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A microspectrophotometer was used to elucidate the local optical properties of cholesteric liquid crystals (CLCs) in cells with interdigitated electrodes as a function of applied voltage. The spectra collected from a spatially selective and micron-sized sampling area allow for new insights into the spectral properties of CLCs in the gaps between patterned electrodes. The microscopic electro-optic response is shown to be highly dependent on the cell thickness and the electrode periodicity. Specifically, the helix unwinding of the CLC superstructure does not always occur uniformly in the sample, as a result of field gradients through the cell thickness: for cells with relatively narrow gaps and electrodes, the redshift occurs initially only in the CLC layers closest to the substrate with the electrodes, leading to broad reflection spectra and different reflection colors depending on which side of the cell is illuminated. Theoretical estimates of the expected shift in the reflection band gap based on the critical field for a given CLC material and the spatial variation of electric field in the cell are found to be in good agreement with the complex behavior observed experimentally. In contrast, in thin cells with wider gaps, the pitch increase affects the whole CLC layer uniformly, because the electric field gradient is small., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Polymer electrolyte membranes (PEMs) with high and controlled ionic conductivity are important for energy-related applications, such as solid-state batteries and fuel cells. Herein we disclose a new strategy to fabricate long-range ordered PEMs with tunable ion conducting pathways using a holographic polymerization (HP) method. By incorporating polymer electrolyte into the carefully selected HP system, electrolyte layers/channels with length scales of a few tens of nanometers to micrometers can be formed with controlled orientation and anisotropy; ionic conductivity anisotropy as high as 37 has been achieved., (© 2011 American Chemical Society)

Layered polymer/nanoparticle composites have been created through the one-step two-beam interference lithographic exposure of a dispersion of 25 and 50 nm silica particles within a photopolymerizable mixture at a wavelength of 532 nm. The polymerizable mixture is composed of pentaerythritol triacrylate (monomer), 1-vinyl-2-pyrrolidinone (monomer), and photoinitiator. In the areas of constructive interference, the monomer begins to polymerize via a free-radical process and concurrently the nanoparticles move into the regions of destructive interference. The effects of exposure time, power density, nanoparticle size, and periodicity on the final nanocomposite structure were measured with transmission electron microscopy to determine the mechanism for particle segregation. Diffraction from the sample was monitored as well, though its magnitude was not a good predictor of nanostructure in this relatively low index contrast system. Exposure time did not have a strong effect on the final structure. The best nanoparticle sequestration was observed at reduced laser power density, smaller interferogram periodicity, and decreased nanoparticle size, indicating that particle segregation is dominated by diffusion-limited nanoparticle transport directed by a matrix containing a gradient of polymerization kinetics.

We report on the development of photoaddressable cholesteric liquid crystal (CLC) mixtures capable of large range color tuning as well as direct on-off electrical switching. The continuously photoaddressable CLC mixtures are based on a high HTP azo-containing chiral material mixed with an off-the-shelf nematic liquid crystal (QL9/E44). By polymer stabilizing the QL9/E44 mixture, it is demonstrated that the photoaddressable reflection of the notch can be switched on and off with an AC voltage. The novel combination of these effects has potential utility in lasing, dynamic notch filters, and spatial light modulators.

Droplet deformation and alignment are achieved in holographic polymer-dispersed liquid-crystal reflection gratings by applying an in situ shear during recording. High diffraction efficiency (99%) is obtained for light polarized parallel to the shear, with nearly zero efficiency for perpendicular polarization, and no increase of incoherent scattering. Permanent polarization dependence is related to stress-induced morphology changes of liquid-crystal droplets that are frozen by polymerization. The system is studied by electron microscopy and modeled by anisotropic coupled-wave and scattering theory. The morphology is consistent with the theory of small deformations of liquid droplets in fluid flow. Diffraction efficiency measurements are in agreement with theory incorporating this morphology as well as concomitant orientation and alignment of liquid-crystal molecules.

We report the combination of top-down and bottom-up nanomanufacturing techniques to fabricate active, hierarchically structured volume reflection gratings. Holographic polymerization (H-P) formed lamellar structures of approximately 200 nm in thickness, confining a block copolymer (BCP) to approximately 100 nm domains. Subsequently, the BCP self-assembles into nanolayers with a period of approximately 21 nm. We envisage that this approach opens a gateway to fabricating hierarchical nanostructures at different length scales.

Holographic photopolymerization (H-P) is a simple, fast and attractive means to fabricate one-, two- and three-dimensional complex structures. Liquid crystals, nanoparticles and silicate nano-plates have been patterned into submicron periodical structures. In this article, we report fabrication of a one-dimensional reflection grating structure by patterning a semicrystalline polymer, polyethylene glycol (PEG), in Norland resin (thiol-ene based UV curable resin) matrix using the H-P technique. Sharp notches observed in the reflection grating of this Norland/PEG system indicate a finite Δ present in the system due to spatial segregation of the PEG and Norland resin. The notch position red shifts upon heating and the diffraction efficiency (ratio between diffraction and incident light intensity, DE) increases from ∼20% to 60% for the Norland 65/PEG 4600 grating. This dynamic behavior of the reflection grating is also fully reversible. The unique thermal switching behavior is attributed to the melting/formation of PEG crystals during heating/cooling. By employing different molecular weight PEGs which have different melting temperatures, a series of switching temperatures have been achieved. Since PEG can be easily coupled with a variety of functional groups, this research might shed light on fabricating multifunctional Bragg gratings using the H-P technique.

Abstract- In order to develop new systems that couple photochromism with molecular conformation change, a series of spiropyrans having different ring substituents were attached to poly(L-glutamic acid). The polypeptides were dissolved in hexafluoroisopropanol and dark adapted so that the dye was in the merocyanine form. Following adaptation by white light and dye photoconversion from the merocyanine to spiropyran forms, polypeptide dark-adaptation kinetics were monitored by circular dichroism (CD) and dye dark adaptation was monitored by UV/ visible. Light adaptation caused a light-induced coil-to-helix transition, with dark adaptation resulting in relaxation back to the coil. The dark-adaptation rate constant measured by UV/visible was equal to that measured by CD, demonstrating close coupling between dye state and polypeptide conformation. By varying the substituents on the spiropyran ring, dark-adaptation half lives were varied from less than a minute to 2 h, representing nearly three orders of magnitude.

To determine the maximum range of coupling between side-chain photochromism and polypeptide conformation change, we modified the carboxylate side chains of succinylated poly(L-lysine) with a spiropyran to form polypeptide I. The extent of modification was determined to be 35.5%. The spacer group length between the polypeptide alpha-carbon and the dye was 12 atoms, providing minimum polypeptide-dye interaction. Conformation changes were monitored by circular dichroism as a function of light adaptation and solvent composition (hexafluoroisopropanol [HFIP] vs trifluoroethanol [TFE]). Under all solvent compositions, the dark-adapted dye was in the merocyanine form. Light adaptation by visible light converted the dye to the spiropyran form. When dissolved in TFE, I adopted a helical conformation insensitive to light adaptation. With increasing percentage HFIP, a solvent-induced helix-to-coil transition was observed around 80% (vol/vol) HFIP. At 100% HFIP, both light- and dark-adapted forms of I were in the coil state. Near the midpoint of the solvent-induced helix-to-coil transition, light adaptation caused conformation changes. Applying helix-to-coil transition theory, we measured a statistically significant difference in coil segment-HFIP binding constant for light- vs dark-adapted solutions (6.38 +/- 0.03 M-1 vs 6.56 +/- 0.03 M-1), but not for the nucleation parameter sigma (1.2 +/- 0.4 10(-3) vs 1.3 +/- 0.3 x 10(-3). The small binding constant difference translated to a light-induced binding energy difference of 17 cal/mol/monomer. Near the midpoint of the helix-to-coil transition, collective interactions between monomer units made possible the translation of a small energy difference (less than RT) into large macromolecular conformation changes.(ABSTRACT TRUNCATED AT 250 WORDS)

We report storage and electrical switching of holographic image data in an economical polymer-dispersed liquidcrystal material. The hologram is recorded in a fast, single-step process and can be reversibly erased and restored repeatedly by the application of fields of approximately 10-15 V/ microm, with a response time of 22 micros and a relaxation time of 42 micros. Simple (quasi-sinusoidal) holographic transmission gratings also are studied with switching fields of <5 V/ microm and with response and relaxation times of 25 and 44 micros, respectively.

An argon-ion laser interferometer (0.3638 or 0.488 microm) wrote photoreversible holographic grating (fringe spacing 0.39-100 microm) on spin-cast thin films (5 microm thick) of poly(spiropyran l-glutamate).

We have observed double-peaked phase-conjugate pulses in the degenerate four-wave mixing of nanosecond laser pulses in solutions of diphenyl polyenes. We attribute this to the superposition of fast and slow gratings, where the slow grating is due to two-photon absorption. This is supported by the results of time delay and polarization experiments as well as evidence of a second spatial harmonic grating.

An investigation of kinetics of the helix to coil dark reaction of light adapted poly(spiropyran-L-glutamic acid) (PSLG) dissolved in hexafluoroisopropanol was performed. The reaction was associated with the spiropyran (SP) to merocyanine (MC) ring opening. The ring opening reaction monitored with UV/VIS spectroscopy showed first order kinetics. Chromophore and polypeptide backbone circular dichroism data fit to an expression consistent with a single intermediate series mechanism. By FTIR, we monitored the polypeptide alpha-helix amide I, the MC chromophore--C = C--stretch and the protonated unmodified carboxylate C = O stretch bands. During the first step of the series mechanism, changes in the hydrogen bonding of the unmodified carboxylate groups occurred, suggesting breakup of polypeptide aggregates. The second step of the proposed series mechanism was dominated by the helix to coil transition and the ring opening of SP to MC. The CD spectrum of MC in the dark adapted PSLG was red shifted and had a narrower bandwidth than the UV/VIS spectrum. The kinetic and spectroscopic data suggested that a fraction (population I) of the MC chromophores experienced optical activity induced by the chiral polypeptide environment, while the remainder of the MC chromophores (population II) were solvated and enantiomeric.

The optical-limiting behavior of a series of trans-alpha, omega-diphenyl polyene compounds was observed in solutions of chloroform. The influence of planarity, the substitution of donor and acceptor groups, and the extent of pi-electron delocalization on the nonlinear thresholds of the diphenyl polyenes in an optical-limiting geometry were examined. A saturation effect of optical-limiting nonlinear thresholds, consistent with the theoretical work, was observed at 10-11 pi-electron bonds. The temporal profiles of the transmitted laser pulses and the power dependence of the nonlinear thresholds as a function of spot size and wavelength were examined. These examinations led to the conclusion that the predominant nonlinear mechanism was quasi-steady-state self-focusing. Nonlinear thresholds an order of magnitude lower, and thus effective n(2) and X(3) values an order of magnitude higher, than the well-known self-focusing medium CS(2) were observed. Our studies demonstrate that this series of polyenes consists of efficient broadband nonresonant optical-limiting materials.