Spectroscopic discrimination of resonant and nonresonant contributions to the nonlinear refractive index in ion doped solids

This work intends to distinguish resonant and nonresonant contributions to the nonlinear refractive index in ion doped solids using a two-color nonlinear technique. In this experiment, the alexandrite metastable excited state (²E) is populated via the ⁴T₂ absorption band using a 514 nm pump beam (Ar⁺ laser), while the refractive index change is probed by a tunable dye laser, in resonance with the R lines (from ground to metastable transition at λ ∼ 680 nm). The absorption spectrum with the two characteristic R lines is also shown just for comparison of the spectra. Observations reveal that an absorption saturation signal follows the linear absorption spectrum.