Investigation of the population dynamics of the $^4I_{11/2}$ level of Nd$^{3+}$ ions in glasses by determination of the absorption at 0.66 and 1.06 $\mu$m wavelengths

When subnanosecond laser radiation ($\lambda$ = 1.064 $\mu$m) was self-focused in phosphate and silicate neodymium glasses, absorption steps were found at $\lambda$ = 0.66 and 1.054 $\mu$m. The absorption reached 0.1–0.15 cm$^{-1}$ and the relaxation time was $\gtrsim 10^{-8}$ s. Absorption was not recorded at $\lambda$ = 0.56 $\mu$m and in glasses without neodymium. The selective absorption was associated with the lower level of the active transition of the $^4I_{11/2}$ neodymium ion populated by nonlinear processes caused by self-focusing.