One- and two-photon induced phase transition behavior of low-molecular-weight and polymer liquid crystals (M) was observed. When a photochemical reaction of a two-photon absorbing dye was induced in a nemnatic (N) LC, N-to-isotropic (I) phase transition was induced with high spatial resolution. As a result of the phase transition, the refractive index of the materials was changed by two-photon processes. Furthermore, the I phase induced in the polymer LC was thermally stable and it still appeared after heating above N-I phase transition temperature. The results reported here are the first observation of the photochemical phase transition of the LCs induced by a two-photon process. (c) 2005 Elsevier B.V. All rights reserved.