Stimulated terahertz emission in a system of excitons in photoexcited silicon

Stimulated terahertz emission owing to population inversion between exciton states in silicon crystals at intense interband photoexcitation is detected. The terahertz gain spectrum exhibits 13.7- and 15.5-meV lines, the gains of which reach 0.5 and 1 cm$^{-1}$, respectively. The 13.7-meV line is due to the population inversion between highly excited states and the ground state of free excitons. The 15.5-meV line can be associated with the population inversion between the two-exciton and biexciton states. The terahertz gain values make it possible to expect the possibility of creating a new type of a terahertz laser on transitions between free excitons in silicon under interband photoexcitation.