Growth and properties of LiGaX₂ (X = S, Se, Te) single crystals for nonlinear optical applications in the mid‐IR

Single crystals LiGaX₂ (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, LiGaS₂ and LiGaSe₂, have a wurtzite‐type structure whereas LiGaTe₂ is tetragonal (chalcopyrite lattice). The three refractive indices were measured in the whole transparency ranges of LiGaS₂ and LiGaSe₂ and n_a and n_c were found to be very close (quasi‐uniaxial optical anisotropy) with a crosspoint at 6.5 μm (LiGaS₂) and 8 μm (LiGaSe₂). 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 LiGaS₂ and LiGaSe₂.