We report on the photoactuation of high modulus liquid crystalline polymers with side chain azobenzene mesogens (azo-LCP). Photomechanical deformation is actuated with the 442 nm line of a He-Cd laser. Early results show that this system achieves displacement of up to 85° towards the actinic source when the He-Cd polarization is parallel to the nematic director and approximately 30-40° when the polarization is orthogonal to the nematic director. A wide range of materials have been synthesized by the photopolymerization of a liquid crystal diacrylate monomer (RM257) and an azobenzene-containing monoacrylate liquid crystalline monomer. Interestingly, azo-LCP formed with holographic photopolymerization have faster bending rates than polymers formed with flood-lit illumination of equivalent intensity.

Oblique evaporation of inorganic materials has long been used to induce alignment in liquid crystals, often for the purpose of controlling the pretilt angle in a liquid crystal cell. These alignment layers are relatively dense, keeping the liquid crystals above the surface of the inorganic layer. By evaporating at increasingly oblique angles (> 80°), the alignment layer can be made porous, allowing liquid crystals to infiltrate the film and to align to individual nanostructures. By coupling simultaneous computer controlled substrate motion during evaporation, a process known as glancing angle deposition (GLAD), the nanostructures can be grown in a variety of useful shapes, including helices, polygonal spirals, zigzags and periodically bent S-shaped columns. Alone, these films exhibit properties such as linear and circular polarization selective Bragg reflection, and full three-dimensional photonic bandgaps. By infiltrating liquid crystals into the voids of the film, one can align liquid crystals in three dimensions, as well as tune and switch the film's optical properties. Additionally, the GLAD film can be used to template polymerizable liquid crystals for subsequent monomer infiltration. In this work, using spectroscopic ellipsometry, we examine the effects of liquid crystal infiltration on various film structures made from a variety of metal oxides, for both varying film thickness and deposition angle. Techniques for filling a porous film with a known volume of liquid crystals are also presented. Additionally, we examine the switching behaviour for these films under applied electric fields. Finally, we compare experimental and simulated results used to predict and optimize the optical properties of these hybrid films.

We introduce and demonstrate a novel tunable optical filter that is insensitive to input polarization. While the most obvious application of this novel filter is in compact spectroscopy, all technologies that are dependent on tunable passband filters can benefit from it. Analogous to Lyot and Solc filters, this filter is constructed of multiple liquid crystal polarization gratings (LCPGs) of different thicknesses. LCPGs are switchable, anisotropic, thin diffraction gratings which exhibit unique properties including diffraction at visible and infrared wavelengths that can be coupled between only the zero- and first-orders, with nearly 100% and 0% experimentally verified efficiencies. Most relevant to the filter concept introduced in this work, the transmittance of the LCPG zeroth order is independent of the incident polarization. When combined with an elemental spatial filter, polarization-independent bandpass tuning can be achieved with minimum loss. The unique filter design enables a high peak transmittance (~90%) that is difficult in competing polarizer-based technologies. In this work we derive the core principles of the tunable filter, present preliminary experimental data, and discuss the capabilities of the filter in terms of finesse, 3dB bandwidth (full-width at half-maximum), and free-spectral-range. We will also evaluate the most likely practical limitations imposed by material properties and fabrication.

A nonlinear optical medium results by the collective orientation of liquid crystal molecules tightly coupled to a transparent photoconductive layer made of a BSO photorefractive crystal. The nonlinear medium, called photorefractive liquid crystal light-valve, gives a large two-wave-mixing gain, thus, when inserted in a ring cavity, it results in an unidirectional optical oscillator. Dynamical regimes with many interacting modes are made possible by the wide transverse size and the high nonlinearity of the liquid crystal gain medium. In particular, we show the generation of spatiotemporal pulses, coming from the random superposition of many longitudinal and transverse modes simultaneously oscillating in the cavity.

The photoalignment of a series of monodisperse glassy-nematic oligofluorenes was investigated on coumarin-containing polymer films in the parallel regime. The orientational order parameter of a spin-cast oligofluorene film, S_OF, was found to decrease with an increasing oligofluorene length because of the increasing demand on Y_dS_d, the product of coumarin dimers' concentration and orientation order parameter. In addition, an increased annealing temperature demanded by a longer oligofluorene caused an orientational relaxation of coumarin dimers to a greater extent. Nonetheless, under favorable conditions S_OF values of penta, hepta-, and nonafluorenes are comparable to those achieved on rubbed polyimide films.

We have studied the tunability of the reflection notch of a cholesteric filter containing a negative dielectric anisotropy LC in a planar alignment. For this purpose, we studied physical, optical and electro-optical characteristics of mixtures containing chiral dopant S811 and the negative dielectric anisotropy liquid crystal ZLI-2806. Interestingly, smectic A phases were seen at room temperature for S811 loadings >20% by weight of ZLI-2806. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) studies confirmed the formation of a cholesteric phase above room temperature. A phase diagram was constructed by varying S811from 9-50% by weight in the mixture. Reflection notchs were not seen at room temperature for compositions of S811 >20%. On heating, the selective reflection notch of the cholesteric phase appeared and blue shifted with temperature. Various methods of tuning the reflection notch were examined. Thermal tuning from 2200 nm to 450 nm was observed over the temperature range 23 to 55° C. Application of a DC field led to electrical tuning (~50 nm) of the notch. The notch was also tuned (>500 nm) optically by exposing a dye doped cholesteric cell to laser lines at either 532 or 647 nm.

A shutter-based free-space optical switching core has been proposed as a promising technology for constructing Storage Area Networks (SANs) over an optical network. A vital component of this switch architecture is the use of a spatial light modulator (SLM) which can enhance the SANs performance. New optical materials are utilized to raise the switching speed and ferroelectric liquid crystals (FLCs) or transparent lanthanum-modified lead zirconate titanate (PLZT) used as an SLM have been compared. Both are capable of reaching the 3 usec target, by either raising the temperature or switching voltage, which is acceptable for SANs since the performance is dominated, not so much by switching speed, but more by reliable robust switching throughput. A six-by-six free-space 12-channel multi-mode fiber ribbon switch system using one fixed wavelength has been implemented. The objective of this paper is to demonstrate that multiwavelength operation based on the CWDM band in each fiber can be implemented on the same shutter-based free-space optical switching architecture using a FLC SLM.

We present the results of nonlinear transmission in various ordered and disordered mesophases of liquid crystals, and demonstrate that in bulk or guided wave geometry, they are capable of clamping the transmission of pulsed or cw lasers to below the Maximum Permissible Exposure level of eyes and optical sensors in the entire visible - infrared region.

Super-structures produced in the smectic molecular organization are reviewed, and the origin for the structures discussed, in which it is emphasized that the frustration plays an important role in the emergence of the super-structures. Properties and structures of mysterious smectic phases possessing chirality-induced super-structures are introduced, including smectic blue and smectic Q (SmQ) phases that possess three dimensional (3D) structures. The molecular design on stabilizing the 3D structure is proposed. The molecular orientations in the 3D structures are not so sensitive to the external electric field due to an intricate smectic ordering, thus it has been difficult to imagine practical applications of these structures. The SmQ compound possessing an azobenzene core in the molecular structure is designed and the liquid-crystalline properties and photoresponse investigated. By means of the photoisomerization of the azobenzene moiety, we demonstrate that the 3D structure of the SmQ phase can be controlled by light as an external stimulus, suggesting a possibility for new applications utilizing the liquid-crystalline 3D structure.

Computational chemistry provides unprecedented opportunities to predict the properties of new materials prior to synthesis. One such important property for optics and photonics applications is optical absorbance. The capability to accurately predict, prior to synthesis, the spectroscopic properties of a series of materials as a function of molecular structure would be an extremely powerful tool in the design and development of new liquid crystal materials, dyes, and dopants intended for use in devices for advanced optics and photonics applications. We have applied time-dependent density function theory (TDDFT) calculations for the first time in the prediction of the absorbance spectra of a series of nickel dithiolene near-infrared (IR) dye complexes with a wide variety of terminal functional groups that are designed to enhance their solubility and stability in liquid crystal host mixtures. The TDDFT method was used to compute the excited-state energies of an existing series of nickel dithiolenes with bulk solvent effects taken into account. Excellent agreement between the theoretical and experimental absorbance maxima was achieved for 14 known dyes with an exceptionally low mean absolute error of 0.033 eV. Calculations conducted on 4 new nickel dithiolene dyes predict that the addition of sulfur atoms to the side chains will increase the maximum absorbance wavelength by up to 160 nm. This improved computational method is being applied to the design and synthesis of highly soluble azobenzene-substituted transition metal dithiolene near-IR dyes that can undergo rapid and reversible photoinduced cis-trans isomerization. Such materials could show substantial promise as photoswitchable near-IR dopants for liquid crystal device applications in telecommunications, sensor protection, nonlinear optics, and laser systems.

Analytical (elastic medium approximation) and numerical (Monte Carlo and molecular dynamics) modelling of an inhomogeneous spatial order in nematic liquid crystal (NLC) molecules in confined systems (e.g. in planar or twisted NLC cells) is widely used for molecular engineering of optical devices based on inhomogeneous spatial distribution of refractive index. We present an overview of recent results obtained viaMonte Carlo approach based on model Lebwohl-Lasher Hamiltonian, concerning optical aspects (index of refraction, diffraction efficiency, transmission) of inhomogeneous molecular order in nematic liquid crystals in confined geometry. The role of anchoring forces and of their fluctuations is discussed, in particular for transmission in Twisted NLC (TNLC) cells. Preliminary results on an influence of molecular order on tunable negative- and zero-index nanosphere dispersed liquid crystals (NDLC) are presented. The phase diagram for real part of effective permittivity in variables external electric (magnetic) field - anchoring strength is calculated.

Here we show the simple fabrication of field effect transistor (FET) with a mesophase semiconductor, a derivative of dithienyl naphthalene, which exhibits a fast mobility (10^-1 ~ 10^-2 cm² V^-1 s^-1) of charged carriers in the mesophase. The compound is a mesogen, but with highly ordered layered structure in a triclinic lattice, meaning a 3D-mesophase is formed. The device performance was studied for the transistor mobility, on/off ratio and threshold voltage of device operation, to have 0.14 cm² V^-1 s^-1, 2 x 10³ and -27 V at room temperature (in a crystal phase), respectively, even though the thin film active layer (100 nm thick) does have a multi-domain system. However, the XRD studies indicate the uniformly aligned molecules in each domain, of which long axis is inclined to be ca. 27° against the axis perpendicular to the substrate plane. This implies that a self-assembling nature of mesogenic molecules is a certain merit for thin film device fabrication in organic electronics.

We present the simultaneous electro-optical and electro-mechanical effects in the nematic elastomers swollen by a low molecular- mass liquid crystal (LMMLC). Under electric fields normal to the initial director, the swollen nematic elastomers exhibit a pronounced change in effective birefringence as a result of director rotation, and they also show a macroscopic strain of more than 10%. This paper mainly focuses on the dynamic aspect of this phenomenon. We evaluate the rise and decay times for the electro-optical and electro-mechanical effects in response to "field-on" and "field-off". The rise time for the optical response at sufficiently high fields reaches a few milliseconds which are comparable to that of LMMLCs. The optical decay times lie about 5 ms (independently of field strength) that is more than two orders of magnitude smaller than that of LMMLCs with the similar cell thickness, because the modulus of rubber elasticity overwhelms the Frank elasticity. The mechanical response is slower than the optical response, but the ratio of the two characteristic times (about 10) is almost independent of field strength as well as whether the field is imposed or removed.

A photochemically tunable structural color material was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores in a SiO₂ inverse opal structure. The SiO₂ inverse opal film infiltrated with the polymer LC reflected a light, which is called a structural color, corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation caused the shift of the structural color band to longer wavelength more than 15 nm. This is caused by the formation of uniaxially anistorpic molecular orientation of the polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable for various optical materials such as memory, display so on.

Holographic techniques allow the recording of homogeneous, high resolution, large-area light intensity patterns in photo sensitive organic materials. The choice of proper experimental conditions easily permits the fabrication of 1D, 2D and 3D periodic and quasi-periodic structures. In this work we present a simple way to fabricate planar complex photonic structures characterized by high dielectric contrast values. To this purpose we used polymer dispersed liquid crystals as photosensitive material for the holographic recording, followed by the removal of the liquid crystal from the recorded structures via a specific solvent, thus obtaining large area regular polymer-air patterns. The obtained structures have been simulated, recorded in the substrates and optically characterized in planar light guided configurations. The relevant optical properties have been analyzed by means of a theoretical approach formally derived from the dynamical theory of x-ray diffraction. The presented experimental technique allows easy fabrication of optical integrated devices to be used either as high sensitivity sensors or in the field of optical telecommunications.

Spatial resolution is an important performance characteristic of spatial light modulators (SLM). This parameter depends on the physical properties of the electro-optical material, as well as on the design features of the SLM. One of the key factors affecting the spatial resolution of liquid crystal (LC)-based SLM is the fringing field effect. This effect can be reduced in thin LC cells with corresponding reduction in the electro-optical response. A strong electro-optic response in thin LC layer can be attained using the Surface Plasmon Resonance (SPR) phenomenon. While SPR-based LC SLMs were already demonstrated about 15 years ago, their development has been hampered by the fact that these devices are expected to have a relatively low resolution, due to the finite propagation length (several tens of micrometers) of the surface plasmons (SP). This study is aimed at improving the spatial resolution of the SPR-SLM by optimizing the metal-dielectric structure of the device. In particular, a small-scale patterning of the metal layer supporting the propagation of SPs is considered a possible solution for reducing the spatial blurring associated with long propagation length of SPs. Detailed computer simulations of the spatial resolution of the SPR-based LC SLM structure have been carried out using both the rigorous coupled wave analysis (RCWA) and the finite difference time domain (FDTD) method. These simulations were performed for an SLM structure based on the well-known prism-type, Kretschmann excitation configuration. The SLM performance for various spatial resolutions was simulated by introducing a dielectric layer with periodically modulated refractive index. The RCWA technique was used for an initial estimate of the SP excitation angle and the optimal thickness of the silver layer supporting the SP propagation. The FDTD method was used for detailed analysis of near and far field spatial distribution of the modulated light. The results of this study showing improved resolution LC-SP-SLM are presented here.r

A new type of polymer hydrogel with a unique organic (polymer)/inorganic (clay) network structure has been synthesized by the in-situ free-radical polymerization of N-isopropyl acrylamide (NIPA) in the presence of exfoliated clay platelets in an aqueous medium. The resulting nanocomposite hydrogels (NC gels) consisting of PNIPA and clay (hectorite) exhibit extraordinary optical, mechanical and swelling properties. NC gels also show a clear phase transition due to the coil-to-globule transition of the PNIPA chains. It was observed that the phase-transition temperature (lower critical solution temperature: LCST), defined as the onset temperature of a steep transmittance drop, shifts to a lower or higher temperature than that of pure water (≅ 34 °C) when conditions are altered. When an inorganic salt, such as NaCl, CaCl₂ and AlCl₃, was added to the surrounding water, the LCST of the NC gels generally shifted to a lower temperature, in a manner almost inversely proportional to the salt concentration. On the other hand, when the NC gels adsorbed cationic surfactant, e.g. hexadecyl trimethyl ammonium chloride, the LCST shifted toward a higher temperature, although the shift and its profile strongly depended on the adsorption conditions, such as the surfactant concentration and the adsorption time. Consequently, non-thermo-sensitive NC gel was obtained by using a surfactant aqueous solution with a concentration higher than the critical micelle concentration.

We have proposed the multidomain patterning of a liquid crystal (LC) alignment by controlling the anchoring strength of alignment surfaces. The azimuthal anchoring strength of rubbed polyvinyl cinnamate (PVCi) is increased by the crosslinking reaction under the unpolarized UV light exposure. The multidomain patterning of twist angle from 0 to 90 degree has been successfully demonstrated in the LC cell using the polyimide coated substrate with strong anchoring and PVCi coated substrate with multi anchoring strength, even though both substrate surfaces are uniformly rubbed and rubbing directions cross at right angles. The twist angle of the LC orientation has been calculated using the torque balance equation. The twist angle in the practical LC cell is consistent with the calculated one. The LC director distribution has also been numerically and experimentally analyzed in the LC cell with one-dimensional periodic change of the anchoring strength. The twist angle distribution is strongly affected by the periodic size, as well as the anchoring strength, the cell thickness and elastic strain energies of K₁₁, K₂₂ and K₃₃.

Nanolithographic fabrication techniques may soon enable electrically-driven LCoS devices to be manipulated using ultra-nanoscale CMOS transistors. However, questions as to the switching properties of such LCoS devices arise due to the diminishing dimensions of their transistors. Thus, experimental investigations into the response times and the onset-threshold voltages for LCoS devices were embarked upon. Such measurements were obtained for various electrode dimensions and cell gaps. Furthermore, an interdigitated (IDT) electrode pattern was used to drive the homeotropically-aligned NLC material in a direction parallel to the bounding planes of the cell. Experimental findings revealed that faster response times were achieved when the electrode spacings were decreased. Such results have shown that a 10μm-thick device with an electrode pitch of 2μm can achieve a switch-on time of < 5ms. In addition, decreasing the electrode spacing results in the threshold voltage to drop. The results therefore indicate that improvements in a LCoS device's switching properties can be realised by using smaller electrode dimensions.

Stimulated Orientational Scattering (SOS) uses the angular reorientation of the director axis in liquid crystals to produce cross-polarized light amplification. Akin to photorefractivity, SOS uses grating formation and the resulting phase-matching to scatter incident radiation into a coherent, cross-polarized signal beam. This paper provides a brief review of the theory underlying SOS, a discussion of the simulation of SOS dynamics, and empirical results of the SOS effect acting in a thin film (300 μm) planar sample of the liquid crystal E7 induced by an Argon ion laser at a wavelength of 488 nm.

We present a theoretical model and some experiment demonstrations of all-optical passive switching processes with 90° twist-aligned nano-doped nematic liquid crystal cells sandwiched between two crossed polarizers. The photosensitive dopants give rise to laser induced dye-assisted director axis reorientation and order parameter modifications, which in turn produce an intensity dependent polarization switching and hence a transmission modulation capability. Experimental observations are in good agreement with our expectation derived from modified Jones matrix analysis and also demonstrate the feasibility of an efficient [microwatt power] low threshold polarization and fast switching [microseconds] all optical limiting device for visible as well as infrared lasers or bright light sources.

New liquid crystals applications are being developed apart from optics, imaging and display systems. A liquid crystal cell can be modeled as a nonlinear capacitor, being its capacitance varied as a function of the applied electric field. A feedback resonator composed of a capacitance and an inductance presents an oscillation frequency depending on the value of these components. A tunable feedback resonator based on a nematic liquid crystal cell as variable capacitance is presented. The circuit has been electrically modeled and validated with experimental measurements. Oscillation frequencies in the range of kHz have been obtained for the combination of the inductance and the liquid crystal cell (capacitance) used in the proposed circuit. New applications for this circuit are currently under study.

Metamaterials are of substantial current interest because they may exhibit unusual and/or configurable optical responses. We studied the optical properties of gold and silver nanoparticles dispersed in different organic liquids in the visible to near-IR. Calculation of the refractive indices of metallic nanospheres or metallic-coated silica spheres in liquid crystals show the possibility of tuning and varying the refractive index by reorientation of the liquid crystal molecules. Measurements of the refractive indices of gold nanoparticles in dodecane were experimentally studied by using spectroscopic ellipsometry and a reasonable agreement with the theoretical results based on Mie scattering was obtained. Finally, the effect of gold and silver nanospheres on the nonlinear absorption properties of an organic liquid (L34, a 4,4'-dialkyl phenyleneethynylene) was studied. The results suggest that metallic nanoparticles dispersed in a host organic fluids can be good materials for fabrication of low and tunable index materials in the visible to near-IR wavelength range, and for the enhancement of the nonlinear absorption of liquids used in switching applications.