On the mechanism of absorption and recovery of radiation transmission in the channel of small-scale self-focusing of short laser pulse in neodymium glass

When diagnosing a small-scale self-focusing (SSSF) channel of a 0.5-ns laser pulse with an intensity of 3–5 GW/cm$^2$ in neodymium glasses, an absorption jump was detected (with a front of $\approx$ 0.5 ns) at wavelengths of 1.06 and 0.66 $\mu$m in addition to filament-like damage typical of SSSF, spectral broadening, and laser radiation scattering. The absorption coefficient reached 0.15 cm$^{-1}$, and transmission in the medium was restored during 15–35 ns for phosphate glass and 5–10 ns for silicate glass. The physical bases of this effect of the appearance and relaxation of absorption in glasses have not been previously presented. An analysis of the experimental data showed that the absorption jump is due to the fast population of the $^{4}I_{11/2}$ level of Nd$^{3+}$ ions in glasses when nonlinear processes occur in the SSSF channel such as stimulated Raman scattering and broadening of the laser pulse spectrum. The relaxation time of the population at the $^{4}I_{11/2}$ level and the restoration of transmission in glasses after the termination of the laser pulse are determined by the characteristic sizes of the Nd$^{3+}$ ions excitation regions arising in the samples upon interference of the laser and scattered radiation, as well as the thermophysical characteristics of the glasses.