Dynamics of two-photon picosecond absorption in ZnWO4 and PbWO4 crystals

A method has been proposed to analyze the dynamics of interband two-photon absorption in a nonlinear medium excited by a sequence of picosecond laser pulses of variable intensity and continuous probe radiation. Induced absorption leading both to hysteresis in the dependence of the absorption on the intensity of laser pump radiation and to the opacity of crystals at the pump wavelength has been revealed in initially transparent ZnWO₄ and PbWO₄ crystals irradiated by a train of 523.5-nm pulses with a duration of 20 ps at pump intensities of 5 to 140 GW/cm². The kinetics of an increase in absorption and its subsequent relaxation at a 523.5-nm picosecond excitation of the crystals have been measured with continuous 633-nm probe radiation. An exponential component of the increase in absorption with the time constant τ = 2−3.5 and 8–9.5 μs depending on the direction of the linear polarization of pump radiation has been revealed at 300 K in ZnWO₄ and PbWO₄ crystals, respectively. The absorption relaxation kinetics in the crystals are complicated and approach an exponential at a late stage with the constant τ = 40−130 and 12–80 ms for the ZnWO₄ and PbWO₄ crystals, respectively.