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

Abstract

The chalcopyrite crystal LiGaTe 2 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 LiGaTe 2 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. LiGaTe 2 is a positive uniaxial crystal that possesses sufficient birefringence for phasematching. 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.

Lattice dynamics of chalcopyrite semiconductors LiAlTe2, LiGaTe2 and LiInTe2

“…The similar discrepancy between theory and experiment [16] is available for LIT, as is seen in Table 4. Our results are of a high level of convergence and are rather close to experiment [6] for all other modes. Probably, the theory and experiment discrepancy observed at about 300 cm -1 is caused by anharmonicity or structural defects that appreciably influence the LGT phonon spectrum, as will be shown below.…”

“…For LGT and LIT we can compare our results with the experimental ones, obtained by Raman and IR spectroscopy techniques [6,16]. The same data serve as a criterion of reliability of the theoretical calculations for LAT because of the lack of experiments for it.…”

“…As seen from Table 4, such a comparison for the majority of frequencies is practically obvious and our calculations enable us to treat almost all experimental results reliably enough. At the same time, in Raman and IR spectra [6] there are three lines at 296 cm -1 , 304 cm -1 and 309 cm -1 that are difficult to compare with the theoretical data -the nearest frequency of B 2 mode at 327 cm -1 is essentially larger. The similar discrepancy between theory and experiment [16] is available for LIT, as is seen in Table 4.…”

“…The crystallographic data for samples, grown by the Bridgman-Stockbarger technique in a vertical setup, are presented in [1,2,6] for LGT and [2] for LAT and LIT. The authors of these papers have produced stable and chemically pure compounds LAT, LGT and LIT, being crystallized into a chalcopyrite structure, which is not changed up to the melting point.…”

“…The same data serve as a criterion of reliability of the theoretical calculations for LAT because of the lack of experiments for it. Unfortunately, the symmetry analysis of vibrations in [6,16] has not been made, therefore the comparison of the calculated frequencies with the measured ones has been fulfilled by their greatest proximity. As seen from Table 4, such a comparison for the majority of frequencies is practically obvious and our calculations enable us to treat almost all experimental results reliably enough.…”