Growth and characterisation of single crystals of ternary chalcogenides for laser applications

Bulk single crystals up to 20 mm in diameter and 40 mm long for LiInS₂ end up to 10 mm, 20 mm, respectively, for LiInSe₂ have been grown. Their colour changed from colourless to rose for the first one and from yellow to dark red for the other. All crystals have wurtzite-type lattice (Pna2₁ space group), lattice parameters were determined. A band gap was found tobe 3.72 and 357 eV for LiInS₂ and 3.02, 2.86 eV for LiInSe₂ at 80 and 300 K, respectively. Colour variations are due to point defects, first of all to interstitial sulfur, resulting in additional wide absorption bands in the shortwave part of transparency range. For LiInS₂ the SHG phase matching conditions were found to be similar for samples of different colour and some difference from Boyd's predictions of 1973 was shown: for XY plane Δφ≃ +3° at 2.6 μm and Δφ∼ -3 to -5° at 4-5 μm. Nonlinear susceptibility for LiInS₂ was estimated: d_eff(XY) ∼3.4 pm/V relative to Boyd's value as 10.6 pm/V. A proper illumination gives a photoinduced change of LiInSe₂ colour from dark red to yellow as a result of changes in point defects charge state.