Spectral and nonlinear optical characteristics of nanocomposites of ZnO–CdS

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.

Nonlinear optical characteristics of nanocomposites of ZnO–TiO2–SiO2

“…There is a change in absorption with the ZnO content and as the volume fraction of ZnO increases, the excitonic peak exhibits its signature. It is seen that the absorption edge corresponding to the nanocomposites gets red shifted and the exciton oscillator strength increases as a function of the ZnO content consistent with published reports [11,12]. The structure and size evolution of the nanocomposites may also have some relation with optical characteristics in addition to the composition and the study is in progress.…”

Reverse saturable absorption in nano colloidal Ge28Sb12Se60 chalcogenide glass

“…Larger particles have better optical limiting performance. The reverse saturable absorption, which is generally associated with a large cross section of absorption from excited levels, brings about optical limiting effects in colloidal solutions [22]. In this case, the optical limiting is due to two-photon absorption.…”

Synthesis of ZnO/titanate nanocomposites with highly photocatalytic activity under visible light irradiation

“…The band gap energy for the titanate nanotube was calculated to be 3.84 eV, which is closer to that of 3.87 eV of TiO 2 nanotubes reported by Bavykin et al [27]. Irimpan et al have reported that the absorption spectra of CdS shifted towards higher energies with respect to its bulk material in the CdS/ZnO nanocomposites [28]. Wang and co-workers also observed a blue shift of the absorption band edges of CdS nanoparticles after coupled with titanate nanotubes and ascribed it to the quantum size effect [20].…”