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Topical Meeting on Optical Bistability (OB)

Study of Phase Coherence Times in CuCl

M.J.M. Gomes, Bernard J. Kippelen, B. Hönerlage, R. Lévy

Published Jan 1, 1988• 0 citations
Publisher

Abstract

We perform time-resolved degenerated four-wave mixing experiments on thin CuCl platelets (thickness of about 30 μm) and epitaxially grown layers (thickness of 5 to 10 μm) at low temperature (2K). The light source is a tunable picosecond dye-laser of 20 ps duration (FWHM) and of 0.3 meV spectral width (FWHM). It is synchronously pumped by the third harmonics of a mode-locked Nd:YAG laser. The dye-laser beam is split into two parts (pump and test) with an intensity ratio of about 8 to 1. The optical delay between both pulses is varied.

DOI
10.1364/obi.1988.wf.5

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Journal of the Optical Society of America B•Nov 1, 1994•1 citation
We perform time-resolved degenerated four-wave mixing experiments on thin CuCl platelets (thickness of about 30 μm) and epitaxially grown layers (thickness of 5 to 10 μm) at low temperature (2K). The light source is a tunable picosecond dye-laser of 20 ps duration (FWHM) and of 0.3 meV spectral width (FWHM). It is synchronously pumped by the third harmonics of a mode-locked Nd:YAG laser. The dye-laser beam is split into two parts (pump and test) with an intensity ratio of about 8 to 1. The optical delay between both pulses is varied.
We report experiments on CuCl at 2 K with temporal, spectral and spatial resolution. The samples used are either vapor phase grown platelets or polycristalline films deposed on silicon substrates. They are excited by a nanosecond pulsed dye laser whose transmission is analysed temporally using a streak camera with picosecond resolution. The photon energy of the laser is varied around half the biexciton energy [1]. The spatial resolution is achieved using films of 2,7, 10 and 13 μm and platelets of about 30 μm thicknesses. We are thus able to study the nonlinear propagation of pulses for different wavelengths and intensities, through samples of different thicknesses.
Transient four-wave-mixing experiments reveal that the phase relaxation time T2{T}_{2}T2​ of an exciton in CuCl spherical quantum dots embedded in glass is very long at low temperatures, e.g., 130 ps ($\ensuremath{\approx}10\ensuremath{\mu}\mathrm{eV}$ in homogeneous spectral width) at 2 K. The measurement at very weak pumping power enables us to extract the intrinsic value of T2,{T}_{2},T2​, typically two orders of magnitude longer than the reported ones. The T2{T}_{2}T2​ value is substantially governed by the energy relaxation time T1{T}_{1}T1​ at 2 K and then becomes shorter with increasing temperature. The time-integrated signal behaves such that the temperature dependence is found to be analogous to that in a system of dye molecules in a polymer.
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.
Abstract Using a picosecond light source and a streak camera as detection system, the temporal evolution of the signal generated throughout degenerate four‐wave mixing (DFWM) experiments is analysed in CuCl at low temperature. The signal emission shows different temporal behaviour depending on whether the test pulse reaches the surface of the sample before, during, or after the pump pulse. This enables one to distinguish the light‐induced grating contribution from the induced decay of quasi‐particles. The latter process can then be used to determine their dephasing time.
Abstract We present a study of optical phase conjugation through degenerate four-wave mixing in CuCl, as a function of light frequency, intensity, polarization and sample temperature. The phase-conjugate signal exhibits two maxima at the frequencies of the T5 exciton (one-photon resonance) and of the biexciton (two-photon resonance). By appropriate combinations of the input beam polarizations, it is possible to single out the various terms contributing to the signal (small-spaced, large-spaced population grating and two-photon coherent excitation). The absolute value of X(3) responsible for the two-photon resonance has been determined ( x(3) (-w,w,w, -w) = 3.10-7 e.s.u.). It leads to ‘mirror’ efficiencies of the order of 10% for input pump intensities I0 » 105 W cm -2 and sample thickness d» 10-4 cm. At higher I0, a saturation of the reflection takes place. The variation of the conjugate beam with temperature has been studied in the range 15 K < T <70 K. Finally, the phase-conjugate nature of the signal has been verified by inserting an aberrator in the path of the input probe beam, and by checking the reconstruction of the reflected beam.
We present a time-resolved study of the diffracted pulse in degenerate four-wave mixing experiments with short-response-time materials. The first step is the validation of a theoretical model that takes into account the temporal characteristics of the energy level (relaxation time T1 and dephasing time T2) and of the incident pulse (duration and spectral phase) in a simple particular case. Two methods are used for characterizing the diffracted beam: the amplitude correlation with the reference pulse and the spectrum intensity measurements. We discuss the experimental results, which show good agreement with the theoretical model and evidence the strong influence of the incident pulse characteristics.