ABSTRACT
This work focuses on experimentally demonstrating the modification in diffusion kinetics, formation of holographic polymer dispersed liquid crystal gratings and an improvement in its electro optic response by doping them with multi-walled carbon nanotubes. Results indicate a faster rise and fall times which is attributed to the reduction in size of the liquid crystal droplets formed and a reduction in switching voltage due to change in dielectric properties of the medium as manifested by a rise in capacitance. Real time diffraction efficiency measurements reveal a time delay in the appearance of the diffracted order due to non-participation of the nanotube in the polymerization induced phase separation process. An analysis of this effect is presented based on the Stoke-Einstein's diffusion equation incorporating shape anisotropy of the nanotubes.
ABSTRACT
We show proof of concept of a transmission-mode wavelength filtering device consisting of layered holographically formed polymer dispersed liquid crystal (H-PDLC) cells. H-PDLC cells were fabricated from a thiolene based polymer composite to have transmission notches in the near-IR wavelength range. Wavelength filtering was achieved by stacking four H-PDLC cells with transmission notches spaced at 10 nm intervals. Results show a broad transmission notch spanning the spectral width of the constituent cells. With bias applied to an individual cell within the stack, the transmission notch of the cell inverts and the overall transmission envelope changes shape. Using a transmitted energy sensing device and a lineshape mapping algorithm, spectral content can be determined to a resolution of 0.1 nm for narrow banded signals. Applications for this switchable wavelength filtering device include serial detection of spectral content for telecom data signals or chemical and biological sample identification through absorption or emission spectroscopy.
ABSTRACT
Using deuteron nuclear magnetic resonance (NMR) and dynamic light scattering, we study the orientational order and dynamics of a BL038-5CB liquid-crystal mixture in a holographic polymer dispersed liquid-crystal material (HPDLC) as used for switchable diffractive optical elements. At high temperatures, where the liquid crystal is predominantly in the isotropic phase, the HPDLC deuteron NMR linewidth and transverse spin-relaxation rate T-12 are two orders of magnitude larger than in the bulk. The analysis shows that the surface-induced order parameter in HPDLC is significantly larger than in similar confining systems and that translational diffusion of molecules in the surface layer is at least two orders of magnitude slower than in the rest of the cavity. The unusual temperature dependence of T-12 upon cooling suggests the possibility of a partial separation of the 5CB component in the liquid-crystal mixture. The onset of the nematic phase in HPDLC occurs at considerably lower temperature than in the bulk and takes place gradually due to different sizes and different content of non-liquid-crystalline ingredients in droplets. Parts of the droplets are found isotropic even at room temperature and the structure of the nematic director field in the droplets is only slightly anisotropic. We point out the capability of NMR to detect the actual state of liquid-crystalline order in HPDLCs and to contribute in this way to the improvement of the switching efficiency of diffraction gratings.