Front Matter: Volume 8642

Lyotropic liquid crystals exhibiting nematic phases were obtained from the mixtures potassium laurate/alkali sulfate salts (M₂SO₄)/1-undecanol (UndeOH)/water and sodium dodecyl sulfate (SDS)/M₂SO₄/1-dodecanol (DDeOH)/water, where M₂SO₄ represents the alkali sulfate salts being Li₂SO₄, Na₂SO₄, K₂SO₄, Rb₂SO₄ or Cs₂SO₄. The birefringences measurements were performed via laser conoscopy. Our results indicated that cosmotropic and chaotropic behaviors of both ions and head groups are very important to obtain lyotropic biaxial nematic phase. To obtain the biaxial nematic phase, surfactant head group and ion present in lyotropic mixture have relatively opposite behavior, e.g. one more cosmotropic (more chaotropic) other less cosmotropic (less chaotropic) or vice versa.

Defects in liquid crystals have been studied over decades to disclose information and knowledge on the structure of LC phases. More recently, LC defects have been identified as a tool to implement new physical functions useful in optical films for polarization conversion or mechanical actuators able to adopt novel exotic shapes. In the present paper we describe a general methodology to engineer different defect patterns by combining the use of linear photopolymerizable polymers and liquid crystals.

An overview on recent applications of the finite-element method Maxwell-solver JCMsuite to simulation tasks in nanooptics is given. Numerical achievements in the fields of optical metamaterials, plasmonics, photonic crystal fibers, light emitting devices, solar cells, optical lithography, optical metrology, integrated optics, and photonic crystals are summarized.

Various liquid crystal (LC) phase shifters that operate in the super-high-frequency electromagnetic-wave regions have been investigated using planar-type excellent waveguides such as the microstrip line (MSL) and coplanar waveguide (CPW). First planar-type LC phase shifters were constructed using MSL, which was developed as an excellent planar waveguide for super-high-frequency electromagnetic waves. CPW-type LC phase shifters have attracted continued attention, because when they are used, all the signal and ground electrodes are at the same surface, which leads to ease in integration for constructing various functional devices. However, they suffer from an essential drawback of degradation in the phase shift magnitude, which is because the propagating electromagnetic waves encounter the permittivity of both the substrate and the LC materials, which reduces the modulation effect of the LC materials to less than half. In this work, a novel MSL-type LC phase shifter is investigated to achieve excellent phase shifting performance while maintaining ease in integration, as offered by the CPW-type phase shifter. Several device structural parameters are investigated to improve the transmission and phase shifting properties. Some LC materials are also tested for further improvement in the high-frequency operation extended to the millimeter-wave region.

Polymer network liquid crystal (PNLC) is attractive for many photonic applications because of its fast response time and large phase modulation. However, the voltage-on state light scattering caused by multi-domains of LC molecules hinders its applications in the visible and near infrared regions. To reduce domain sizes and eliminate scattering for λ=1.06 μm and 1.55 μm, we studied the effect of LC viscosity on domain sizes. PNLCs based on five different LC hosts were prepared. The LC host was first mixed with 6% reactive mesogen and then filled into a 12-μm cell with homogeneous alignment. After UV curing, we measured the on-state transmission spectra of these five PNLCs. By fitting the transmission spectra with Rayleigh-Gans-Debye model, we can estimate the average domain sizes. We found that the domain sizes of PNLC are inversely proportional to the rotational viscosity of the LC host. This finding can be explained by the Stokes-Einstein equation. As a result, PNLC with a slower diffusion rate would cause smaller domain sizes, which in turn lead to faster response time. To achieve a slower diffusion rate, we cured the PNLC samples at a lower temperature. By selecting a high viscosity and high Δε LC host, we demonstrate a scattering-free (<3%) 2π phase modulator at λ=1.06 μm and λ=1.55 μm. Temperature affects the PNLC performance significantly. As the operation temperature increases from 25oC to 70oC, the response time drops from 220 μs to 30 μs. 2π operating voltage for λ=1.06 μm slightly increases from 65V to 85V. Meanwhile, hysteresis decreases from 7.7% to 2%. For λ=1.55μm, operating voltage is 100V. If reflective mode is employed, operating voltage can be reduced to 55V.

During the past decade, photonic band edge lasers based on cholesteric liquid crystals (CLCs) have attracted considerable interest as self-assembled coherent, tunable lights sources. We report on recent progress towards practical applications: (i) Electrical fine tuning of laser emission by in-plane electric fields: the field-induced distortion of the cholesteric helix allows for a controllable, continuous and reversible shift of the band edge resonances. (ii) Improvement of lasing performance by application of an electric field along the helical axis of a system with negative dielectric anisotropy: the electric field stabilizes the soft photonic structure against heating-induced distortions. (iii) PDMSenclosed LC lasers for lab-on-a-chip applications: We demonstrate the formation of a uniform planar cholesteric texture between polydimethylsiloxane (PDMS) substrates and narrow-band laser emission of a PDMS-enclosed LC laser. With PDMS being the standard material for the fabrication of microfluidic devices, this opens a simple and flexible route for the integration of coherent light sources in lab-on-a-chip designs.

High-damage threshold liquid crystal optical elements are used for high-power laser applications as mirrors, waveplates and optical power limiters. Several nonlinear optical effects under high-power, nanosecond laser irradiation of liquid crystals will be outlined: (1) athermal helical pitch dilation and unwinding of cholesteric mirrors by the field of a light wave (in free space and in a laser resonator); (2) dependence of nonlinear refraction of liquid crystal on the laser beam diameter in presence of two-photon absorption; (3) cumulative effects in nonlinear absorption and refraction at low repetition rate (5-10 Hz); (4) feedback-free kaleidoscope of patterns in dye-doped liquid crystals (hexagons/stripes).

In this paper, the electrically-tunable liquid crystal (LC) lenses and the applications are reviewed. We introduce the basic mechanism of LC phase modulation first. LC lenses are categorized based on different operating principles: 1) Gradient Index (GRIN) LC lenses with a homogeneous cell gap, 2) non-GRIN LC lenses with an inhomogeneous cell gap, 3) diffractive LC lenses, and 4) LC lenses controlled by polarizations. To remove the polarization independency, we also summarize polarization independent LC phase modulations. Many promising applications based on LC lenses are also summarized, such as imaging system, pico projectors, optical zoom systems, ophthalmic applications, and solar systems.

Several liquid crystal (LC) modes, such as twisted nematic, vertical alignment (VA), and in-plane switching, have been in competition with each other in the LC display market. Among them, the VA mode has been widely used because of the high contrast ratio. Since the LC molecules are aligned perpendicular to the substrate in the initial state, an excellent dark state can be obtained at normal viewing direction. However, effective phase retardation of LC layer at oblique viewing direction differs greatly from that at normal viewing direction. Thus, gamma distortion phenomenon occurs at oblique view direction. To reduce the gamma shift in the VA mode at oblique viewing direction, multi-domain VA modes were proposed. Although gamma shifts of these modes are smaller than that of the single domain VA mode, the problems still remain. Recently, several technologies for 8-domain alignment have been proposed to decrease the gamma shift at off-axis. However, additional driving circuits are required to realize the eight-domain structure. In this paper we report technologies for the multi-domain VA mode with no additional driving circuits. By using the proposed technologies, we can obtain the dual threshold voltage in each sub-pixel to realize the multi-domain VA mode with no decrease of contrast ratio.

In this work we show resonant transfer of light from a planar polymer waveguide into a high index solid microsphere (BaTiO3) or nematic liquid crystal microdroplet. BaTiO3 spheres were deposited on the waveguide surface either in dry form or as dispersion in pure water. On the other hand nematic liquid crystal (NLC) droplets were dispersed in a 10 mM sodium dodecyl sulfate (SDS) in water that promoted perpendicular surface anchoring of 5CB and therefore radial droplet configuration. Planar waveguides were produced by spinning a high refractive index polymer (1.68 at 632 nm) onto a soda lime glass. We used two different sources of light, either 671 nm diode laser or the supercontinuum (SC) laser for the mode launching into the thin film waveguide using a prism film coupler. The resonant tunneling of light from the waveguide into the high index spheres and LC microcavities was observed in the case of SC illumination, because the spectrum of light radiated from the both microcavities clearly showed whispering gallery modes.

This study demonstrates all optical switches between the four diffractive light levels of a body-centered tetragonal photonic crystal. The sample is based on holographic polymer-dispersed liquid crystals that are fabricated using a twobeam interference with multiple exposures. The switching mechanism bases on the effective index modulation of the PC that contains a liquid crystal/azo-dye mixture could be controlled by two pumping laser beams. The switching time between the blue-laser-pumped and the blue-and-green-laser-pumped levels is fast.

A serial of LC gratings are fabricated mainly based on photoalignment, which include (1) Nematic LC grating with alternating 90° twisted nematic (TN) regions and homogeneous alignment (PA). Both 1D and 2D diffraction gratings are demonstrated by periodic photoalignment of sulfonic azo-dye (SD1) films with a linearly polarized light beam. (2) A polarization independent of 1D/2D LC gratings with alternate orthogonal homogeneously aligned regions. No polarizer is employed. (3) A polarizer-free submillisecond response grating employing dual-frequency LC (DFLC) together with patterned hybrid aligned nematic (HAN) structures. To obtain instantly controllable LC microstructures rather than simple gratings, a digital micro-mirror device (DMD) based a micro-lithography system is developed. It may generate arbitrary micro-images on photoalignment layers. Besides normal phase gratings, more complex 2D patterns including quasicrystal structure are demonstrated, which give us more freedom to develop microstructured LC based photonic devices.

We studied highly anisotropic periodic structure formed by liquid crystal and polymer phase separation, or holographic polymer dispersed liquid crystal (HPDLC), by θ−2θ optical diffractometry particularly employing the high order Bragg diffractions. Then, we revealed the relationship between microscopic structure and diffraction properties of the HPDLCs prepared at several different conditions with regard to the compositional ratio of the raw materials and the exposure temperature. The high order Bragg diffractions are closely connected to periodic boundaries between LC droplets and polymer, and is potentially able to be designed by HPDLC fabrication processes. The analysis with the information from the higher order Bragg diffraction is expected to allow you to observe more detailed and quantitative HPDLC internal structure.

Modern holographic techniques based on Spatial Light Modulators get serious benefits from providing uniform intensity distribution of a laser beam: more predictable and reliable operation, higher efficiency of laser energy usage, more simple mathematical description of diffraction transformations, etc. Conversion of Gaussian intensity distribution of TEM00 lasers to flattop one is successfully realized with refractive field mapping beam shapers like piShaper, which operational principle presumes transformation with high flatness of output wavefront, conserving of beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field, negligible residual wave aberration, and achromatic design provides capability to work with several laser sources with different wavelengths simultaneously. Applying of these beam shapers brings serious benefits to the Spatial Light Modulator based techniques like Computer Generated Holography, Dot-Matrix mastering of security holograms, holographic data storage. This paper will describe some design basics of refractive beam shapers of the field mapping type and optical layouts of their applying in holographic systems. Examples of real implementations and experimental results will be presented as well.

The electromagnetic response of metamaterial can be managed by combining resonances and interferences of different materials and on different lengths scales. In our contribution we study composite metamaterials containing resonant plasmonic metallic nanoparticles that show organization. The material bases its non-conventional properties on short distance self-organization by mesogens that form a liquid crystal material. We analyze the properties of such materials with a structural model containing organized nanoparticles. Theoretically insight of the electromagnetic properties is provided and we give details on their optical properties.

Over the last decades, scientists have paid growing attention towards the terahertz properties of liquid crystals. On the one hand, the dielectric properties of liquid crystals are relatively unexplored at terahertz frequencies, and the observed low-energy phenomena are not yet well understood. On the other hand, terahertz technology requires switchable devices, in which liquid crystals could potentially serve as a base material. This paper gives an overview of the research done so far on the properties and applications of liquid crystals in the terahertz frequency range. The path from first liquid-crystal terahertz experiments to comprehensive studies of their structure-property relation is outlined. Furthermore, the evolution from basic concepts to first liquid-crystal terahertz devices is sketched, and prospects as well as future challenges are discussed. Due to the development of compact and cost-efficient components, terahertz spectrometers matured from room-filling laboratory instruments to compact, reliable scientific tools. Modern terahertz systems are thus also covered in this report. Liquid-crystal devices could help terahertz technology continue this trend, and pave the way to a wider range of application.

Nematic liquid crystal (NLC) is one of most useful soft matters. Because the molecular orientation can be controlled electrically, NLC is widely applied to display devices. It is known that NLC exhibits strong second-harmonic-generation (SHG) due to its orderly arranged molecules. The strength of SHG is strongly dependent on the angle between the incident beam polarization and the NLC molecular orientation, so the SHG in NLC can be switched on/off by rotating the NLC director. However, it is very difficult to control the orientation of NLC director electrically within a micrometer spatial domain. In this report, we demonstrated the orientation control of NLC with sub-micrometer spatial resolution based on optical Freedericksz transition (OFT) combined with a high-numerical-aperture objective. We used azo-dye doped NLC to reduce the intensity threshold of OFT with 473-nm excitation. Interestingly, we found that the threshold of OFT increases with tighter focuses. This effect can be explained by the intermolecular forces from the NLC molecules around the focal spot. By incorporating both the blue laser and a femtosecond near-infrared laser into an optical scanning microscope, we have successfully demonstrated switch of SHG inside a NLC thin film. Note that SHG is confined within femtoliter focal volume due to its intrinsic nonlinearity. That is, we have achieved an ultrasmall switch of nonlinear optical signal in NLC. This work will find applications in optical communication as well as optical-base storage system.

The doping of nanoparticles of PγCyD-ZrO2 and those hydrophobic SiO2 and TiO2 into brings tremendous changes in their optical properties such as the reduction of birefringence, the creation of an optical homogenizing effect and the enhancement of the performance of FSC-LCD that produces a large value of Luminance Efficiency; this paper reports these novel optical effects in LCDs.

We propose a method for fast switching of nematic liquid crystals with neither alignment materials nor alignment process. A three-terminal electrode structure is used to apply in-plane and vertical electric fields to randomly-aligned liquid crystals. A vertical field is applied to align liquid crystals vertically for the dark state, whereas an in-plane field is applied to align liquid crystals homogeneously for the bright state. We obtained the turn-on time of 1.2 ms and turn-off time of 0.5 ms in the three-terminal electrode structure with neither alignment materials nor alignment process. However, three-terminal electrode structure with neither alignment material nor alignment process shows low transmittance. For higher transmittance, we mixed reactive mesogen and nano-particles with anisotropic molecular shape to liquid crystals. As a result, we obtained a transmittance similar to the conventional fringe field switching mode and achieved the total response time of less than 3 ms.

The concentration level of bile acids is a useful indicator for early diagnosis of liver diseases. The prevalent measurement method in detecting bile acids is the chromatography coupled with mass spectrometry, which is precise yet expensive. Here we present a biosensor platform based on liquid crystal (LC) films for the detection of cholic acid (CA). This platform has the advantage of low cost, label-free, solution phase detection and simple analysis. In this platform, LC film of 4-Cyano-4'-pentylbiphenyl (5CB) was hosted by a copper grid supported with a polyimide-coated glass substrate. By immersing into sodium dodecyl sulfate (SDS) solution, the LC film was coated with SDS which induced a homeotropic anchoring of 5CB. Addition of CA introduced competitive adsorption between CA and SDS at the interface, triggering a transition from homeotropic to homogeneous anchoring. The detection limit can be tuned by changing the pH value of the solution from 12uM to 170uM.

Liquid crystal mixtures dedicated to microwave application have been developed. They have high tuning ranges, low dielectric losses and high reliability against the heat stress. These mixtures help to realize agile LC-based microwave components and devices, such as phase shifter, tunable filter, phased array- or reflect array antennas.

Microcavity resonance is demonstrated in nanocrystal quantum dot fluorescence in a 1-D chiral photonic bandgap cholesteric liquid crystal host. The resonance demonstrates coupling between quantum dot fluorescence and the cholesteric microcavity. Observed at a band edge of a photonic stopband, this resonance has circular polarization due to microcavity chirality with 4.9 times intensity enhancement in comparison with polarization of the opposite handedness. The circular polarization dissymmetry factor ge of this resonance is ~1.3. We also demonstrate photon antibunching of a single quantum dot in a similar glassy cholesteric microcavity. These results are important in cholesteric laser research, in which so far only dyes under pulsed excitation were used, as well as for room-temperature single-photon source applications.

We report on dispersion of a small amount of colloids in a blue phase liquid crystal, and the blue-phases were found to exhibit extended temperature range. The stabilized temperature range of the blue phase was a function of their most effective concentration, different sizes of colloid particles and shape of colloidal particles. The temperature range was probed and determined and the temperature range of the blue phases was found to decrease as the colloidal particle size increased. Additionally, the temperature dependent of Bragg wavelength peak was found to redshifted in the colloidal-BP mixtures. The electro-optical results, especially the switching voltage and response time suggested that concentration and size of dispersed colloids modified the elastic energy of the blue phase liquid crystal composites and led to a lower switching voltage.