Picosecond dynamics of degenerate orthoexcitons inCu2O

Previous time-resolved luminescence studies of orthoexcitons in ${\mathrm{Cu}}_2$O have shown a ‘‘quantum saturation’’ behavior. Basically, it was found that for 10-ns laser pulses, which are comparable to the exciton lifetime, the exciton-gas temperature rises along the phase boundary for Bose-Einstein condensation as the density is increased. The microscopic processes that lead to such an unusual effect remained unresolved. To isolate the various kinetic processes, we have now conducted a study using 100-ps pulses, which are short compared with the orthoexciton lifetime. Combined with an increased time resolution, this short-pulse excitation has allowed us to observe some of the intrinsic rates of thermalization and decay of the orthoexcitons. At low-to-intermediate excitation levels, the decay of the gas temperature is found to agree with a model of acoustic-phonon emission, and the deduced exciton-phonon coupling is in basic agreement with that obtained from diffusion experiments. Another principal result is that the decay time of the gas temperature is significantly increased with increasing degeneracy, i.e., at high excitation level. After the short excitation pulse the orthoexcitons follow the Bose-Einstein phase boundary in density and temperature in a way similar to their behavior during the ‘‘steady-state’’ conditions of the long-pulse excitation. We conclude that the temperature rise is not simply due to insufficient thermalization caused by a shortening of the exciton lifetime; at high density there must be a heating mechanism such as Auger recombination.