Generation kinetics of nonequilibrium charge carriers in crystals with deep impurities involving two-center transitions between band and impurity states

A new and efficient mechanism of nonlinear photoexcitation of a transparent crystal with deep impurity centers is proposed. It is hypothesized that the energy of a quantum of light is smaller than the energy gap between the bottom of the conduction band and the impurity level, but is larger than the gap between the impurity level and the top of the valence band. The kinetics of nonlinear cascade generation of nonequilibrium charge carriers is considered taking into account two-center processes in which the energy transfer and the photon absorption occur in one and the same elementary event. The dependences of the quasi-equilibrium concentrations of nonequilibrium charge carriers in the bands and of the occupancy of impurity states on the laser-radiation intensity are obtained. It is shown that the generation process of nonequilibrium electron–hole pairs is of a threshold nature. Depending on the concentration of impurity centers, the threshold intensities can be $\sim$ 10$^{5}$–10$^{7}$ W/cm$^{2}$, while the setting time of the quasi-equilibrium occupancies of electronic states is $\sim$ 10–0.1 ns.