Specific mechanisms of bulk and surface absorption of ultrashort mid-infrared laser pulses in optical materials

Low photon energy of mid-infrared radiation supports generation of non-equilibrium free-carrier plasma in transparent optical materials via nonlinear absorption and associated inter-band electron excitation. The collision rate of the plasma may be low so that the free carriers experience from zero to very few collisions per electron per pulse for pulses shorter than 200 fs. Those features of the free-carrier plasma support specific mechanisms of light absorption. We consider solid-state analogs of the Brunel absorption at interfaces, Landau damping, and nonlinear absorption by free-carrier stimulated inverse bremsstrahlung. Parametric scaling of those mechanisms is compared against that of the traditional models.