LiGaTe2: A New Highly Nonlinear Chalcopyrite Optical Crystal for the Mid-IR

Abstract

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 μm 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 μm, 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 d36 was estimated by phase-matched second harmonic generation to be 43 pm/V ± 10%. It is only the third crystal belonging to the AIBIII 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.