422 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, NO. 3, MAY/JUNE 2006 Effects Leading to Colossal Optical Nonlinearity in Dye-Doped Liquid Crystals Liana Lucchetti, Matteo Gentili, and Francesco Simoni, Member, IEEE Abstract—In this paper, we report our most recent results about the huge optical nonlinearity of methyl red (MR)-doped 5CB thin cells. The basic concepts of the surface-induced nonlinear effect interpretation are recalled and possible methods to obtain a better control of the samples response are discussed. Index Terms—Colossal optical nonlinearity, dye-doped liquid crystals (LCs), surface-induced nonlinear effect (SINE). I. INTRODUCTION T HE nonlinear optical properties of liquid crystals (LCs) in the nematic phase have been studied for more than 20 years, since the first observation of the giant optical nonlin- earity (GON) in 1980 [1]. This effect led to an optical nonlin- earity several orders of magnitude higher than that of isotropic liquids and was due to the peculiar property of LCs of keeping the molecular anisotropy on a macroscopic scale. It is well known that the optical field of an electromagnetic wave is able to produce a nonlinear optical response in liquids because of its interaction with the permanent or induced dipoles of the liquid molecules. Thus, even if the medium is macro- scopically isotropic, its anisotropic molecules reorient under the action of the optical field leading to a field-dependent change of the molecular polarizability. Such a molecular reorientation also occurs in LCs; but in their case much stronger effects take place related to the macro- scopic anisotropy and the collective behavior of these materi- als. GON results due to the collective molecular reorientation induced by the optical field and is about 10 6 times stronger than the nonlinearity induced in conventional liquids (the cor- respondent nonlinear coefficient, i.e., the change of the re- fractive index relative to light intensity, is of the order of 10 −5 cm 2 /W). After the discovery of GON, LCs became the subject of in- tense research activity and several effects leading to sensitive enhancement of their nonlinear optical response have been ob- served. The first of these observations was made by Janossy [2]. He found that doping nematic LCs with antraquinone dyes lowered the threshold for the optical Freedericks transition by two orders of magnitude. Several later experiments [3] pointed out that this effect was a manifestation of a dye-induced en- hancement of the LC nonlinear optical response that can be phenomenologically accounted for as an additional torque pro- portional to the ordinary optical torque through the relation Manuscript received July 29, 2005. The authors are with the Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio, Universit` a Politecnica delle Marche, 60131 Ancona, Italy (e-mail: l.lucchetti@univpm.it). Digital Object Identifier 10.1109/JSTQE.2006.872055 Γ dye = ηΓ opt ,η being a parameter characteristic of the dye, generally of the order of 10 2 . Further experiments [4] have pointed out that a similar en- hancement of the optical torque can be obtained with azo dyes also, provided that the two isomers cis and trans are treated as two different dopants and are associated with different enhance- ment factors. Another development concerning optical nonlinearities of LCs started with the first observation of the photorefractive ef- fect in these materials [5]. More recently, a very strong enhance- ment of the nonlinear optical response characterized by a non- linear coefficient n 2 of the order of 1 cm 2 /W has been detected in a particular mixture of nematic and azo-dye, namely, pentyl- cyanobiphenil 5CB doped with a small amount of the azo-dye methyl red (MR) with homeotropic orientation. The first obser- vation was made by Khoo et al. [6], who defined the observed behavior as a “supranonlinear” response and gave a first in- terpretation based on a photorefractive-like effect due to the photo-induced space-charge field generated by the spatial mod- ulation of conductivity and dielectric anisotropy of the sample, sustained by an internal dc field. Several further experiments, including some performed by the authors of this paper [7], [8], have confirmed the supranon- linear behavior of MR-doped 5CB cells. In particular, the crucial role played by the surface in determining such a high nonlin- earity has been recently highlighted and the effect has been phenomenologically described as a surface-induced nonlinear effect (SINE) [7], [9], [10]. This effect is based on the close dependence between LC bulk orientation and cell-anchoring conditions; that is, any light-induced change of the latter will produce a bulk reorientation, which in turn affects the light propagation with the consequent onset of a nonlinear optical response. Moreover, to induce the elastic deformation of the whole sample it is enough to excite a thin layer near the surface, and this allows the use of very low pump intensity to induce the nonlinear response. In the following sections, we are going to first recall the basic ideas underlying the SINE and then report all our recent results concerning the huge nonlinear optical response of MR-doped 5CB samples. II. SURFACE-INDUCED NONLINEAR EFFECT The phenomenon leading to the transient interfacial modifi- cation responsible for the surface nonlinear effect is connected to the light-induced adsorption and desorption of MR molecules at the irradiated surface. It is well established that in MR-doped nematics it is possible to modify the surface conditions by “writ- ing” an easy axis, which produces a new steady orientation in the 1077-260X/$20.00 © 2006 IEEE