By combining polymer-dispersed liquid crystal (PDLC) and holography, holographic PDLC (H-PDLC) has emerged as a new composite material for switchable or tunable optical devices. Generally, H-PDLC structures are created in a liquid crystal cell filled with polymer-dispersed liquid crystal materials by recording the interference pattern generated by two or more coherent laser beams which is a fast and single-step fabrication. With a relatively ideal phase separation between liquid crystals and polymers, periodic refractive index profile is formed in the cell and thus light can be diffracted. Under a suitable electric field, the light diffraction behavior disappears due to the index matching between liquid crystals and polymers. H-PDLCs show a fast switching time due to the small size of the liquid crystal droplets. So far, H-PDLCs have been applied in many promising applications in photonics, such as flat panel displays, switchable gratings, switchable lasers, switchable microlenses, and switchable photonic crystals. In this paper, we review the current state-of-the-art of H-PDLCs including the materials used to date, the grating formation dynamics and simulations, the optimization of electro-optical properties, the photonic applications, and the issues existed in H-PDLCs.

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Holographic Polymer-Dispersed Liquid Crystals: Materials, Formation, and Applications

We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization suggests that the orientation of a liquid crystal within the ellipsoidal droplets is axial. The distribution of droplet orientations appears to vary for different grating spacings. The switching properties of the gratings are investigated and analyzed with a sinusoidal dielectric tensor modulation model. The analysis also takes into account orientational distribution of average directors and spatial distribution of the droplets. Temperature dependence of diffraction efficiency is tentatively explained by a structural transition within the droplets and the presence of impurities in droplets.

Characterization of holographic polymer dispersed liquid crystal transmission gratings

We modelize the photoinduced formation of switchable diffraction gratings in polymer-based liquid-crystalline composite materials. The model assumes that redistribution of molecules is due to mass diffusion and incorporates a realistic kinetic description of polymerization processes. Numerical simulations predict two different kinds of structure that have already been experimentally observed. Two parameters governing the phenomenon are pointed out, which determine the structure that will be formed after the curing process has been completed.

Model for the photoinduced formation of diffraction gratings in liquid-crystalline composite materials

The temporal evolution of an anisotropic reflection grating produced in a holographic polymer-dispersed liquid crystal film is investigated. We find that this type of grating is preceded in time by an isotropic concentration grating, and that the development of the anisotropic grating can be delayed until several seconds after laser exposure. The formation of an anisotropic grating is nearly coincident with the onset of phase separation of liquid crystal and implies a macroscopic ordering of liquid crystal droplet directors. Detailed knowledge of grating evolution may allow in situ control over the polarization sensitivity of the hologram.

Evolution of anisotropic reflection gratings formed in holographic polymer-dispersed liquid crystals

Effect of surfactant on the electro-optical properties of holographic polymer dispersed liquid crystal Bragg gratings

The underlying physical phenomena behind the increased reflection/diffraction efficiency of holographically formed polymer-dispersed liquid crystal gratings formed from blended oligomers are investigated. Using parameters determined by in situ diffraction measurements during grating formation, a phenomenological diffusion model shows that a composition modulation in the polymer network results in blended oligomer systems. Scanning electron microscope studies reveal polymer morphologies consistent with this hypothesis.

C. C. Bowley

Papers

Diffusion kinetics of formation of holographic polymer-dispersed liquid crystal display materials