Mechanism of negative nonlinear absorption effect in erbium doped yttrium aluminum garnets

Negative input-output characteristics were obtained at modulation frequencies of up to 1 GHz in erbium doped yttrium aluminum garnets. The effect was seen at wavelengths of 785.9 to 788.9 nm in transmitted spectra that showed a peak corresponding to the transition from ⁴I_13/2 to ²H_11/2 in Er³⁺.