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Quantum Optoelectronics

Radiationless Stimulated Exciton Emission

E. Benson, Emery Fortin, A. Mysyrowicz

Published Jan 1, 1997• 0 citations
Publisher

Abstract

Experiments investigating the process of Bose-Einstein condensation of excitons in Cu2O will be reviewed. The exciton flux reaching the back surface of mm thick single crystals is investigated following creation of a high density of excitons close to the front surface with a pulsed laser. An anomalous propagation regime is observed at. sufficiently low crystal temperatures and sufficiently high particle densities, in the form of a ballistic packet progressing at a velocity close to the crystal sound velocity. The ballistic packet is attributed to an excitonic superfluid, the result of a Bose-Einstein Condensation (BEC). Conditions for appearance of the ballistic excitonic transport are found to be in good agreement with the predictions of the weakly interacting Bose gas model.

DOI
10.1364/qo.1997.qthd.1

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Experiments investigating the process of Bose-Einstein condensation of excitons in Cu2O will be reviewed. The exciton flux reaching the back surface of mm thick single crystals is investigated following creation of a high density of excitons close to the front surface with a pulsed laser. An anomalous propagation regime is observed at. sufficiently low crystal temperatures and sufficiently high particle densities, in the form of a ballistic packet progressing at a velocity close to the crystal sound velocity. The ballistic packet is attributed to an excitonic superfluid, the result of a Bose-Einstein Condensation (BEC). Conditions for appearance of the ballistic excitonic transport are found to be in good agreement with the predictions of the weakly interacting Bose gas model.
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Pulsed-laser excitation of naturally grown crystals of Cu2{\mathrm{Cu}}_{2}Cu2​O produces a dense gas of excitons, which exhibits Bose-Einstein quantum statistics. Both orthoexcitons and paraexcitons, split by a 12-meV electron-hole exchange energy, are formed. At high excitation levels, the time-resolved luminescence of orthoexcitons yields a density-temperature relation that follows the phase boundary for Bose-Einstein condensation, n=CT3/2{\mathit{CT}}^{3/2}CT3/2, but no striking evidence for a condensed fraction is observed. We have now measured the time-resolved spectrum of the lower-lying paraexcitons whose luminescence is 1500 times weaker than the orthoexciton luminescence. At intermediate power levels, the paraexciton distribution is more degenerate than the orthoexcitons. The relative intensities of the two species indicate that at high excitation levels the paraexciton density exceeds the critical density for condensation. At these high densities the paraexciton spectrum develops a structure that cannot be explained in terms of the thermalized Bose-Einstein distribution. Time-resolved spatial measurements of the luminescence indicate a rapid nondiffusive expansion of the gas at supersonic velocities that approach the expected ballistic velocities of the excitons. The rapid transport and the concurrent changes in the spectral distribution of paraexcitons may be the first observation of excitonic Bose-Einstein condensation and superfluidity.
Experimental results are presented on excitonic effects related to the Bose-Einstein condensation (BEC) of excitons in Cu2O. Such effects include resonant light polarization dependent absorption, exciton creation and diffusion by absorption of scattered light, and resonant high density exciton packet induced absorption. These results are then utilized, in conjunction with similar results on excitonic BEC, to develop a plausible model for condensate amplification. Condensates are initially created at T = 1.8K with high intensity ND:YAG pulses (lambda = 532 nm) and then stimulated by a secondary source tuned around the 1S orthoexciton resonance (lambda = 609.51 nm); the latter induces the observed condensate amplification. By careful analysis of the various interactions between the secondary light source, the crystal, and the condensate itself, an improved understanding of how the secondary pulse excitations interact with the passage of a high density exciton packet is obtained.
Experiments on Bose-Einstein condensation (BEC) of excitons in the semiconductor Cu2O started over 30 years ago, as one of the first serious attempts at exciton BEC. Early claims were based on spectroscopic signatures and transport data which have since been reinterpreted, in large part because the Auger recombination process for excitons was not well understood. Understanding of the Auger process has advanced, and recent experiments have made significant progress toward exciton BEC. We review the history of experiments on exciton BEC in Cu2O, the Auger recombination process, and the prospects for observing exciton BEC in this system in the near future.
In a first part, a brief review of some excitonic properties is presented, with the accent put on their relevance with respect to the possibility of Bose-Einstein condensation. In a second part, recent experimental evidence is described, which supports the idea that excitonic particles may form a highly quantum fluid. In Cu20, the analysis of the exciton decay spectrum shows a gradual evolution of the gas from a classical regime at low densities up to a strongly degenerate one at high densities, with a chemical potential µ ~ 0. In CuCl excitons are unstable against formation of molecules (biexcitons). It is possible to generate directly a high density gas of biexcitons with momentum K ~ 0 by giant two-photon absorption. At low excitation, the observed molecular emission is in agreement with Maxwell -Boltzmann statistics. At high densities, strong deviations occur. In particular, the appearance of a sharp emission line is attributed to the presence of a Bose-Einstein condensate of excitonic molecules.