Four-wave mixing in alexandrite crystals

Degenerate four-wave mixing was observed in alexandrite crystals (BeAl₂{O}₄$: ${Cr}³⁺$), and the signal beam efficiency and decay rate were measured as functions of pump beam-crossing angle, wavelength, and power. The results are consistent with scattering from excited-state population gratings related to the difference in dispersion of the Cr³⁺ ions in the ground and metastable states. These gratings can be selectively established with Cr³⁺ ions in the inversion or mirror sites depending on the excitation wavelength. Strong scattering occurs only for pump beams polarized parallel to the b direction of the crystal.