COHERENT NUTATION OF EXCITONS, PHOTONS, AND BIEXCITONS IN SEMICONDUCTORS.

A new method is presented for the detection of biexcitons, based on the nutation of coherent excitons, photons, and biexcitons in the M band luminescence region of semiconductors for times shorter than the exciton and biexciton relaxation times. Two branches of polariton-type dispersion relations are found for optical transitions in the M band region; their position and shape depend on the crystal excitation level. It is found that the interconversion frequency from excitons and photons to biexcitons and back varies considerably with the initial particle densities. When the initial exciton and photon densities are the same, this frequency is zero. As the difference between the photon and exciton densities increases in absolute value, the nutation frequency increases as the square root of this difference.