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physica status solidi (b)

Optical tracking of high‐density cooled excitons in potential traps in Cu2O

N. Naka, N. Nagasawa

Published Jul 21, 2003• 21 citations
Publisher

Abstract

Abstract The density distribution of cold exciton clouds generated into a strain‐induced potential well by two‐photon excitation in Cu2O is studied at 2 K. The potential shapes of the double well are examined by two‐photon diagnostics. The motion of the exciton clouds is tracked by time‐resolved luminescence imaging and spectral imaging. We find that an anomalous spike appears on the center of the well in a specific excitation condition, i.e. ΔE = −5.0 meV, where ΔE is the detuning of the two‐photon excitation energy from the orthoexciton level under zero stress. The origin of the spike has been discussed in view of bosonic stimulation of the excitons into the ground state.

DOI
10.1002/pssb.200303152

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Abstract The density distribution of cold exciton clouds generated into a strain‐induced potential well by two‐photon excitation in Cu2O is studied at 2 K. The potential shapes of the double well are examined by two‐photon diagnostics. The motion of the exciton clouds is tracked by time‐resolved luminescence imaging and spectral imaging. We find that an anomalous spike appears on the center of the well in a specific excitation condition, i.e. ΔE = −5.0 meV, where ΔE is the detuning of the two‐photon excitation energy from the orthoexciton level under zero stress. The origin of the spike has been discussed in view of bosonic stimulation of the excitons into the ground state.
There is a 40-year long history in the search for Bose-Einstein condensation (BEC) of excitons in semiconductors. This thesis presents research directed toward this goal in bulk crystal Cu2O in three dimensions and in GaAs-based coupled quantum wells (CQW) in two dimensions.The Auger recombination process in Cu2O plays a major role in limiting the density of the excitons. We find that the rate for this process increases with applied stress and lattice temperature. We create paraexcitons in Cu2O through a resonant two-photon excitation in a harmonic potential trap with the Auger recombination process as small as possible (at low temperature and low stress), and find that the exciton creation efficiency in the resonant two-photon excitation is greater for one-beam excitation than for two colliding pulses, but the colliding pulse method may be useful for direct creation of a condensate in the ground state. The paraexciton density in this work is about thirty times less than the required density for BEC of paraexcitons. One promising direction for BEC of excitons in Cu2O is that with higher laser power from stronger IR laser sources, or at lower temperature, the critical density can be approached under one-beam two-photon excitation resonant with the paraexciton state.In two dimensions, the CQW structure has been modified with four design strategies: highest possible barriers, introducing into the barriers a superlattice of 60 angstrom GaAs wells, p-i-n doping, and wider quantum wells, which provides indirect excitons low disorder and high mobility. With a cold near-resonant excitation, we conclude that the excitons act as a free gas, travelling distances of hundreds of microns. We also present observations of a narrow beam of emitted light, when the indirect excitons are confined in a two-dimensional harmonic potential trap, in a way quite similar to the first observations of BEC in alkali atoms. A beam-like emission has been suggested as a telltale for BEC of excitons. This opens the door to a whole range of investigations, including attempts to observe coherence of the emitted light, proof of superfludity of the excitons, and other fascinating effects.
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.
We demonstrate absorption imaging of 1s1s1s paraexcitons in a strain-induced trap potential in bulk semiconductor Cu2O{\mathrm{Cu}}_{2}\mathrm{O}Cu2​O. Using excitonic $1s\text{\ensuremath{-}}2p$ induced absorption in the midinfrared region, clear images of exciton ensembles are taken at liquid-helium temperatures. The observed spatial spread and redshift of the $1s\text{\ensuremath{-}}2p$ resonance prove that the images are of a thermalized gas of trapped 1s1s1s paraexcitons. With its high sensitivity, midinfrared absorption imaging is especially important for characterizing quantum degenerate paraexcitons including Bose-Einstein condensates at near 100 mK temperatures, as they do not luminesce because of the principle of momentum conservation.
Various optical spectra such as time-resolved luminescence spectra are studied for excitonic molecules in CuCl under two-photon resonance excitation by using picosecond and nanosecond frequency-tunable laser pulses. Two sharp emission lines observed under resonance excitation are ascribed to the radiative decay of “cold” excitonic molecules created directly at k ∼2 k 0 , while the broad emission band on the low energy side of the sharp line to the luminescence of “hot” molecules created through single excitons. The stimulated emission process gives a considerable influence on both the sharp luminescence lines and the hot molecule band, and further on two-photon resonant excitonic Raman lines as well. Discussions are made on the Bose condensation of the excitonic molecules and also on whether the emission under just-resonance excitation is due to Raman scattering or luminescence.
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