Single crystals LiGaX 2 (X = S, Se, Te) of optical quality were grown, with transparency ranges at 5 cm -1 absorption level of 0.32-11.6 µm, 0.37-13.2 µm and 0.54-14.2 µm, respectively. The first two, LiGaS 2 and LiGaSe 2 , have a wurtzite-type structure whereas LiGaTe 2 is tetragonal (chalcopyrite lattice). The three refractive indices were measured in the whole transparency ranges of LiGaS 2 and LiGaSe 2 and n a and n c were found to be very close (quasi-uniaxial optical anisotropy) with a crosspoint at 6.5 µm (LiGaS 2 ) and 8 µm (LiGaSe 2 ). Sellmeier equations were fitted and phase-matching conditions for second harmonic generation (SHG) were calculated: the 1.467-11.72 µm spectral range for the fundamental is covered by LiGaS 2 and LiGaSe 2 .

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Lithium thioindate (LiInS2) is a new nonlinear chalcogenide biaxial material transparent from 0.4 to 12 µm, that has been successfully grown in large sizes and good optical quality. We report on new physical properties that are relevant for laser and nonlinear optics applications. With respect to AgGaS(e)2 ternary chalcopyrite materials, LiInS2 displays a nearly-isotropic thermal expansion behavior, a 5-times larger thermal conductivity associated with high optical damage thresholds, and an extremely low intensity-dependent absorption allowing direct high-power downconversion from the near-IR to the deep mid-IR. Continuous-wave difference-frequency generation (5-11 µm) of Ti:sapphire laser sources is reported for the first time.

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.

Analysis of new, promising wide-band-gap nonlinear crystals for the mid-IR is given. We focused on ternary and quaternary chalcogenides and based on their physical and chemical properties, which in addition to linear and nonlinear optical properties, determine the efficiency of crystal usage in certain laser experiments. The correlation between the composition, structure and output parameters as well as the trends in changes of the main characteristics such as band gap, nonlinear susceptibility, laser damage threshold, etc, are demonstrated. Vibrational spectra of Li-containing nonlinear crystals and their application in THz radiation generation are discussed. Using an example of LiGaS 2 , the defect effect on spectroscopic characteristics is shown. High-temperature annealing in a proper atmosphere allows one to remove extended and point defects and to improve the transparency. Energy structure and parameters of main point defects such as anion vacancy (F-center) and cation antisite defect Ga Li calculated from the first principles agree well with the experimental results.

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Single crystals of lithium selenoindate (LiInSe2) are grown by the Bridgman–Stockbarger technique up to sizes of 10 mm in diameter and 20 mm in length. The different phases have color from yellow (as grown) to dark red (after annealing) but all have the same wurtzite type structure mm2 with slightly differing cell parameters. The band gap for the yellow phase at 300 K is at 2.86–2.87 eV. The red color is attributed to point defects and can be removed by proper illumination. Sellmeier equations are constructed for the 0.5–11 μm range and their validity is checked with second harmonic generation which provides first estimations of the nonlinear coefficients of LiInSe2. The potential of LiInSe2 is compared to that of the widely spread and technologically mature AgGaS2.