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Le Journal de Physique Colloques

STRONG NONLINEAR OPTICAL RESONANCES IN CdSe WITH PICOSECOND RESPONSE TIME

Claus Dörnfeld, J. M. Hvam

Published Jun 1, 1988• 3 citations
Publisher

Abstract

We have performed coherent light scattering in CdSe at low temperatures and found very strong nonlinear optical resonances of excitonic origin. The coherent nonlinear response time, or dephasing time, is in the picosecond range.
DOI
10.1051/jphyscol:1988248

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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.
The Bose condensation of many exciton system is observed experimentally for the first time. It is found that excitonic molecules in CdSe exhibit the Bose condensation when the excitation is given at 1.8∼4.2 K by pico-second light pulses from a model-locked neodymium glass laser. This is concluded from the experimental facts that (1) an extremely sharp luminescence line attributed to originate from the k =0 condensed excitonic molecules is observed, (2) this sharp line is accompanied by the side-band on the low energy side and the spectrum is well understood by the theory developed by one of the authors, and (3) this line is produced within a limited range of excitation density which is wider at 1.8 K than at 4.2 K. Some arguments are presented on the importance of the use of the picosecond pulse excitation for the realization of the condensation.