Abstract
Terahertz photoluminescence from silicon crystals due to radiative transitions between the energy levels of free excitons has been studied up to intensities of interband photoexcitation significantly exceeding the threshold for the formation of electron-hole liquid droplets. At temperatures above 20 K and a photoexcitation density in excess of 7 W/cm2, the appearance of stimulated intraexciton terahertz radiation was clearly demonstrated. Terahertz gain spectrum was obtained. The gain spectrum indicates that population inversion arises between the highly excited states of excitons and sublevels of the ground state of free excitons, as well as possibly between two-exciton and biexciton states. The performed calculations of the times of energy relaxation of exciton states show the fundamental possibility of the appearance of population inversion between the states of excitons in the process of their energy relaxation.
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