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 .

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.

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Lithium selenoindate (LiInSe 2) is a new nonlinear chalcogenide biaxial crystal, related to LiInS 2 and transparent from 0.54 to 10 µm at the 50% level (10 mm thickness), that has been successfully grown in large sizes and with good optical quality. We report on new physical properties that are relevant for laser and nonlinear optical applications and summarize all relevant characteristics, both from the literature and such measured in the present work. With respect to AgGaS(e) 2 ternary chalcopyrite materials, LiInSe 2 displays a nearly isotropic thermal expansion behavior with 3 to 5-times-larger thermal conductivities associated with high optical damage thresholds, and low intensity-dependent absorption, allowing direct high-power down-conversion from the near-IR, especially 1064 nm, to the deep mid-IR. Continuous-wave difference-frequency generation (5.9-8.1 µm) of Ti:sapphire laser sources is reported for the first time as well as nanosecond optical parametric oscillation with Nd:YAG laser as a pump source at 100 Hz and idler tuning between 4.7 and 8.7 µm.