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Physical review. B, Condensed matter

Time-resolved exciton-biexciton transitions in CuCl

R. Lévy, B. Hönerlage, J. B. Grün

Published Feb 15, 1979• 40 citations
Publisher

Abstract

The time-resolved induced absorption due to exciton-biexciton transitions is observed in CuCl, using an intense ultraviolet picosecond pulse excitation and a picosecond continuum as a probe beam. In the same experimental conditions, time-resolved luminescence is also recorded. We have obtained information on the time evolution of the exciton and biexciton populations. Studying the dynamic equations of the system, we have determined the radiative lifetimes of biexcitons and transverse excitons.

DOI
10.1103/physrevb.19.2326

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Time-resolved investigations of induced absorption due to exciton-biexciton transitions in CuCl

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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.

Physical review. B, Condensed matter•Feb 15, 1983•81 citations
The time-resolved induced absorption due to exciton-biexciton transitions is observed in CuCl, using an intense ultraviolet picosecond pulse excitation and a picosecond continuum as a probe beam. In the same experimental conditions, time-resolved luminescence is also recorded. We have obtained information on the time evolution of the exciton and biexciton populations. Studying the dynamic equations of the system, we have determined the radiative lifetimes of biexcitons and transverse excitons.
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.
Abstract The induced absorption due to exciton‐biexciton transitions is observed in CuCl with an experimental set‐up using two dye lasers. The shape of the absorption band is studied. The oscillator strength per exciton of this absorption is estimated and compared to theoretical evaluations. The excitation spectrum of this induced absorption is also measured and its features are tentatively explained by the existence of a superficial layer on the samples, where excitons have a very short lifetime.
Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.
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.
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.