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We confirm the existence of a biexciton in ZnSe-based quantum wells using transient four-wave-mixing spectroscopy. It is found that a new spectral line appears below the heavy-hole-related exciton resonance, corresponding to the exciton-biexciton transition. Through the analysis, we have determined the dephasing time of the biexciton, as well as the biexciton binding energy.

An experiment is described which aims at detecting the coherency associated with the presence of a Bose condensate of biexcitons in CuCl. The optical phase-conjugation signal at the two-photon resonance of theK = 0 biexciton is detected in the presence and in the absence of a high density pool of randomly injected biexcitons. An increase in the magnitude of the phase-conjugate signal together with a small blue shift of the biexciton resonance are attributed to the Bose condensate.

Coherent propagation of quadrupole polaritons in Cu2O is demonstrated by time resolved spectroscopy. Clearly resolved oscillations with time-dependent period are observed. The experimental results are quantitatively explained as resulting from quantum beats between the two branches of the exciton-polariton. The analysis yields the homogeneous line width of 6.5x10−7 eV and an oscillator strength of f=3.6x10−9. Measurements at intensities higher than 108 W/cm2 lead to faster decay and a clear reduction of the beat amplitude.

We have photographed the emission spot due to electron-hole liquid recombination in GaP and examined the picture under strong magnification (x150). Under our excitation conditions, a single large drop is formed, with a penetration depth inside the crystal l ≈ 80 μm well exceeding the penetration depth of carriers under low excitation (⩽10 μm ).

The Mott transition in CuCl from a metallic phase of free electrons and holes towards an insulating phase of bound particles (excitons or biexcitons) has been studied by time-resolved luminescence with a resolution slightly better than 1 ps. The phase change takes place in a very short but finite time (about 3 ps) at a carrier density N ⋍ 10 19 cm -3 .