Z-SCAN STUDIES ON ZNSE, ZNS AND GAP

Abstract

We report the measurements of the nonlinear optical properties of direct-band-gap semiconductors ZnSe, ZnS and indirect-band-gap semiconductor GaP using Z-scan technique. The nonlinear optical properties include third-order effects of two-photon absorption and bound-electronic refraction, and fifth-order effects of free-carrier absorption and refractive index change due to the generation of photo-excited free carriers. The relaxation times of photo-excited free carriers are also determined. Both nanosecond and picosecond laser pulses are used for conducting Z-scan experiments. All the results are obtained by analyzing our experimental data through computer simulation. We begin in Chapter I with a brief introduction to nonlinear optical materials. A review of past works studying the nonlinear optical properties of these materials is presented. The purpose of our research work and the outline of the thesis are also given. Chapter 2 concentrates on the details of the Z-scan technique that we used for studying the optical nonlinearities in ZnSe, ZnS and GaP. We describe our experimental set-up and present the theoretical analysis. Chapter 3 presents our study on the nonlinear optical properties of ZnSe and ZnS. We first present a new solution to solve the equations involved in optical nonlinearities in iv semiconductors, then use ZnSe as a standard sample to testify the reliability of our Z-scan system and analysis. The nonlinear optical properties of ZnSe obtained by us are compared with the published results by other research groups. Finally we report our results for ZnS. In addition to measuring the free-carrier absorption cross section and refractive index change due to the excited carriers at 532 nm, we use nanosecond and picosecond lasers together to determine the relaxation times of two-photon-excited free carriers in ZnSe and ZnS at 532 nm. Chapter 4 details our studies on optical nonlinearities in GaP. We first measure the linear absorption by using a spectrophotometer and analyze the linear refraction by using literature survey. The optical nonlinearities are investigated by using nanosecond laser pulses. We determine the free-carrier absorption cross section from 532 to 650 nm. We also obtain the change in the refractive index due to the generation of an electron-hole pair. Chapter 5 summarizes all the important experimental findings and the conclusions.