Bragg gratings yield a single diffracted order when irradiated by a coherent beam at the appropriate Bragg angle. In many cases, nearly all of the energy of the incident beam can be coupled to the diffracted beam. Hence these gratings can form many useful optical elements, and this has been realized in 1-D, 2-D, and 3-D photonic crystals. Bragg gratings made with liquid crystals offer the added dimension of dynamic properties through the large electro-optical effect in liquid crystals. Applications for spatial light modulators are numerous, including optical switches, modulators, active optical elements (e.g., lenses), laser sources, and tunable filters. We have been exploring a number of approaches for making liquid crystal Bragg gratings, including holographic polymer-dispersed liquid crystals, cholesteric liquid crystals, and homogenous nematic liquid crystals in hybrid devices. We have studied the dynamic properties of these Bragg gratings by electrical, thermal, and optical stimulation. Modification and control of optical and dynamic properties have been obtained through combinations of liquid crystals with polymers, combinations of various dopant materials, and interactions of liquid crystals with organic and inorganic interfaces. We discuss the materials, fabrication, characterization, and physics of liquid crystal Bragg gratings and present the results of various devices we have studied in our lab. We will also discuss potential applications.

Polarization properties and electro-optical switching behavior of holographic polymer-dispersed liquid-crystal (HPDLC) reflection and transmission gratings are studied. A theoretical model is developed that combines anisotropic coupled-wave theory with an elongated liquid-crystal-droplet switching model and includes the effects of a statistical orientational distribution of droplet-symmetry axes. Angle- and polarization-dependent switching behaviors of HPDLC gratings are elucidated, and the effects on dynamic range are described. A new type of electro-optical switching not seen in ordinary polymer-dispersed liquid crystals, to the best of the author’s knowledge, is presented and given a physical interpretation. The model provides valuable insight to the physics of these gratings and can be applied to the design of HPDLC holographic optical elements.

T he use of holographic materials is increasing at a dramatic rate in research and consumeruse systems. A recent report estimated that the worldwide holographic market for 1998 was as high as $2.8 billion.1 While static holograms are by far the most widely used, many applications would bene® t greatly by being able to turn the holograms off and on like an optical switch. This article describes the development of holographically recorded liquid crystal/polymer composites known as holographic polymer-dispersed liquid cr ystals (HPDLCs). H-PDLCs are prepared by causing inter ference between two coherent laser beams in a photosensitive monomer/liquid crystal (LC) mixture contained between two sub-

Gratings formed by anisotropic phase separation of liquid crystals show tremendous promise for switchable optical elements. Formation of useful gratings is linked to the control of the resulting nano-scale morphologies. In an attempt to decrease scatter and improve the electro-optic properties of polymer-dispersed liquid crystal (PDLC) volume transmission gratings we have evaluated the results of altering the relative concentrations of the components in the pre-polymer mixture. Discussion is made concerning the expected effects on polymerization and phase separation and how these changes are reflected in the film morphologies and electro-optical properties. Initial results indicate that changes in the concentration of co-initiator, chain extender, and effective monomer functionality have a significant effect on the size and shape of the LC domains at constant LC loading.

Holographic gratings formed through the anisotropic phase separation of liquid crystals show promise as switchable optical elements. In order to form useful elements, however, it is necessary to control the nanoscale morphologies within the grating films. In this manuscript, we evaluate the role of monomer functionality on the morphology and the electro-optical properties of both gratings and conventional scattering polymer-dispersed liquid crystal (PDLC) films. Both of these structures are formed using polymerization-induced phase separation (PIPS) of liquid crystals from a cross-linked polymer formed through free-radical photo-polymerization. Floodlit (uniform illumination) films and holographic gratings (from non-uniform illumination caused by the interference of two laser beams) were made using monomers with 2–5 acrylate groups, while keeping the LC concentration constant in the syrups. The morphologies of these films were examined using low voltage scanning electron microscopy (LVSEM). In all cases, very small LC domains were formed with little indication of growth or coalescence. Lowering monomer functionality reduced the volume fraction of phase-separated domains in the floodlit samples. For the grating samples, the local volume fraction and the LC domain sizes decreased substantially as the monomer functionality was decreased. Using detailed image analysis, differences in the anisotropy of the domains was also probed. A much stronger tendency to form anisotropically-shaped domains was observed for the higher functional syrups. These domain anisotropy differences are correlated with the number of reactive double bonds per monomer and are suggestive of local environmental differences exerted at the time of the domain formation.