Molecular-Field-Enhanced Optical Kerr Effect in Absorbing Liquids

Abstract

A large enhancement of the optical Kerr nonlinearity associated with light-induced molecular orientation is observed in a liquid of anisotropic organic molecules when it is doped with a small amount of dye. The effect is ascribed to a photoinduced intermolecular force field, whose orienting action adds to the direct optical field action. The experimental results are in agreement with a model based on such interpretation. The proposed mechanism is in principle very general, and should work in all absorbing liquids made of anisotropic molecules. [S0031-9007(96)01885-6] PACS numbers: 34.20.Gj, 42.70.Nq The most effective mechanism for optical Kerr effect in transparent organic liquids at the nanosecond time scale is light-induced molecular orientation [1]. It results from the torque that the optical electric field exerts on the molecules due to their anisotropic polarizability. In absorbing liquids other mechanisms for large nonlinear optical response are present, but they are mostly isotropic. Light-induced molecular orientation is therefore still the main origin of nonlinear birefringence but, as we have found, its nature can be largely modified by light absorption.We turned a transparent liquid into absorbing by mixing it with small amounts of a dichroic dye. A large enhancement of its nonlinear birefringence was then observed.The enhancement factor was about 20 for dye concentrations of 0.26%. The material chosen for our experiments has a liquid-crystalline nematic mesophase a few degrees below our working temperatures. Pretransitional phenomena [2] thus allowed us to single out the orientational effects from other competing mechanisms, showing that it is indeed light-induced molecular orientation which is enhanced.A similar increase of the optical nonlinearity had been reported a few years ago for dye-doped liquid crystals in the nematic phase [3], and a model for it has been recently proposed [4]. An analogy between the isotropic liquid and nematic phases is not straightforward, because of the complex collective response characterizing the liquid crystal mesophase. Nevertheless, we developed a model for the orientational nonlinearity of absorbing isotropic liquids, based on the same main idea of Ref.[4], which explains well our experimental results.According to our picture, the enhanced optical Kerr effect is the result of two processes. The first one is electronic excitation of molecules, induced by light absorption. This process is anisotropic, since its probability is proportional to cos 2 u, where u is the angle between the optical electric field and the molecule transition dipole. Therefore, it generates an oriented population of excited molecules and a corresponding hole in the orientational distribution of ground-state molecules. Absorption alone, however, does not change the total orientational distribution of liquid molecules, which remains isotropic. The second process is provided by intermolecular interactions. Each molecule has an average orientational energy U͑u͒ which depends on the orientati...

Dynamical Study of Guest-Host Orientational Interaction in LiquidCrystalline Materials

“…The dyes that cause the effect are known not to undergo photoinduced conformational changes, as in the case with the azo dyes [16]. The enhanced optical reorientation due to dye-doping was also found to be not limited to the liquid crystalline phase, but also present in isotropic liquids, seemingly with the same mechanism at work [17,18,19]. Thus the effect is quite general.…”

“…The same physical underpinnings of the model apply to dye-doped LC in the nematic phase [17,18,19]. It has been suggested that a change in rotational diffusion upon photo-excitation, as observed in our study, could also be the driving force in the optical reorientation of azo-dye systems, working either in conjunction with or, in certain cases, instead of the well-known photo-isomerization of the azo dyes [63].…”

“…16) We see that each term on the right hand side (RHS) can be written in the form: 17) describing the mean-field reorientational effect that the j population has on the i population.…”

Mechanisms of giant optical nonlinearity in light-absorbing liquid crystals: a brief primer

“…(iv) A dye-induced enhancement of similar magnitude has been observed also in the isotropic liquid phase, where the orientational nonlinearity is a standard optical Kerr one [19,20,21].…”

Fluorescence-Based Investigation of the JÁnossy Effect Anomalous Wavelength Dependence

“…This enhancement is sometimes called ''J a anossy effect'', from the name of its discoverer. This effect is actually not even limited to the liquid crystalline phase, as closely related phenomena have been observed also in dye-doped isotropic liquids [2,3].…”