Mid-infrared (2.75-6.0 /spl mu/m) second-harmonic generation in LiInS/sub 2/

Summary form only given. We present second-harmonic generation (SHG) measurements on two different LiInS/sub 2/ crystals. The material has enjoyed renewed interest because of its attractive optical properties, such as the large transparency range from 0.4-12 /spl mu/m and the high (estimated) nonlinear susceptibility d/sub 33/ of about 18 pm/V, which is higher than that of the widely used AgGaS/sub 2/. Recently, the physical properties for a set of LiInS/sub 2/ crystals were studied as a function of their composition. Both \´yellow\´ (as grown) and \´rose\´ (annealed in sulphur vapour) LiInS/sub 2/ crystals were found to have an identical lattice structure of the so-called "wurtzite" type (orthorhombic, Pna2/sub 1/) and a similar energy gap E/sub g/=3.59 eV at 300 K. The picosecond pulses from the free-electron laser (FEL) for infrared experiments (FELIX) are used as the pump in the wavelength range from 2.75 to 6.0 /spl mu/m. Both crystals (\´rose\´ 5 /spl times/ 3 /spl times/ 2 mm, \´yellow\´ 3 /spl times/ 2 /spl times/ 2 mm) are cut for type-II (eoe) phase matching in the XY plane. The phase matching angles in the range from 2.75 to 6.0 /spl mu/m were measured by tuning the FEL wavelength, while the data points around 2.5 /spl mu/m wavelength were taken from measurements on a 5-mm thick \´ rose\´ LilnS/sub 2/ crystal with a nanosecond OPO. The measurements show several degrees difference with calculations based on Boyd\´s refractive index data.