Anisotropy of second-order nonlinear optical susceptibilities of arsenic triiodide–sulfur addition complex: AsI3 · 3S8

We report the anisotropic linear and second-order nonlinear optical (NLO) properties of arsenic triiodide–octa-sulfur (1:3) adduct, AsI3 · 3S8, which spontaneously crystallizes in the trigonal rhombohedral non-centrosymmetric space group R3m. The trigonal symmetry of the AsI3 · 3S8 molecule coincides with the crystal symmetry. The crystals are optically uniaxial with no/ne of about 1.2 and show dichroism at the UV-visible wavelengths. Second harmonic generation (SHG) tensor elements were determined from Maker fringes measured with an Nd:YAG laser (∼10 ns pulse, 1064 nm). The coefficient d222,eff = 32.0 pm/V for the light polarized parallel to the layers of iodine and sulfur atoms in the AsI3 · 3S8 crystal ab plane is markedly larger than d333 = 11.6 pm/V for the polarization of light parallel to the crystal c axis. The anisotropy parameter, defined as the d222,eff/d333 ratio, is about 2.7 for the AsI3 · 3S8 crystal, smaller than that for the isomorphous CHI3 · 3S8 crystal (7.3) but larger than for the SbI3 · 3S8 crystal (0.7). Highly anisotropic components of the first hyperpolarizability tensor of an AsI3 · 3S8 molecule, β, were derived from the quadratic nonlinear susceptibility tensor of the crystal.