LiGaTe2:: A new highly nonlinear chalcopyrite optical crystal for the mid-IR

The chalcopyrite crystal LiGaTe2 was grown by the Bridgman-Stockbarger technique with sufficient size and optical quality that allowed the characterization of its linear (dispersion and birefringence) and nonlinear optical properties. X-ray structural analysis was performed on single crystals. The transmission was recorded in the 0.5-24 mu m range, and Raman and IR-spectra were recorded in the 0-400 cm(-1) and 180-400 cm(-1) ranges, respectively. The clear transparency range of LiGaTe2 extends from 2.5 to 12 mu m, the band-gap at room temperature is at 2.41 eV (515 nm), residual absorption limits the transmission near the band-edge, and the upper limit for the transmission is determined by the onset two-phonon absorption. LiGaTe2 is a positive uniaxial crystal that possesses sufficient birefringence for phase-matching. Its nonlinear coefficient d(36) was estimated by phase-matched second harmonic generation to be 43 pm/V +/- 10%. It is only the third crystal belonging to the A(I)B(III)C(2)(VI) chalcopyrite family for which phase-matched nonlinear frequency conversion could be demonstrated, and of this group it exhibits the highest nonlinearity and figure of merit for nonlinear optical applications in the mid-infrared.