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ABSTRACT Electrically switchable volume reflection holograms were written by inhomogeneous illumination of a prepolymer syrup containing a nematic liquid crystal and a multifunctional acrylate monomer. Switchable holograms are diffractive optics structures and the diffraction efficiency can be controlled by the application of an electric field. Reflection gratings with grating spacing varying between 0.16-0.27 ýtm were made during the phase separation of liquid crystals from the fast curing prepolymer syrup. The reflection efficiency of the holograms were electrically modulated with the applied field of - 10-15V/gim. Real time study of the grating formation revealed that the maximum efficiency is reached in -15 seconds. The shrinkage of the host polymer during grating formation resulted in the blue shift of the reflection notch. The response time of the grating in an electric field is -50 gs. Low voltage scanning electron microscope studies showed the presence of discrete nematic droplet domains of sizes 30-60 nm in liquid crystal rich region.

INTRODUCTION Holographic polymer dispersed liquid crystals (H-PDLC) are formed by laser illumination of a pre-polymer syrup containing a nematic liquid crystal in an interference fringe pattern. This inhomogeneous illumination results in a spatial modulation in the density of the liquid crystal droplets through the polymer host. The droplets are of submicron size and unlike the PDLC's formed by blanket exposure, there is very little scattering. In the field off state, there is random orientation of the droplet directors which leads to index mismatch between the polymer rich and liquid crystal rich regions. In the off state, diffraction of light due to the presence of a holographic grating is observed. When an electric field is applied, the LC directors align with the field and the index modulation disappears and all the light is transmitted. The electrical switchablity of HPDLC's have potential applications in diffractive optics including spatial light modulation devices. The simplest Bragg hologram is a planar volume grating. Both transmission and reflection type holograms can be written in PDLC's. Electrically switchable reflection holograms form the building blocks for applications in optical image information displays and also dynamic filters. Recently, we have described our work on the development of switchable transmission holograms in PDLC systems(I-7). The diffraction gratings are formed by phase separation of liquid crystal and the grating consists of alternating planes of PDLC and solid polymer. The transmission holograms with spacing of the order (0.5-.Sim) exhibited high diffraction efficiencies(>80%), low switching voltage(5-8V/gm) and fast response(-30gs). Studies at small Bragg spacing

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Mat. Res. Soc. Symp. Proc. Vol. 559 0 1999 Materials Research Society

(0.13-0.22gm) are also of interest for reflection holograms as they exhibit reflection colors in the visible region. Recently, there are some reports of switchable reflection gratings(8,9). In this pap