Abstract

The most widely spread techniques to produce coherent and tunable radiation in the mid-IR are based on second order nonlinear optical processes in birefringent non-centrosymmetric crystals. All commercially available mid-IR nonlinear crystals with transparency extending well beyond 5 μm are uniaxial. LiInS₂ was the first wrurtzite type (mm2) mid-IR crystal whose potential was evaluated almost three decades ago.¹ Recently we demonstrated that LiInS₂ can be grown with sufficient size and quality for real applications² and in 2001 both second harmonic generation (SHG)³ and optical parametric amplification⁴ have been realized. However, the nonlinear coefficients turned out to be much smaller than those of AgGaS₂. We studied now the analogous LilnSe, since the substitution of S by Se is expected to improve the nonlinearity as in the case of the chalcopyrites AgGaS₂ and AgGaSe₂. Here we describe the synthesis of single crystals of LiInSe₂, the results of its structural analysis and present measurements of its transmission, band edge, index of refraction and the nonlinear coefficients. The substitution of the Ag-ion by the lighter Li-ion increases the frequencies of the crystal lattice vibrations and the Debye temperature. Thus improved thermal conductivity and increased optical damage threshold are expected. The wurtzite type crystal lattice permits to avoid the difference in sign of the thermal expansion coefficients along different directions and facilitates the doping for engineering of noncritical phase-matching conditions.

© 2002 Optical Society of America