DYE DOPED NEMATIC LIQUID CRYSTAL REORIENTATION IN A LINEAR POLARIZED LASER FIELD: THRESHOLD EFFECT NICOLETA ESEANU* 1 , CORNELIA UNCHESELU 2 , I. PALARIE 3 , B. UMANSKI 4 1 Department of Physics, ''Politehnica'' University of Bucharest, 313 Splaiul Independentei, Bucharest 2 Department of Physics, Technical University of Civil Engineering of Bucharest, 122-124 Ave. Lacul Tei, Bucharest 3 Faculty of Physics, University of Craiova,13 A. I. Cuza Str., Craiova 4 SSC RF “NIOPIK” (Organic Intermediate & Dyes Institute), B. Sadovaya 1/4, 103787 Moscow *nicoletaeseanu@ yahoo.com (Received July 15, 2003) Abstract. An experimental study of the molecular orientation induced by a polarized Ar + laser beam in a dye doped nematic liquid crystal (DDNLC) has been done. A sandwich type sample filled with 5CB+MR was aligned approximately planar by two glass surfaces, only one surface being rubbed. By measuring He-Ne probe beam intensity in time a threshold effect has been observed when modifying the Ar + laser power. This could be explained by using the percolation theory. Key words: dyes, liquid crystals, molecular orientation, parallel alignment, threshold effect. 1. Introduction The operation principle of the electro-optical liquid crystal devices (such as liquid crystal displays - LCD) combines bulk properties and surface interactions. Therefore controlling the alignment of the liquid crystal (LC) molecules is an attractive research field. It is well known that a laser beam passing through a transparent nematic liquid crystal (NLC) produces the reorientation of the nematic director. This effect is explained by the optical torque acting on NLC molecules due to their anisotropic optical polarizability [1]. Recently it was demonstrated that a small amount (less than 1%) of a dye dissolved in NLC can enhance the reorientation by almost two orders of magnitude and, in some cases, can even change its sign [2]. The explanation proposed by Janossy [3] is connected with the electronically excited metastable states appearing in the molecules of absorbing dye. The interaction of the excited dye molecule with the host LC molecules is different from the one of the dye molecule in the ground state. The Janossy model is suitable for the reorientational bulk effect. A surface driven reorientation effect as a result of the light action on the bulk of a light-sensitive NLC-azodye mixture was reported by Voloschenko and co-workers. The explanation was the adsorption of dye phototransformed molecules onto the aligning photopolymer surface [4].