We present a new bistable scattering effect in a filled nematic liquid crystal, and its application to a high resolution laser-addressed display. The filled nematic consists of a small fraction of inorganic particles of nm size in a nematic liquid crystal with positive dielectric anisotropy. The scattering ON state is reached by locally writing with a laser. The transparent OFF state can be obtained by combining a moderate voltage with selective application of the laser to the part to be erased. The key parameters controlling the processes and the possible physical mechanisms involved are discussed.

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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...

To explain the large photoinduced molecular reorientation phenomena observed in dye-doped liquids and liquid crystals, the hypothesis was formed that the rotational mobility of dye molecules is strongly altered during their electronic excitation. Here, we report the direct measurement of a 30%-50% mobility decrease of photoexcited anthraquinone dye molecules dissolved in a cyanophenyl liquid host. This mobility reduction is ascribed to an excited-state reinforcement of intermolecular hydrogen bonding. These results provide fully independent evidence for the validity of current models of the photoinduced reorientation and a working demonstration of the design concepts of "fluctuating-friction" molecular motors. We propose that a light-induced modulation of molecular mobility associated with electronic photoexcitation is of general relevance to the behavior of photosensitive organic materials, currently investigated for applications in optical data storage, liquid-crystal displays, and organic optoelectronic devices.