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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Coherent nonlinear optical resonances in II-VI semiconductors

J. M. Hvam, Claus Dörnfeld, C. R. Paton

Published May 1, 1990• 2 citations
Publisher

Abstract

Coherent nonlinear resonances due to extended and localized excitons in CdSe and CdSeS1_ are investigated by picosecond time resolved degenerate four-wave mixing. Large nonlinear coefficients (X(31O_9cm2/V2) are found, with coherence times in the picosecond range. Preliminary results on picosecond optical switching in CdSe are presented, and the possibility of a purely coherent optical switching with picosecond response time is discussed.

DOI
10.1117/12.18121

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Coherent nonlinear resonances due to extended and localized excitons in CdSe and CdSeS1_ are investigated by picosecond time resolved degenerate four-wave mixing. Large nonlinear coefficients (X(31O_9cm2/V2) are found, with coherence times in the picosecond range. Preliminary results on picosecond optical switching in CdSe are presented, and the possibility of a purely coherent optical switching with picosecond response time is discussed.
A picosecond time-resolved four-wave mixing technique is used to study the dephasing of nonlinear optical resonances in CdSe. At low temperature strong coherent, nonlinear signals at the free exciton resonance, at the biexciton resonance, and at the exciton-biexciton transition are observed. The observed correlation traces, i.e. the diffracted intensity versus the delay between the interfering light pulses, yield information on the dephasing process. Dephasing times for excitons as long as 40 ps, decreasing with increasing excitation density and temperature, are observed. A two-level model is applied to describe theoretically the dynamical behavior of the resonances, and to extract their homogeneous broadening, as well as their inhomogeneous broadening.
We have observed high-speed optical switching based on the optical bistability of CdSe at low temperatures due both to induced absorption and to dispersion changes. Using a streak camera to record the response of a thin CdSe platelet to high-intensity dye-laser pulses at near-band-gap energies, we have studied these phenomena on a picosecond time scale and measured switching times of several hundred picoseconds. We attribute our observations to nonlinearities caused by exciton interactions in the semiconductor material. Finally, we discuss the part played in these phenomena by thermal and other dynamic effects.
Picosecond relaxation processes of excitons, including dephasing processes, are systematically studied in CuCl, CdSe and GaAs/AlAs MQW (multi-quantum well structures) by observing time-resolved induced absorption, luminescence and non-degenerate four-wave mixing. By studying the transient induced absorption from the exciton (polariton) state to the excitonic molecule state, time-resolved energy distribution of excitons in CuCl is determined (Fig.1).1) Another technique, i.e. time-resolved luminescence measurement, is applied to excitons in CdSe and GaAs/AlAs MQW (Figs.2 and 3).2,3) By analyzing the above-mentioned experiments the distribution of excitons in the two-dimensional energy-time coordinates is successfully determined. Thus, energy relaxation processes of excitons are clearly visualized. Prior to energy relaxation, coherently generated excitons are considered to be dephased. Phase relaxation of excitons in CuCl is directly measured for the first time in the picosecond time domain by means of time-resolved non-degenerate four-wave mixing (Fig.4).4) These new techniques will be demonstrated and the dephasing as well as energy relaxation mechanism of excitons and excitonic polaritons will be discussed. In the above experiments characteristics of a picosecond tunable optical parametric oscillator pumped by a mode-locked Nd3+:YAG laser as well as a picosecond tunable dye laser synchronously pumped by a mode-locked ion laser are complementarily utilized.
Coherent optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the decay of the spectrally resolved non- linear signal as a function of the delay between the incident pulses in a two-beam TFWM experiment, and from the real time analysis of single speckles in RRS experiments (pure dephasing). From the density- and temperature- dependence of the dephasing times the exciton-exciton and the exciton-phonon interactions are determined. The degree of coherence of the secondary emission is determined from the speckle analysis.