Strong optical nonlinearities in gallium and indium selenides related to inter-valence-band transitions induced by light pulses

Abstract

A nonlinear optical effect is shown to occur in gallium and indium selenides at photon energies of the order of 1.5 eV. It corresponds to transitions from a lower-energy valence band to the uppermost one when a nonequilibrium degenerate hole gas is created in the latter by a laser pulse. This inter-valence-band transition is allowed by crystal symmetry. Its oscillator strength is estimated through the $f$-sum rule and turns out to be about two orders of magnitude higher than that of the fundamental transition. The intensity of this effect is stronger when the pump pulse photon energy is close to that of the inter-valence-band transition; a condition that can be fulfilled only in indium selenide. The transient behavior of the sample transmittance is shown to be controlled by the balance between absorption and stimulated emission, which depends on the hole quasi-Fermi level and the gap renormalization due to Coulomb interaction in the electron-hole gas generated by the pump.