Electro-optical properties and dynamic light scattering studies of polymer-stabilized cholesteric diffraction gratings

Polymer-stabilized cholesteric liquid crystal diffraction gratings (PSCDGs) are electrically responsive, transmission diffraction gratings, which are formed when an electric field-induced light diffracting state of a cholesteric liquid crystal is stabilized by an internal polymer network. The planar diffracting state can then be stabilized by photopolymerization of a low concentration (∼3–5 wt%) of reactive diacrylate monomer and photoinitiator components, which are added to the initial mixture. Under UV irradiation, these components react to form a solid network that anchors the liquid crystal diffracting state. PSCDGs have current and potential applications for electro-optic displays, wavelength tunable reflectors, and electrically controllable diffraction gratings.; In the first part of this dissertation, we study the optical morphology and electrooptical properties of PSCDGs in a “microspot” geometry, where 200–400 μm diameter gratings are stabilized against a normal state background by photopolymerization using a focused UV laser beam. A comparison of the optical morphology based on mesogenic versus non-mesogenic monomers reveals an interesting difference in the fidelity with which the distorted state of the liquid crystal can be captured. The electro-optical properties demonstrate efficient phase grating diffraction, and electrically-switchable grating-“on” and “off” states with fields of a few V/μm and 1/e relaxation times of order 10 msec. The radius of the stabilized micrograting is studied as a function of UV exposure time, and we show that for the laser intensities used, the polymerization process occurs in the “reaction-depletion” rather than “diffusion” limit.; The second part of our work concerns a dynamic light scattering (DLS) study of the liquid crystal director fluctuation modes in the PSCDG system. In addition, we present results on a homeotropically-aligned, pure cholesteric, where a planar diffracting state is also observed. Two basic issues are addressed: (1) the effect of distortion of the helical structure on the measured time correlation function of the scattered intensity and on the dispersion of the fluctuation modes; and (2) the potential effects of confinement on the fluctuations in a PSCDG system due to the presence of narrowly-spaced, spatially-periodic and orientationally-ordered domains of the polymer network.