Polymers containing 6- and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV-irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of the extent of dimerization, X. The experimental data for X as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. The kinetic model was employed to describe the evolutions of coumarin dimer's and monomer's orientational order. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and the late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation was successfully interpreted by considering three factors: the relative abundance of coumarin dimers to monomers, their orientational order parameters, and the energetics of molecular interaction.

We have developed liquid crystalline retardation films to improve quality of images of LCDs such as their viewing angle performance and coloration. We have achieved to make many types of optical retardation films by using rod-like liquid crystalline polymer (LCP). The resulting liquid crystalline polyesters film has several advantages over conventional uni- or biaxial stretched retardation film. Optical well-controlled structures such as twisted nematic, hybrid nematic and homeotropic structures could be stabilized for ideal compensation of various LCD modes including TN, STN, ECB, VA and IPS modes. Twisted nematic film is effective to cancel coloration in STN mode that is a fatal drawback for color representation. Hybrid nematic film is quite unique film because the film works not only as a wave plate but also as a viewing angle compensator for TN and ECB modes. By using rod-like LCP, it is also possible to make negative-C plate and positive-C plate. Negative-C plate could be realized by using a short pitch cholesteric alignment and positive-C plate could be realized by using homeotropic alignment. Viewing angle performances of various LCD modes compensated with the LCP films are reported in this study.

The superprism and lasing devices were demonstrated using holographic polymer-dispersed liquid crystal (HPDLC) films. The HPDLC film for superprism application was designed and fabricated using three coplanar beams. The fabricated HPDLC film contained two-dimensional (2D) ordered nano-sized LC domains (~150nm in diameter) embedded in a polymer matrix; its periodicity was estimated using a scanning electron microscope to be ~350nm. The dispersion of white light from this HPDLC superprism was ~50°, and the deflection of light output from it was consistent with the theoretical value obtained by the pland wave expansion method. HPDLC for laser application was fabricated using two counter-propagation laser beams similar to those used in standard holography. The structure has a periodicity in the range of optical wavelength, and reflects light selectively as governed by Bragg reflection. Doped with a laser dye whose emission spectrum overlaps the reflection spectrum of the grating, the HPDLC reflection grating can be lased at the band edges of the reflection band gap. The details of the experiments, results will be reported.

Traditionally, the light receptor and light modulation aspects of Optically Addressed Spatial Light Modulators (OASLMs) occur in separate layers. Due to the progress that has been made in the study of nonlinearity in liquid crystal cell doped with chromophores in the past 20 years, it is appropriate to consider in what ways they themselves may be useful as OASLMs. The light reception and modulation aspects coexist within the same layer in these cells. We have been studying a variety of chromophore-doped systems (azo and anthraquinone dyes, buckminsterfullerene, and carbon nanotubes) over the past four years. Dynamic holographic grating formation is observed under conditions of low power laser light both with and without external fields. The majority of the samples are planar aligned and normal incidence of light can be used. They possess very good lifetime stability and no degradation even under high write light intensities. We understand how to avoid permanent recordings using appropriate alignment surfaces. This is important in OASLM applications where real-time updating of written information is required (dynamic holography, all-optical switching). The resolution of the devices is superior to the thickness of the liquid crystal layer, and comparable to the best traditional OASLMs. We are currently working on understanding the dynamics in order to address the issue of speed of response. The report will include latest results on diffraction efficiency from our OASLM characterization set-up.

Usually when optically pumped, dye-doped cholesteric liquid crystal (CLC) laser generates circularly polarized laser light in the same handedness as the cholesteric helix. On a distributed feedback basis, laser light at photonic band edge comes out from both sides symmetrically. In this work, we incorporated a metallic mirror reflector to the CLC laser on one side so that laser light only emits from single direction and hence the extracted output can be enhanced by ~2-5X. Furthermore, upon reflection the mirror reflector introduces a π phase change. Therefore, two different types of CLC lasers with different polarization states are demonstrated by putting the mirror at different substrates. With a mirror attached at the outer side of the liquid crystal substrate, we obtained a nearly unpolarized CLC laser based on incoherent supposition of two orthogonal circularly polarized beams. With mirror coated at one of the inner surfaces of the liquid crystal cell, we obtained a linearly polarized CLC laser based on coherent combination of two orthogonal circularly polarized beams. For these two cases, the output power and polarization states are compared and the physical mechanism is discussed correspondingly. Moreover, the tuning of the linear polarization direction is demonstrated.

Polymer cholesteric-liquid-crystal (PCLC) flakes suspended in a fluid are used as the active medium in a novel particle-based, electro-optic technology. The motion of PCLC flakes is controlled with an electric field so that PCLC flake devices are brightly reflective in their "off" state and appear dark when an electric field is applied, causing the flakes to reorient 90°. Basic devices using a mildly conductive host fluid such as propylene carbonate are not bistable, and flakes relax to their original position within tens of seconds to minutes after the electric field is removed. We seek to control flake orientation by designing waveforms that follow the initial drive voltage. Shaped pulses were investigated to accelerate flake relaxation. The optimal pulse for motion reversal was found to be a 1.5-s sawtooth pulse with a 3-V amplitude. We also examined the use of holding voltages, which follow the driving voltage, but have amplitudes a fraction of the driving-voltage magnitude. The holding voltage prevents flakes from relaxing, while saving on power consumption. Cells driven at several volts were found to retain their brightness with the application of a holding voltage between 0.4 to 0.5 V.

Generally, optical feedback and/or two counter-propagating beams are necessary to form high-definition patterns in the cross section of a laser beam after passing through a nonlinear medium. In this paper we present an observation of pattern formation in liquid crystal media in a single laser beam without any external feedback. We found that after irradiation of a dye-doped liquid crystal cell with repetitive nanosecond pulses, the beam coming out of the liquid crystal cell exhibits a spectacular kaleidoscopic change of beam patterns in the far field. The patterns vary from pulse to pulse in an ordered manner cycling through a variety of complicated forms. We speculate that localized phase separation of the dye from the liquid crystal host occurs in the focal region of the beam in our experiments, and that the observed far-field patterns result from the laser-beam diffraction on these absorptive and refractive inhomogeneities.

Statistical mechanics of an inhomogeneous NLC molecular alignment in NLC cells with complicated intermolecular and molecule-wall (anchoring) interactions can be easily studied using Metropolis Monte Carlo simulation method for Lebwohl-Lasher lattice system.¹ Those studies have elucidated some microscopic aspects of the role of anchoring forces for local and global molecular ordering, leading to novel features in the external electric field (E) - anchoring strength (α) phase diagram for refractive index or an unexpected maximum of diffraction efficiency for low values of anchoring strength α in planar NLC cells.² Quite recently, this approach was used for a study of transmission of light through a twisted NLC cell as a function of applied voltage and light wavelength, for some values of α.³ First results show interesting generalizations of known analytical results of Yeh and Gu.⁴ In this paper we address a question of the importance of fluctuations of local rubbing directions and of fluctuations of anchoring strength for optical properties of NLC based optical systems. Preliminary results for chosen systems are presented.

Advances in computational chemistry hardware and software now make it possible to accurately model and predict physical properties (e.g., electronic spectra and chirality) in terms of hours or days instead of the weeks or months of intensive effort that were required only a few years ago. Previously, we reported on the effectiveness of computational modeling methodology in predicting the helical twisting power (HTP) for both well known chiral dopants and a series of novel nickel dithiolene IR dyes in a liquid crystal host. This earlier work, we showed that that correlation between the computationally derived weighted, scaled-chirality index G_0SW and HTP for materials with rigid molecular structures was excellent, but the high dependence of G_0SW on conformational energy in flexible molecular systems results in an inadequate representation of the true system conformational energy if only a single computed energy-minimized conformer is used in the calculations. By taking into account additional contributions to G_0SW through Monte Carlo simulation of a large number of energy-minimized conformers, a new multiconformer model was developed for flexible molecular systems that showed a 67% improvement in the predictive accuracy for G_0SW and HTP for a series of chiral dopants previously evaluated by the single-conformer model. The single-conformer model was also applied successfully to a series of rigid azobenzene molecular systems to accurately predict HTP for both geometric isomeric forms, which to our knowledge, is the first time that any quantitative chirality calculations have been attempted with compounds that exhibit a strong relationship between HTP and geometric isomerism.

We demonstrate the use of an acousto-optic modulator to enhance the refresh rate and dynamic properties of a liquid-crystal spatial light modulator (SLM). The useful area of the SLM surface is split in several zones which are addressed separately, and read in a sequence by a steered laser beam. This configuration allows to increase the refresh rate by five orders of magnitude. Furthermore, improvements on the nature of the transition between different holograms are experimentally shown. The advantages of this technique are discussed in the particular context of cold atom manipulation with holographic optical tweezers.

We have investigated the physical and optical properties of the left-handed chiral dopant ZLI-811 mixed in a nematic liquid crystal (LC) host BL006. The solubility of ZLI-811 in BL006 at room temperature is ~24 wt%, but can be enhanced by increasing the temperature. Consequently, the photonic band gap of the cholesteric liquid crystal (CLC) mixed with more than 24 wt% chiral dopant ZLI-811 is blue shifted as the temperature increases. Based on this property, we demonstrate its applications in thermally tunable band-pass filters and dye-doped CLC lasers. In addition, we also demonstrated a spatially tunable laser emission by generating a one-dimensional temperature gradient along the dye-doped cholesteric liquid crystal (CLC) cell. The lasing wavelength is widely tunable from 577 nm to 670 nm. The lowest excitation energy and maximum lasing efficiency occur at λ~605 nm which corresponds to the peak fluorescence emission of the dye.

A series of ferroelectric liquid crystals consisting new glassy liquid crystals (GLCs) as chiral dopants were prepared and evaluated for their potentials in fast switching ability less than 1 ms. The properties of pure ferroelectric glassy liquid crystals (FGLCs) and mixtures were reported in this paper. In particular, the novel FGLC possessing wide chiral smectic C mesophase over 100 °C is able to suppress smectic A phase of host. The mixture containing 2.0 % GLC-1 performs greater alignment ability and higher contrast ratio than R2301 (Clariant, Japan) in a 2 μm pre-made cell (EHC, Japan). These results indicate that novel FLC mixtures consisting glassy liquid crystals present a promising liquid crystal materials for fast switching field sequential color displays.

Liquid crystal on silicon (LCoS) devices have been exploiting the ever-diminishing CMOS silicon process, which is pushing towards 45nm dimensions. Consequently, such fine metallic structures are bound to influence the alignment of the liquid crystal material. To illustrate this, a number of 1D metal patterns with differing mark-to-space ratio were fabricated using an Electron-Beam exposure technique. The results confirmed Dwight Berreman's topological alignment theory regarding the pitch of the surface topography and how this influences the quality of the planar alignment. Patterns with a metal to spacing ratio of 1:1 were shown to yield higher contrast ratios and hence better planar alignment. Such findings could be useful for developing non-intrusive alignment methods for nanoscale LCoS devices.

One of the key factors affecting the performance of liquid crystal devices is the fringing field effect. This effect is the principal cause for the current resolution limitations of LCDs as well as the reduction in both the maximum deflection angle and the diffraction efficiency of beam steering devices. Recent studies in the reduction of the fringing field effects will be presented with applications in the development of ultra-small pixel sizes in LCD's and high performance LC-beam steering devices. A particular implementation using Gires-Tournois structure will be discussed. Another area of research to be discussed is a study of the fundamental limits of LCs and other electro-optic materials, with respect to their electro-optic coefficient. Fundamental physical limitations based on material stability considerations will be presented.

In a liquid crystal light-valve with optical feedback we show that localized structures can be stored as elementary pixels lying on matrices that are impressed in a small pretilt angle of the nematics contained in the valve. The molecular reorientation, which is induced by the optical feedback, is bistable and may be controlled by a suitable phase profile superimposed on the input beam.

We report our recent experimental results on a new polarization-independent, liquid crystal (LC) spatial light modulator (SLM). Based on a periodic nematic director profile, the modulator acts as a switchable diffraction grating with only 0^th- and ±1^st-orders at efficiencies of ≥ 99%, manifests contrast ratios ~600:1 (for laser light), switching times of ~2ms, and threshold voltages of < 1V/μm. Results of modulating broadband, unpolarized light from light-emitting-diodes (LEDs) indicates that contrast ratios are ~100:1 so far. Note that incoherent scattering for visible light is very low, and that samples are typically completely defect-free over large areas. An important feature of this diffractive polarization-independent SLM compared to its predecessors is its potential to achieve much larger diffraction angles, which enables a larger aperture (and etendue). In addition to describing the fabrication and characteristics of this SLM in general, we report on our initial progress in implementing a projection display system. All of the surprising and useful results from this grating arise from its continuous nematic director, which is most properly classed as a switchable polarization grating (PG). The SLM described here offers the inherent advantages polarization-independence at the pixel-level and fairly fast switching with nematic LCs, while maintaining similar switching voltages, cell thickness, contrast ratios, and materials.

We report on the initial development of a visible initiator for thiol-ene photopolymerization using the 647 nm radiation from a Krypton ion laser. The photoinitiator system consists of the dye oxazine 170 perchlorate and the co-initiator benzoyl peroxide. Electron transfer occurs between the singlet excited state of the oxazine dye and benzoyl peroxide with subsequent decomposition of the peroxide yielding benzoyl oxy radicals capable of free radical initiation. We demonstrate that this photoinitiation system enables holographic patterning of HPDLC gratings as initial Bragg transmission gratings with a periodicity less than one micron using 647 nm radiation. These gratings were electrically switchable between a diffractive and transmissive state. Morphology studies using bright field transmission electron microscopy (BFTEM) indicate the phase separation of nearly spherical shaped nematic liquid crystal droplets of several hundred nanometers in diameter. This demonstration suggests that reflection gratings can be written using this photoinitiator system and 647 nm radiation which have switchable notch wavelengths approaching 2 microns.

A review is given of recent theoretical and experimental studies on the liquid crystal (LC) infiltration of 3D photonic crystal (PC) structures so as to obtain tunable Bragg reflection and transmission characteristics. It is shown that large-pore and non-close-packed inverse opals formed by sintering, or by a multiple-layer conformal deposition technique, provide a simple and effective dielectric scaffold for liquid crystal infiltration. The dynamic optical properties are strongly dependent on the scaffold structure and the dielectric contrast between the scaffold and the LC. Experimental structures were fabricated using precise, conformal, low temperature atomic layer depositions of Al₂O₃ and TiO₂ to create inverse opals and non-close-packed inverse opals, which were subsequently infiltrated with the nematic liquid crystals 5CB and MLC2048. The dependence of the visible/infrared reflectance and transmittance were investigated as functions of applied electric field amplitude and frequency for applications in optical modulation and switching.

Holography is one of the most promising techniques that enables ultra-high density optical data storage with a simple and small optical setup. The key component in holographic devices is a holographic material that meets various requirements such as high diffraction efficiency, fast response, and stability. Furthermore, additional problems, rewritability and reversibility, must be solved in rewritable holographic materials which are more convenient. In this paper, we successfully achieved rewritable holograms with 55 multiplicities by a simple formulation of optically transparent copolymer films containing azobenzene and mesogenic moieties. The recorded hologram was stable for more than six months and was rewritable over 300 times. In addition, holograms with angle and polarization multiplicity were recorded by controlling the incident angle and the polarization direction of writing beams independently.

In a photorefractive liquid crystal light valve with optical a feedback we show pattern formation and we use the properties of the optical structures to perform a measurement of the spatial resolution of the device. Then, we show experimentally and theoretically how the Talbot effect can be used to enhance the nonlinear response of two or more light valves in cascade.

Dye-doped nematic liquid crystal samples where the vector director is not pre-aligned exhibits simultaneously positive and negative nonlinear refractive index under cw illumination at room temperature, regardless the polarization state of the illuminating beam. However, its relative contributions are polarization dependent. Experimental Z-scan curves for 100 μm thick methyl red doped 5CB nematic liquid crystals, demonstrate that negative nonlinearity is an order of magnitude larger than the positive. The polarization state of the transmitted beam is change to elliptical carrying information about the positive and negative nonlinearities.

We describe and analyze laser trapping of small colloidal particles in a nematic liquid crystal, where the index of refraction of colloidal particles is smaller compared to the indices of the liquid crystal. Two mechanisms are identified that are responsible for this anomalous trapping: (i) below the optical Freedericksz transition, the trapping is due to the anisotropic dielectric interaction of the polarized light with the inhomogeneous director field around the colloidal particle, (ii) above the optical Freedericksz transition, the optical trapping is accompanied by the elasticity-mediated interaction between the optically distorted region of a liquid crystal and the particle. In majority of the experiments, the trapping above the Freedericksz transition is highly anisotropic. Qualitative agreement is found with a numerical analysis, considering nematic director elastic distortion, dielectric director-light field coupling and optical repulsion due to low refraction index colloid in a high index surroundings.

We demonstrate how liquid crystal cladded metallo-dielectric and all-dielectric frequency selective surfaces (FSS) can perform as broadband tunable optical filters and planar negative index optical materials. Structures are designed using a genetic algorithm technique that takes into account the specifics of nanofabrication techniques to maximize the effective optical material response and minimize losses as the birefringent nematic liquid crystal (NLC) overlayer is tuned.

The critical role of buffing strength of poly(vinyl cinnamate) (PVCi) surface for liquid crystal (LC) alignment is reported. It has been found that weakly and strongly rubbed PVCi films provide LC alignment which is perpendicular and parallel to the rubbing direction, respectively. Surface-specific sum-frequency vibrational spectroscopy have been used to investigate the structure of poly(vinyl cinnamate) (PVCi) surface on a molecular level. Quantitative analysis provides an approximate orientation distribution function (ODF) for the aligned cinnamate moieties and the polymer backbones. Weak rubbing aligns the phenyl ring of the cinnamate side-chains perpendicular to the rubbing direction, with the main axis along the surface normal. Strong rubbing orients the phenyl rings toward the plane normal to the surface along the rubbing direction.

We study the phenomena of laser beam propagation in azobenzene liquid crystals (azo LCs) in waveguiding configuration. We found that spatial solitons can be formed at microwatt power levels of a He-Ne laser beam. We have observed several well-known processes of nonlinear propagation such as undulation of solitons, their interaction and merging. In addition, we have shown that due to the memory of the nonlinear waveguide in azo NLC, the solitons are steered following mechanical displacement of the LC cell, a phenomenon that can be used for stabilized coupling of radiation into optical fibers. Cis-trans isomerization of azobenzene molecules and related change in the LC order parameter is the underlying mechanism of optical nonlinearity that makes possible formation of solitons. This mechanism is present in the mesophase as well as in the photoinduced isotropic phase of the material. We have shown that the photoinduced isotropic phase may be locally induced to form a waveguide that steers the solitons to large angles without noticeable attenuation of the beam at distances ~ 1 cm. We have presented a simple 3-level theory for describing the complexity of the effects of laser beams on azo LCs.

Poly(phenylacetylene)s and poly(1-alkyne)s containing chiral sterol pendant groups with molecular structures of -[HC=C-C₆H₄-CO₂-R]_n-, -[HC=C-C₆H₄-O(CH₂)₁₀-CO₂-R]_n- and -[HC=C(CH₂) _mCO₂-R]_n-, (where R = cholesterol, stigmasterol, ergosterol and m = 2, 3, 8} are designed and synthesized. The monomers are prepared by esterifications of acetylenic acids with cholesterol, stigmasterol, and ergosterol and exhibit cholestericity at high temperatures. Polymerizations of the monomers are effected by WCl₆-Ph₄Sn, MoCl₅-Ph₄Sn, and organorhodium catalysts, giving high molecular weight (M_w up to 8.0 × 10⁵) polymers in high yields (up to 99%). The structures and properties of the polymers are characterized and evaluated by IR, NMR, TGA, DSC, POM, X-ray, UV, and CD analyses. All the polymers are thermally stable (greater than or equal to 300 °C). Polymers with long flexible alkyl chains form smectic and cholesteric phases at elevated temperatures. With an increase in the spacer length in poly(1-alkyne)s, the packing arrangements of the mesogenic pendants in the mesophases change from bilayer or mixed mono- and bilayer into homogeneous monolayer structures. Few poly(phenylacetylene)s show CD bands in the absorption region of the polyacetylene backbones, revealing that the main chains are helically rotating with a preferred screw sense.

Phase contrast microscopy, is a technique that can be used to produce high-contrast images of transparent objects. The technique employs a phase mask, at the object Fourier transform plane, to create a synthetic reference wave that interferes with the object wave at the image plane. However, the fabrication and alignment of these masks is an expensive and delicate process. In this work, we present a nonlinear phase contrast microscope that can be implemented with a conventional optical microscope using a low power CW coherent light source to illuminate the specimen. An intensity dependent refractive index material is used to photoinduce the filter. Therefore, the aligning procedure is greatly simplified. The nonlinear material is a thin cell of dye doped liquid crystal where it is possible to produce a tunable phase delay depending on the incident light intensity, the light polarization, and the temperature. Due to these characteristics the resulting setup is relatively inexpensive, easy to implement, and extremely robust.

We report a numerical analysis of the liquid crystal polarization grating (LCPG) as an electro-optically controlled, polarization independent light modulator. The 2D finite-difference time-domain (FDTD) modeling for periodic anisotropic structures has been developed as a numerical tool to study optical properties of anisotropic gratings. Both normal and oblique incidence cases are successfully implemented for wide-band analysis. Nematic director profiles of the LCPG are obtained from elastic free-energy calculations using a commercial software tool, called LC3D. A study of the essential diffraction characteristics of the LCPG is presented, which manifests pixel-level light modulation with a nearly 100% efficiency on unpolarized light. The effect of an off-axis input and the grating regime on the LCPG diffraction is investigated. Finally, we present a study of the electro-optical response of the LCPG when an electric field applied for both static and dynamic cases. The FDTD results show that a highly efficient, polarization-independent light modulation with capability of an electrical switching/tuning is possible by the LCPG.