Excited-state absorption studies of Cr⁴⁺ ions in several garnet host crystals

An experimental study of saturable absorption and excited-state absorption (ESA) in several inorganic saturable absorbers, Cr⁴⁺:YAG, Cr⁴⁺:GGG, and Cr⁴⁺:YSGG, is presented. We provide the theoretical background of absorption characteristics in saturable absorbers that exhibit ESA, with some new results: approximate analytical solutions are proposed for the optical transmission in the case of a slow absorber, and for various light intensity conditions of spatially or temporally Gaussian beams in fast and slow absorbers. Experimentally, partial bleaching of the first excited state itself could be observed in Cr⁴⁺:YAG at λ = 1064 nm, yielding the higher excited-state lifetime as τ* = (0.55 ± 0.1) ns. The regular transmission bleaching curve was measured in Cr⁴⁺:GGG, for the first time in this material, yielding σ_gs = (58 ± 5) × 10^-19 cm², and σ_es = (13 ± 2) × 10^-19 cm² at λ = 1064 nm. ESA spectra were measured for the three materials between ∼700 and 900 nm. All three exhibit crossing between saturable absorption at longer wavelengths and inverse saturable absorption at shorter wavelengths.