Litty, Irimpan; Deepthy, A; Bindu, Krishnan; Nampoori, V P N; Radhakrishnan, P(Springer, 2008)
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Abstract:
In the present work, we have investigated the nonlinear
optical properties of self-assembled films formed from
ZnO colloidal spheres by z-scan technique. The sign of the nonlinear
component of refractive index of the material remains
the same; however, a switching from reverse saturable absorption
to saturable absorption has been observed as the material
changes from colloid to self-assembled film. These different
nonlinear characteristics can be mainly attributed to ZnO defect
states and electronic effects when the colloidal solution is
transformed into self-assembled monolayers. We investigated
the intensity, wavelength and size dependence of saturable and
reverse saturable absorption of ZnO self-assembled films and
colloids. Values of the imaginary part of third-order susceptibility
are calculated for particles of size in the range 20–300 nm
at different intensity levels ranging from 40 to 325MW/cm2
within the wavelength range of 450–650 nm.
V P N Nampoori; Litty, Irimpan; Radhakrishnan, P(Journal of Applied Physics, American Institute of Physics,, 2008)
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Abstract:
In this article, we present the spectral and nonlinear optical properties of ZnO–CdS nanocomposites
prepared by colloidal chemical synthesis. The optical band gap (Eg) 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 Eg. 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.