Deviations from the reciprocity theory of McCumber at low temperature in Nd3+-doped glasses

The reciprocity theory of McCumber [Phys. Rev. 136, A954 (1964)], which relates optical absorption and emission spectra between two energy level manifolds, was tested at temperatures down to 130K on the F3∕24→I9∕24 transition in Nd-doped glass. Deviations between calculated and measured spectra were observed for temperatures below 230K in Nd:silicate and below 170K in Nd:fluorozirconate glass. Below these temperatures, the calculated absorption is skewed to shorter wavelength and the calculated emission is skewed to longer wavelength. The magnitude of the deviation depends on the ratio of inhomogeneous linewidth to thermal energy kBT.