Electrically switchable Bragg reflection gratings, comprised of periodic monolayers of liquid crystal nanodroplets embedded in a polymer, have historically displayed limited diffraction efficiency (< 90%). Attempts to increase diffraction efficiency by increasing the liquid crystal concentration lead to undesirable levels of incoherent scattering. We have developed a model of liquid crystal Bragg gratings to study and predict these properties for reflection filters. Examining the effective medium theory of inhomogeneous nanodroplet layers, we have investigated the enhancement of properties that lead to higher diffraction efficiency without increasing incoherent scattering. In particular, we focus on properties of nanodroplet ensembles that contribute to the permittivity modulation of the grating, leading to coherent diffraction, and permittivity variance, leading to incoherent scattering. Using the liquid crystal BL037 in a commercially available polymer as an example, we illustrate how a diffraction efficiency of 99% can be achieved, without simultaneously increasing the level of incoherent scattering, and compare this to the previous state of the art with these materials. We then demonstrate this approach experimentally, confirming the viability of the technique and the predictions of the model.

Urea derivative with double long alkyl chains and the corresponding rare-earth metal complex were prepared and their thermotropic liquid-crystalline behavior was evaluated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffractometry, electron microscopy. It was found that in spite of the compounds without disk-like molecular shape, they exhibit a columnar phase with two-dimensional rectangular lattice in which each column is built up from one-dimensional molecular chains, i.e., non-covalently linked polymer architectures on the basis of one-dimensional hydrogen bond. In the columnar liquid-crystalline materials, we predicted that delocalization of protons along the one-dimensional self-assembled molecular chains is induced, resulting in useful channels for prototropy. To verify this prediction, we evaluated preliminarily the electric conductivity of the urea liquid-crystalline thin films by electrochemical impedance spectroscopy. The solid-state films retaining a columnar structure showed a protonic conductivity even under dry conditions.

A new electro-optic waveguide platform, which provides unprecedented electro-optical phase delays (> 1mm), with very low loss (< 0.5 dB/cm) and rapid response time (sub millisecond), is presented. This technology, developed by Vescent Photonics, is based upon a unique liquid-crystal waveguide geometry, which exploits the tremendous electro-optic response of liquid crystals while circumventing historic limitations of liquid crystals. The exceedingly large optical phase delays accessible with this technology enable the design and construction of a new class of previously unrealizable photonic devices. Examples include: a 1-D non-mechanical, analog beamsteerer with an 80° field of regard, a chip-scale widely tunable laser, a chip-scale Fourier transform spectrometer (< 5 nm resolution demonstrated), widely tunable micro-ring resonators, tunable lenses, ultra-low power (< 5 microWatts) optical switches, true optical time delay (up to 10 ns), and many more. All of these devices may benefit from established manufacturing technologies and ultimately may be as inexpensive as a calculator display. Furthermore, this new integrated photonic architecture has applications in a wide array of commercial and defense markets including: remote sensing, micro-LADAR, OCT, laser illumination, phased array radar, optical communications, etc. Performance attributes of several example devices are presented.

Nonlinear phase contrast microscopy is an optical technique that uses an intensity-dependent refractive index material to produce high-contrasted images of transparent specimens. Earlier proposal of liquid crystals as phase filters for phase contrast applications used optically addressed spatial light modulators fabricated with photoconductive film. Here, we propose the use of a simpler planar nematic liquid crystal cell doped with 1% wt methyl red. Owing to their polarization dependent enhancement factor a tunable phase filter can be photoinduced efficiently. Thus, images of different degree of contrast (and even contrast reversal) can be obtained either by rotating the polarization vector. All optical real-time imaging of dynamic events can be performed and image processing such as edge enhancement is demonstrated.

Azobenzene liquid crystals (azo LCs) have been proven to possess the highest optical nonlinearity, bounding with photosensitivity, for cw laser beams. We show here that azo LCs are highly nonlinear for short laser pulses as well. Single as well as multicomponent room temperature nematic azo LCs were used in this study for single nanosecond pulses of the second harmonic of a Nd:YAG laser. These compositions demonstrate sensitivity starting from ~ 10 mJ/cm² and exhibit response time at the nanosecond scale. The effect of material composition, layer thickness and pulse energy on the nonlinear response of a system of crossed polarizers comprising planar oriented LCs are reported.

Various kinds of crosslinked liquid-crystalline polymers were synthesized with acrylate monomers and diacrylate cross-linkers containing azobenzene moieties. The effect of spacer length on photoinduced bending behavior of the polymer films was investigated. The films with long spacers exhibited low glass transition temperatures and underwent bending and unbending behavior at room temperature. Mechanical force generated during the process of UV light irradiation was also measured. It was found that the maximum force increased with the increment of the cross-linking density and the intensity of UV light.

Diamantane, one of the diamondoid family molecules, was introduced to thermally reversible photochromic heliofulgides. They exhibited good thermally reversible photochromism. A chiral photochromic indolylfulgide derivative was used to control the pitch length of cholesteric liquid crystalline state by photoirradiation. However, as the indolylfulgide derivative has absorption in the visible light region, it is not a suitable agent to control the selective reflection wavelength. Chiral benzofurylfulgide derivative, possessing shorter absorption maximum wavelength, was employed for this purpose. Diamantane has a long barrel-like C₃-symmetric structure with the corresponding symmetric axis. Several derivatives, with long alkyl or related substituents on the carbon atoms at both ends of the molecule, showed liquid crystalline properties. Thus, diamantane worked as a thread stitching up photochromism and liquid crystals.

An azobenzene liquid crystal attached to gold nanoparticles with average diameter of 3.5 ± 1.1 nm was synthesized. The gold nanoparticles prepared in this study were soluble in common non-polar organic solvents such as toluene, chloroform and so on, but not soluble in polar solvents. The particles exhibited their absorption maximum at around 500 nm due to surface plasmon resonance. We confirmed that the reversible photochemical and thermal isomerization of the azobenzene moieties could also be induced on the surface of the gold nanoparticles by UV irradiation. Furthermore, the azobenzene LCs used in this study showed LC behavior even if they attached to the gold nanoparticles.

Light-emitting reactive mesogens are promising materials for photolithographically processable organic light-emitting diodes (OLEDs). We study the photopolymerization of a fluorene-based mesogen with methacrylate photoreactive groups. We find that there is no significant change in the photoluminescence quantum efficiency when the mesogen is photopolymerized in a glovebox with oxygen and water concentration at < 20 parts per million. There is significant quenching of luminescence when the photopolymerization is carried out in a less controlled environment. A real-time measurement is used to monitor changes in the FTIR spectrum of the compound during polymerization. No photo-initiator is added to the mesogen suggesting that the photo-polymerization is self-initiated by the chromophore. We show that the self-initiation does not proceed by fragmentation of the aromatic core and suggest an alternative mechanism.

The recently developed fluorescence confocal polarizing microscopy (FCPM) imaging technique allows 3D images of the director structure in a liquid crystal cell to be resolved with sub-micron resolution. Results are presented on imaging the response of 5-micron pitch cholesteric liquid crystals to an in-plane electric field applied between two silver electrodes. The results show, in exquisite detail, how the application of an in-plane field causes the cholesteric helix to tilt through 90° either within, or immediately adjacent to, the electrode gap depending on the sign of the dielectric anisotropy of the liquid crystal. Furthermore, imaging the cholesteric material above the silver electrodes reveals a previously unreported optical intensity enhancement. This phenomenon is discussed along with its possible benefits to the existing imaging technique. The effects of the point spread function of the system are discussed and a ray optics model is used to produce model data highlighting the influence of this phenomenon on the recorded results.

We report a new familiy of polarimetric imaging cameras based on tunable liquid crystal components. Our camera designs use a dual frequency liquid crystal tunable filter that rotates the polarization of incoming light, in front of a single linear polarizer. The unique features of this approach include fast switching speed, high transmission throughput, no mechanical moving parts, broad bandwidth, high contrast ratio, wide viewing angle, and compact/monolithic architecture. This paper discusses these tunable liquid crystal polarimetric imaging camera architectures (time division, amplitude division), the benefits of our design, the analysis of laboratory and field data, and the applicability of polarization signatures in imaging.

We report our experimental success in realizing high efficiency liquid crystal polarization gratings (LCPGs) on reflective substrates, with periods as small as 2.2μm, enabling the largest switchable LCPG diffraction angles reported yet for red light. Moreover, these gratings retain nearly ideal electro-optical properties, including > 95% hologram efficiency, high polarization contrast, sub-millisecond total switching times, and relatively low voltage operation (thresholds ~1.5V). We discuss two different fabrication approaches, each with its own set of advantages, which have resulted in gratings with the above compelling properties. We anticipate broad utility of these diffractive elements in a variety of applications.

We found that 1-alkyl-4-phenyl-2,2,3,3,5,5,6,6,7,7,8,8-dodecamethyl-2,3,5,6,7,8-hexasilabicyclo[2.2.2]octanes exhibited hexagonal columnar phases with an alkyl group ranging from a methyl to a pentyl group. Introduction of a cyano group at para-position of the phenyl group was found to enhance the stability of the columnar phases. The observed columnar phases strikingly contrasted with a nematic phase of 1-alkyl-4-(4-cyanophenyl)bicyclo[2.2.2]octanes. Furthermore, the polysilacage derivatives with such short alkyl groups as n-propyl, n-butyl, n-pentyl, and n-hexyl at 1-position, that contain neither long alkyl groups nor aromatic rings, were also disclosed to exhibit columnar mesophases, indicating that the polysilacage moiety has a high tendency to form columnar structure.

Large-aperture liquid crystal (LC) devices have been in continuous use since 1995 as polarization control devices in the 40-TW, 351-nm, 60-beam OMEGA Nd:glass laser system at the University of Rochester's Laboratory for Laser Energetics. The feasibility of using a noncontacting alignment method for high-peak-power LC laser optics by irradiation of a linearly photopolymerizable polymer with polarized UV light was recently investigated. These materials were found to have surprisingly large laser-damage thresholds at 1054 nm, approaching that of bare fused silica (30 to 60 J/cm²). Their remarkable laser-damage resistance and ease in scalability to large apertures of these photoalignment materials, along with the ability to produce multiple alignment states by photolithographic patterning, opens new doorways for their application in LC devices for optics, photonics, and high-peak-power laser applications.

Triphenylene-containing poly(1-decyne)s with different alkyl chain lengths are synthesized and the effects of the structural variables on their mesomorphic properties are investigated. The monomers [HC≡C(CH₂)₈CO₂C₁₈H₆(OC_mH2m+1)₅; m = 4-9] are prepared by consecutive etherization, coupling, and esterification reactions. The monomers form columnar phases at room temperature. The polymerizations of the monomers are effected by [Rh(nbd)Cl]₂, producing soluble polymers in high yields (up to 84%). The structures and properties of the polymers are characterized and evaluated by IR, NMR, TGA, DSC, POM, and XRD analyses. All the polymers are thermally stable, losing little of their weights when heated to 300°C. The isotropization temperature of the polymers increases initially with the length of alkyl chain but decreases on further extension. Whilst the polymers with shorter and longer alkyl chain lengths adopt a homogeneous hexagonal columnar structure, those with intermediate ones form mesophases with mixed structures.

Liquid crystal (LC) devices exhibit fast and strong tuning and switching capabilities using small voltages and can be miniaturized thus have a great potential to be used with miniature optical imaging systems for biomedical applications. LC devices designed specifically for integration into biomedical optical imaging systems are presented. Using a combination of one or two LC retarders we obtained polarimetric imaging of the skin. LC tunable filters with high dynamic range and large throughput are designed for hyperspectral imaging and for spectral domain optical coherence tomography. The designs are based on several concepts both using the classical stack of retarders and using more modern designs based on single layer in a waveguide or in a Fabry-Perot cavity.

The alignment of liquid crystal by nano-structured surfaces is investigated. In particular we shall examine the new nano-structured inhomogeneous surfaces. It is shown that this type of surface can give excellent alignment properties. Reliable pretilt angles of any value between 0 and 90° can be obtained. These nano-structured alignment layers are very useful for a variety of display applications.

Nematic phase gratings have been studied in which a planar nematic layer of thickness 17.2 μm is sandwiched between two glass substrates coated with an alignment polymer. The upper substrate is a continuous earth plane and the lower substrate has a patterned electrode of interdigitated stripes (electrodes and gaps are both 40 μm wide). Reorientation of the nematic liquid crystal occurs in response to d.c. electric fields applied between the interdigitated electrodes. These nematic reorientation regions have been used to investigate the influence of the flexoelectric polarisation in the nematic liquid crystal by observing the resultant (i) movement of tilt fringes in a Mach-Zehnder interferometer, and (ii) optical diffraction patterns. In the Mach-Zehnder interferometer the periodic variation of the refractive index resulting from the periodic distortion profile is measured directly from the displacement of the tilt fringes. The asymmetry in the response to positive and negative polarities of the d.c. voltage for both measurement techniques is directly related to the sum of the flexoelectric coefficients, e₁ + e₃.

The dielectric dispersion in uniaxial nematic liquid crystals creates a "dielectric memory" effect whereby the polarization induced by the electric field decays exponentially with time rather than instantaneously, as in materials without dispersion. The induced polarization couples linearly with the electric field. This linear coupling allows one to accelerate the director relaxation towards the "off" state by a specially designed electric pulse of a proper polarity and duration We show theoretically and experimentally the possibility of electrically driving the director towards the off state, thereby decreasing the switching time.

Image intensifier tubes, as part of night vision devices, have been the primary devices for the detection and amplification of near infrared light for night vision operations. In this paper, we demonstrate a novel all-optical night vision amplifier device with a potential to replace the image intensifier tube in night vision goggles. This image amplifier is based on a novel structure of semiconductor and spectrally tunable liquid crystal (LC) materials within a thin cell. The LC reacts to near-infrared (NIR) radiation but is unaffected by visible light, allowing see-through capability including visible-wavelength cockpit light. The technology is made very attractive by its high sensitivity, spatial resolution, and contrast without expensive, bulky, and heavy optics or high-voltage components.

This paper provides research progress in the development of fast electro-optic gratings based on liquid crystals for laser beam attenuations. The electro-optic phase grating is formed by the phase separation of ~100nm liquid crystals droplets from a polymerizing organic matrix using holographic interference technique. The formed grating separates the incident laser beam into the output beams: the transmitted and diffracted beam, whose intensities can be electrically adjusted through electro-optic effect. The fast electro-optic gratings have a very fast electro-optic response time of 50 microseconds with diffraction efficiency above 99.8%. Optical receivers used in FSO have a limited dynamic range and there is a need for in-line variable attenuators to keep the signal levels from overloading the receiver. These attenuators should be continuous, provide sufficient attenuation, and also provide a low insertion loss for weak signal reception. The use of electro-optic Bragg gratings is one solution to meet the requirements for an in-line attenuator for FSO.

Thin films of organics/silica nanohybrids synthesized on a substrate have been subjects of extensive investigation. In general, however, the nanostructures of the thin films are aligned only at local levels, but are randomized at larger scales. Development of alignment methods for such nanostructures at larger scales is of great demand to extend applications. We report herein newly developped methods for macroscopic alignment and micropatterning of nanohybrid films synthesized on a photo-crosslinkable liquid crystalline polymer thin film. Two topics are included: First, the photoalignment of ordinary templating systems of 2D hexagonal liquid crystalline surfactant micelles. Mesostructured organic/inorganic hybrid materials are widely studied. Here, the method of photoaligning and characterizations of the resulting materials will be presented. Second, the photoalignment of chromonic columnar liquid crystal of dye aggregates. We have newly developed a procedure to synthesize a nanohybrid consisting of chromonic liquid crystal and silica network. This new type of chromonic nanohybrids is also found to be aligned by the identical photo-crosslinkable polymer film.

We present a new photonic micro-optical device based on an array of electrodes made from vertically aligned multiwall carbon nanotubes used to address a liquid crystal cell. The electrodes create a Gaussian electric field profile which is used to reorient a planar aligned nematic liquid crystal. The variation in refractive index within the liquid crystal layer acts like a graded index optical element which can be varied by changing the applied electric field to the carbon nanotube. Results are presented from a device fabricated with a 10um pitch between the micro-optical elements.

The effects of plasma surface treatment on molecular stacking of a discotic liquid crystal are studied. Glass substrates are bombarded by an obliquely incident O₂ plasma beam. Plasma treatment causes an increase in surface free energy of the substrates, and in addition, the oblique plasma beam generates a preferential direction on the surface processed. The configuration of the cell is found to be crucial to achieve alignment of the discotic columns, and in general, cells with anti-parallel configuration of substrates should be used to achieve uniform alignment of the columnar phase. Homeotropic alignment of dichotic columns can be produced on plasma treated glass substrates with high surface free energy. When the surface free energy of the substrate decreases, the axes of the discotic columns will tilt from the normal of the substrate and towards the preferential direction.

We study the enhancement and control of the refractive index of liquid crystals with dispersed gold and silver nanoparticles. The maximum obtainable variation in the real and imaginary parts of the effective refractive index of the solution by reorientation of the liquid crystal molecules is calculated, and the results obtained with gold and silver nanospheres are compared. The effect of size, concentration, and composition of the nano-particulates (solid spheres vs. silver- or gold-coated silica nanoshells) on the refractive index and its wavelength dependence is also considered.

Photochemically tunable photonic band gap materials were prepared by infiltration of liquid crystal polymers having azobenzene groups into voids of SiO₂ inverse opal films. Linearly polarized light irradiation resulted in transformation from a random to an anisotropic molecular orientation of azobenzene side chains in the voids of the SiO₂ inverse opal film, leading to the reversible and stable shift of the reflection band to longer wavelength more than 15 nm. In order to improve switching properties, we used copolymers with azobenzene monomer and tolane monomer, which indicate higher birefringence, as infiltration materials into the voids. The azo-tolane copolymers were found to show the higher birefringence than azobenzene homopolymers by the linearly polarized light irradiation. Thus, the reflection band of the SiO₂ inverse opal film infiltrated with the azo-tolane copolymers was shifted to long wavelength region more than 55 nm by the irradiation of linearly polarized light.

Color shift on off-axis angles is key issue of the cholesteric liquid crystal (CLC) polarizer. In this paper, color shift issues on various azimuthal and viewing angles of back light unit assembled with CLC polarizers are analyzed by optical simulation. We found that smaller NZ value of quarter-wave plate (QWP) of our samples will compensate the color shift issue more efficiently. Although abnormal dispersion type QWP gives the best performance at the on-axis viewing angles, at large viewing angle its color shift is depending on azimuthal angle and bandwidth of CLC polarizers. Besides, a single layer type CLC polarizer of gradient pitch over a shorter bandwidth has better color coordinate property also be performed.

In this work we derive the birefringence matrix of a twisted nematic liquid crystal using Jones matrix formalism. It is assumed that due to the shape of the molecules and negligible absorption each plane slice of a liquid crystal cell exhibits a uniform intrinsic birefringence (linear or elliptical). Under this scope, it is shown that the anisotropy of the twisted nematic liquid crystal is described by an inhomogeneous birefringence matrix. A polarimetric procedure to verify this result is also proposed.

We report enhanced polarization-independent tunable optical filters based on liquid crystal (LC) and reactive mesogen (RM) polarization gratings (PGs). This new design achieves tunable passbands with significantly smaller bandwidth and allows for potential applications in spectroscopy and beyond. Analogous to Lyot and Solc filters, our filter is constructed of multiple bilayer polarization gratings (BPGs) of varying thicknesses, with the potential for highly compact implementation. BPGs are a hybrid of a switchable/tunable liquid crystal (LC) PG and a reactive mesogen (RM) PG. By adding the RM layer, the BPG provides a significant advantage over LCPGs for the filter application in that it allows very thick gratings to be created with thin active LC layers. As such, the difficulty in fabricating LCPGs with arbirtrarily large thicknesses is much less of a concern. BPGs exhibit the unique properties of PGs, including polarization independent zero-order transmittance, as well as diffraction at visible and infrared wavelengths. Our unique design enables a high peak transmittance (~ 90%) as well as a significantly improved full-width-at-half-maximum (FWHM). Here, we present preliminary data, discuss the unique capabilities and compelling advantages of our filter. We analyze performance in terms of finesse, 3dB bandwidth (FWHM), and free-spectral-range by comparison to theoretical simulation.

We have experimentally demonstrated broadband light modulation by achromatic liquid crystal (LC) polarization gratings (PGs), which manifest polarization-independent modulation with high efficiencies (≥95%). Recently, we introduced achromatic PGs with a unique double-layer, reversed-twist structure as efficient, broadband polarizing beamsplitters. We now report on our successful implementation of electrically switchable achromatic LCPGs on a reflective substrate. To pattern a spiraling, periodically varying LC profile, we utilize polarization holography and photoalignment techniques. Use of reflective substrates enables the same retardation compensation of double-layer achromatic PGs. In addition, perhaps most importantly, the single cell structure allows the electro-optical switching/modulation by applying an electric field across the cell. The achromatic LCPG sample shows steeper voltage responses and less spectral shifts while operating in grayscale with respect to previously reported LCPGs. Relatively faster switching times (~6 msec for 3 μm-thickness) were measured compared to a conventional LCPG with the same thickness (~10 msec). Interesting electro-optical behaviors were also observed including zero-voltage threshold and a hysteresis in the voltage response.

Self-organized liquid-crystal filamentary forms arise in many mixtures of the smectogen compounds at the transition from the isotropic melt. In some mixtures, a subsequent phase transition to the crystal or crystal smectic phase occurs at the core of the filaments. The resulting hard-core fibers act as anisotropic cylindrical lenses composed of the crystalline core surrounded by the nematic shell. In this work, the filaments, referred to as nematoids, have been obtained in several binary mixtures based on five mesogens: 4,4'-dipentylazoxybenzene, 4-dodecyloxy-4'- pentylbiphenyl, 4-hexyl-4'nonyloxybiphenyl, 4-acetyl-4'-dodecylbiphenyl and 4''-pentylcyclohexyl 4-(4'- pentylcyclohexyl) benzoate within a silicone oil as an inert liquid. To characterize the molecular arrangements within the nematoids, we present the microinterferometric measurements of the refractive index distribution within the fibers and its changes at the phase transitions.