1. Some reviews on nonlinear optics in liquid crystals have been appeared recently in the literature: N.V.Tabiryan, A.V.Sukhov, and B.Zel’dovich, `The orientational optical nonlinearity of liquid crystals’, Mol. Cryst. Liq. Cryst., 136, 1–140 (1986);
2. I.C.Khoo, `Nonlinear optics of liquid crystals’, Progress in Optics, XXVI, 107–161 (1988);
3. F.Simoni, `Nonlinear optical phenomena in nematics`, in Physics of liquid crystals, Gordon & Breach, N.Y. 1989;
4. Classical books where all mesophases are considered in some detail is P.G. de Germes, `The physics of liquid crystals’, Clarendon Press, Oxford, 1974; S.Chandrasekhar, `Liquid Crystals’, Cambridge Univ. Press, 1977;
5. A.Saupe and G.Englert, Mol. Cryst. 1, 503 (1966);