Holographic Patterning Of Polymer Dispersed Liquid Crystal Materials For Diffractive Optical Elements

Liquid crystals are a soft condensed matter phase that share properties with both isotropic liquids and crystalline solids. Composite materials formed from both low molecular weight liquid crystals and polymers are of both fundamental and technological importance. One method for preparing dispersions of polymers and liquid crystals is polymerization-induced phase separation. Photo-polymerization induced phase separation utilizes a mixture of a liquid crystal and a photo curable polymer contained between two indium tin-oxide (ITO) coated glass substrates. Irradiation of the homogeneous pre-polymer mixture initiates polymerization, which in turn induces a phase separation between the polymer and liquid crystal. Holographically patterned polymer dispersed liquid crystals (H-PDLCs) are a recent descendent of the conventional PDLCs. Rather than a random dispersion of droplets, holographic exposure is used to selectively pattern the liquid crystal phase in well-defined planes creating diffractive optical elements. H-PDLCs can operate in the transmission or reflection diffractive modes, depending on the experimental geometry, and can be switched on and off with voltages just as conventional polymer dispersed liquid crystals (PDLCs). The realization of a switchable H-PDLC Bragg and Raman-Nath transmission and Bragg reflection gratings written with the 632.8 nm wavelength of He-Ne laser are developed and investigated. The observation of well-defined periodic grating structures, which are characterized with grating periods of 210 nm - 10.5 micron, are discussed in detail. Polymer functionality effects on formation kinetics along with morphology development are examined along with effects of externally applied electric fields on the diffraction efficiency and switching times. Theoretical models for these data are discussed and compared. Future research in this area along with initial experimental results in a new operational mode, the so called "Reverse Mode", are also examined and discussed.