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physica status solidi (b)

Optical Switching Behaviour in Two‐Beam Experiments with CuCl

R. Lévy, J.‐Y. Bigot, Mohammed Frindi, B. Hönerlage

Published Dec 1, 1985• 3 citations
Publisher

Abstract

Abstract An investigation is made of the transmission properties of a test beam through CuCl samples and through Fabry‐Perot etalons containing CuCl under simultaneous excitation of a pump beam. For appropriately chosen photon energies of the two beams, the transmission of the test beam through the sample decreases due to nonlinear absorption, or increases when the sample is in a F.P. resonator. The switching behaviour indicates that the nonlinear absorption and dispersion properties of CuCl depend on the exciton and biexciton population dynamics.

DOI
10.1002/pssb.2221320221

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physica status solidi (b)•Nov 1, 1986•7 citations
Abstract An investigation is made of the transmission properties of a test beam through CuCl samples and through Fabry‐Perot etalons containing CuCl under simultaneous excitation of a pump beam. For appropriately chosen photon energies of the two beams, the transmission of the test beam through the sample decreases due to nonlinear absorption, or increases when the sample is in a F.P. resonator. The switching behaviour indicates that the nonlinear absorption and dispersion properties of CuCl depend on the exciton and biexciton population dynamics.
We study the time variation of the nonlinear absorption due to the creation of biexcitons by a high-intensity nanosecond pulsed laser in CuCl. We show that this variation is different at the beginning and at the end of the laser pulse. This memory effect depends on the photon energy and on the intensity of the laser beam. An explanation based on the time dependence of the population is suggested. We also make a time analysis of a bistable device composed of a sample of CuCl placed in a Fabry-Pérot cavity. The switching times measured are of some hundredths of picoseconds. We stress the importance of the absorption present in the cavity and show that it may destroy the dispersive bistability. Finally, we give some trends of the spatial variation of hysteresis in the transverse section of the laser beam.
The theoretical study of the nonlinear dielectric function of three-level systems has shown a memory effect which depends on the inversion of the exciton and biexciton populations created during the excitation and on the relaxation times of these quasiparticles. This effect is studied at low temperatures (2 K) by measuring the time evolution of the transmission of a beam through a CuCl sample. In addition, it has important consequences on the switching properties of bistable devices using CuCl as the active medium.
We present a thorough investigation of the different relaxation channels opened in a strongly interacting system of excitonic quasiparticles in CuCl for density regimes ranging up to 50 MW/cm2{\mathrm{cm}}^{2}cm2. A spectrally narrow pump beam consisting of 30-ps pulses is tuned around one-half the biexciton energy. It creates an initial distribution of biexcitons through resonant two-photon absorption. A spectrally broad probe beam made of pulses that also have 30-ps duration monitors the excitation-induced variation of the transmission spectrum of CuCl samples near the Z3{\mathit{Z}}_{3}Z3​ exciton resonance. Coherent processes corresponding to parametric emissions of various orders are observed. Absorption and gain bands that reflect the evolution of biexciton and exciton populations on their respective dispersion curves are followed as functions of the time delay between pump and probe pulses.
A comprehensive investigation of the optical properties of excitonic molecules in CuCl films is presented and discussed. The experimental work includes resonant two-photon absorption (TPA), observations of the resonantly excited biexciton emission, and optical pump-and-probe studies. The collision broadening of the TPA implies a collision rate \ensuremath{\sim}${10}^{\ensuremath{-}12}$ sec for the biexcitons, which is sufficiently rapid to establish a quasithermal equilibrium for the particles on a time scale that is short in comparison with the biexciton lifetime and the duration of the laser pump pulse. The biexciton luminescence line shapes obtained with resonant $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}=0$ two-photon excitation have been fitted using a Bose-Einstein thermal distribution of biexcitons. The chemical potential obtained from the fits goes to zero as the biexciton density is increased at a fixed lattice temperature or if the lattice temperature is decreased at fixed density. The sharp luminescence features that are observed at high densities and low temperatures with single-beam, two-photon excitation are interpreted as resulting from a Bose-Einstein---condensed state of the biexcitons at twice the wave vector of the laser pumping photons. Further evidence for a condensed state is obtained from optical-probe measurements in which a small number of probe biexcitons is injected into the sample, with and without the presence of a condensed state produced by an intense laser pump beam. The luminescence of these injected probe biexcitons is modified in the presence of the pump beam such that it exhibits the sharp features of the emission from the condensed state. Measurements of optical gain and stimulated emission in the presence of the pump beam show that the redistribution of the probe luminescence does not result from those processes, but must involve a preferential redistribution of the probe particles in momentum space via collisions. Such a redistribution is expected when additional particles are added to a condensed system of bosons.
We present new experimental results concerning the optical nonlinearities of CuCl microcrystallites embedded in a glass matrix. Using a "pump and probe" technique, the induced absorption is measured in the spectral range of the Z3 exciton. We observe two phenomena: first, a bleaching of the excitonic resonance accompanied by the generation of a new absorption band at higher photon energies when the test probes the sample during the pump pulse; secondly, a weak shift of the excitonic resonance towards higher photon energies when probing the sample during and after the pump excitation. We vary the intensity and the photon energy of the pump beam as well as the delay between the pump and the probe pulses. Thus we can show that the pump beam populates excitonic states, which leads to a complete bleaching of the excitonic absorption. In addition, excitons can be created at higher energies, inducing the new absorption band. Its energy depends on the number of states already occupied in the microcrystallites. The shift of the excitonic resonance, on the contrary, can be explained by the heating of the microcrystallites through this high population of excitons and its non radiative decay during and after the excitation pulse.
Abstract Using nanosecond laser pulses the time behaviour is investigated of degenerate four‐wave mixing close to the biexcitonic resonance in CuCl. The signal emission presents different time evolutions depending on excitation conditions. Its generation rate shows intensity changes and retardation effects. These effects are explained by an attenuation of the pump pulse through a time‐dependent nonlinear absorption and, at the resonance, by a loss of coherence related to collisions with bi‐excitons.