GIANT NONLINEAR LOSSES IN GASE CRYSTALS UNDER DOUBLE 2-PHOTON RESONANT CONDITIONS - VALENCE BAND-STRUCTURE FROM INDUCED FREE CARRIER ABSORPTION

The results of spectral investigation of photo-induced absorption in GaSe crystals under two-photon laser excitation are presented. A sharp increase of the coherent nonlinear losses is observed at the frequencies of probe light near fundamental absorption edge, and is explained by the appearance of the double band resonance effect for the interband two-photon absorption (TPA) with transitions between a remote valence band and a lower conduction band, when the upper valence band is in a quasiresonance with both absorbed quanta. The spectral features of this resonance are influenced by Fano effects due to configurational interaction of the spectrally overlapping contributions of resonance and continuum parts of TPA. A long-wave spectra part of 1.1-1.8 eV is formed by a selective inter-valence-subband absorption by nonequilibrium free carriers generated by TPA of laser radiation, which permits to reconstruct the detail model of GaSe valence band structure and to confirm the conditions for the double resonance of TPA.