Radhakrishnan, P; V P N Nampoori; Litty, Irimpan(Science of Advanced Materials , American Scientific Publishers, 2010)
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Abstract:
The spectral and nonlinear optical properties of ZnO based nanocomposites prepared by colloidal
chemical synthesis are investigated. Very strong UV emissions are observed from ZnO–Ag, ZnO–
Cu and ZnO–SiO2 nanocomposites. The strongest visible emission of a typical ZnO–Cu nanocomposite
is over ten times stronger than that of pure Cu due to transition from deep donor level to the
copper induced level. The optical band gap of ZnO–CdS and ZnO–TiO2 nanocomposites is tunable
and emission peaks changes almost in proportion to changes in band gap. Nonlinear optical
response of these nanocomposites is studied using nanosecond laser pulses from a tunable laser in
the wavelength range of 450–650 nm at resonance and off-resonance wavelengths. The nonlinear
response is wavelength dependent and switching from RSA to SA has been observed at resonant
wavelengths. Such a change-over is related to the interplay of plasmon/exciton band bleach and
optical limiting mechanisms. The observed nonlinear absorption is explained through two photon
absorption followed by weak free carrier absoption, interband absorption and nonlinear scattering
mechanisms. The nonlinearity of the silica colloid is low and its nonlinear response can be improved
by making composites with ZnO and ZnO–TiO2. The increase of the third-order nonlinearity in the
composites can be attributed to the enhancement of exciton oscillator strength. This study is important
in identifying the spectral range and the composition over which the nonlinear material acts
as an RSA based optical limiter. These nanocomposites can be used as optical limiters and are
potential materials for the light emission and for the development of nonlinear optical devices with
a relatively small limiting threshold.