AND CHARACTERIZATION OF SEMI-ORGANIC NONLINEAR OPTICAL CRYSTAL seminar ideas 0 774 21-06-2012, 01:35 PM
Last Post: seminar ideas
  Electrical Characteristics of n-ZnO/p-Si Heterojunction Diodes seminar projects crazy 0 1,860 14-06-2009, 02:07 AM
Last Post: seminar projects crazy
  Luminescence from Surfactant-Free ZnO Quantum Dots seminar projects crazy 0 3,144 14-06-2009, 01:35 AM
Last Post: seminar projects crazy