MANIFESTATION OF EXCITON BOSE-CONDENSATION IN INDUCED 2-PHONON EMISSION AND RAMAN-SCATTERING

The unusual two-photon emission by Bose-condensed excitons caused by s imultaneous recombination of two excitons with opposite momenta leavin g the occupation numbers of excitonic states with momenta p not equal 0 unchanged: (below coherent two-exciton recombination) is investigate d. Raman scattering accompanied by the analogous two-exciton recombina tion (or creation) is also analyzed. The excess momentum equal to the change of the electromagnetic field momentum in these processes can be transferred to phonons or impurities. The processes under considerati on take place if there is Bose condensation in the interacting exciton system, and, therefore, can be used as a method to reveal exciton Bos e condensation. If the frequency of the incident light omega<2 Omega ( Omega is the frequency corresponding to the recombination of an excito n with p=0), the coherent two-exciton recombination with the excess mo mentum elastically transferred to impurities leads to the appearance o f the spectral line 2 Omega - omega corresponding to the induced two-p hoton emission. In this case the anti-Stokes line on frequency omega+2 Omega also appears in the Raman spectrum. If omega>2 Omega, there are both Stokes and anti-Stokes lines on frequencies omega+/-2 Omega in t he Raman spectrum. The induced two-photon emission is impossible in th is case. The spectral Lines mentioned above have phonon replicas on fr equencies \omega +/-(2 Omega-n omega(0)(s))\ corresponding to the tran smission of the excess momentum (partially or as a whole) to optical p honons of frequency omega(0)(s) (n is an integer number). The quantita tive estimation shows that the light corresponding to the coherent two -exciton recombination can be experimentally observed in Cu2O.