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physica status solidi (b)

Optical Gain and Induced Absorption in High‐Density Exciton System in CuCl

J. M. Hvam, Albert Bivas

Published Sep 1, 1980• 7 citations
Publisher

Abstract

Abstract In CuCl at 1.7 K optical gain/absorption spectra are recorded in the spectral range of bound exciton and biexciton emission. Energy, effective temperature, and oscillator strength of biexcitons are determined. The observed kinetics indicates that the total pair density in the exciton‐biexciton system is pinned at about 1.6 × 1016 cm−3 for all excitation densities above 10 kW/cm2. At intense excitation, a Fano‐interference is observed between a discrete impurity absorption line and a continuum absorption tail. The latter is followed by a band of positive gain towards lower photon energies.

DOI
10.1002/pssb.2221010138

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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.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Dec 22, 1989•4 citations
Abstract In CuCl at 1.7 K optical gain/absorption spectra are recorded in the spectral range of bound exciton and biexciton emission. Energy, effective temperature, and oscillator strength of biexcitons are determined. The observed kinetics indicates that the total pair density in the exciton‐biexciton system is pinned at about 1.6 × 1016 cm−3 for all excitation densities above 10 kW/cm2. At intense excitation, a Fano‐interference is observed between a discrete impurity absorption line and a continuum absorption tail. The latter is followed by a band of positive gain towards lower photon energies.
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.
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 measured the optical transitions due to the internal energy levels of an exciton and biexciton in a CuCl single crystal using pump-probe spectroscopy. The transient absorption bands due to the transitions from the 1s1s1s to 2p2p2p and from the 1s1s1s to 3p3p3p exciton states were observed at 3 K, which is consistent with their reported energies. Simultaneously, the transient absorption peak due to the biexciton was observed, which corresponds to a transition from the lowest state (composed of two 1s1s1s excitons) to higher states (composed of 1s1s1s and 2p2p2p excitons). The value of the observed biexciton peak energy was reasonable considering the hydrogen molecule orbitals and the electron-to-hole effective mass ratio. In addition, the transient absorption peaks were broadened at 77 K, because of the increase in the homogeneous width of the 2p2p2p exciton state. The transient absorption spectrum was almost completely determined by this width. Our findings are of importance with regard to the optical phenomena in the infrared region related to the quantum coherence of excitons and biexcitons in semiconductors.
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.