Terahertz Luminescence Induced by Intra‐Exciton Radiative Transitions in Silicon

Abstract

This article reports the results of experimental investigations of a new type of terahertz emission from semiconductors. This emission is due to radiative transitions between energy levels of free excitons. The intra‐exciton radiative transitions arise during the binding of nonequilibrium electrons and holes into exciton states. The experiments were carried out on high‐purity Si crystals at helium temperatures in the conditions of interband photoexcitation. The excitation densities were in the range of 0.1 to 140 W cm−2. Terahertz luminescence spectra demonstrate narrow lines of optical transitions between the energy levels of free excitons. The intra‐exciton optical transitions dominate the emission spectrum under conditions of both low excitation densities and at excitation densities well above the electron–hole liquid formation threshold. The intra‐exciton terahertz emission intensity becomes superlinear as a function of the excitation density at a high level of the photoexcitation and temperatures above 20–25 K. This experimental observation could be explained by stimulated optical transitions between the energy levels of free excitons, which presumably appear under intense interband pumping.