Abstract

We report on a new approach to the study of Bose-Einstein condensation of biexcitons in semiconductors. Instead of relying on a spectral analysis of the molecular luminescence,^[1] we apply nonlinear optical methods to directly address an essential attribute of a Bose condensate, namely its coherency. The temporal and spatial coherency of the ground state of the molecular fluid can be probed by monitoring the fully degenerate phase conjugate signal at the peak of the giant two-photon resonance of the biexciton. In the experiment, we measure the phase conjugate signal in selected high quality CuCl single crystals held at 2 K when the crystal is simultaneously pumped with an intense incoherent source tuned to the free exciton absorption band. Normally, injection of additional random particles results in the destruction or deterioration of a prepared coherent excitonic state. Here the opposite behavior occurs. Instead of a decrease, we observe an increase of the coherent signal under incoherent pumping (see Fig. 1). This surprising effect can be understood if one assumes that a Bose condensation of biexcitons takes place in the crystal. All particles exceeding the critical density n_c(T) collapse into the ground state K = 0, resulting into an increase of the spatially uniform coherent oscillation at E = 2hω.

© 1992 IQEC