Multiphoton generation of electron-hole pairs involving free carriers in an indirect-gap crystal

The paper reports on a study of nonlinear absorption of high-intensity laser radiation in the case where the photon energy is less than one half of the indirect-gap width in a crystal. The deficiency in the energy needed for two-photon excitation of the electron-hole pair is made up by the kinetic energy of free electrons, which was acquired in intraband nonlinear absorption of light. The probabilities of Auger-type indirect two-photon interband transitions involving free electrons have been calculated by perturbation theory. It is shown that, for a free-carrier concentration in the conduction band n _c ? 10¹⁵ cm^?3, and the radiation intensity range of interest for the experiment, j ～ 3–10 GW/cm², the calculated probabilities of such transitions exceed by several orders of magnitude those of “conventional” direct and indirect (involving phonons) multiphoton transitions which can take place in the system considered.