Proceedings Volume 7414

Liquid Crystals XIII

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Proceedings Volume 7414

Liquid Crystals XIII

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Volume Details

Date Published: 20 August 2009
Contents: 8 Sessions, 16 Papers, 0 Presentations
Conference: SPIE Photonic Devices + Applications 2009
Volume Number: 7414

Table of Contents

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Table of Contents

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  • Front Matter: Volume 7414
  • Novel Liquid Crystalline Optical Materials
  • Liquid Crystal Lasers: Imaging and Tuning Devices
  • Optical Tuning and Switching: Microwave Application
  • Opto-optical Switching: Nonlinear Optics
  • Optical Switching, Optoelectronic Devices, Optical Manipulation
  • Wave Mixing, Slow Light, Holographic Devices, Nanomachine
  • Poster Session
Front Matter: Volume 7414
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Front Matter: Volume 7414
This PDF file contains the front matter associated with SPIE Proceedings Volume 7414, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
Novel Liquid Crystalline Optical Materials
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Red-green-blue 2 D tuneable liquid crystal laser devices
H. J. Coles, S. M. Morris, A. D. Ford, et al.
In this paper, we review our recent experimental work on coherent and blue phase liquid crystal lasers.We will present results on thin-film photonic band edge lasing devices using dye-doped low molar mass liquid crystals in self-organised chiral nematic and blue phases. We show that high Q-factor lasers can be achieved in these materials and demonstrate that a single mode output with a very narrow line width can be readily achievable in well-aligned mono-domain samples. Further, we have found that the performance of the laser, i.e. the slope efficiency and the excitation threshold, are dependent upon the physical parameters of the low molar mass chiral nematic liquid crystals. Specifically, slope efficiencies greater than 60% could be achieved depending upon the materials used and the device geometry employed. We will discuss the important parameters of the liquid crystal host/dye guest materials and device configuration that are needed to achieve such high slope efficiencies. Further we demonstrate how the wavelength of the laser can be tuned using an in-plane electric field in a direction perpendicular to the helix axis via a flexoelectric mechanism as well as thermally using thermochromic effects. We will then briefly outline data on room temperature blue phase lasers and further show how liquid crystal/lenslet arrays have been used to demonstrate 2D laser emission of any desired wavelength. Finally, we present preliminary data on LED/incoherent pumping of RG liquid crystal lasers leading to a continuous wave output.
Molecular design of main-chain liquid crystalline polyolefin
Naofumi Naga, Masato Sone, Keiichi Noguchi, et al.
Liquid crystalline features of polyolefin have been clearly observed in a polyolefin of poly(methlene-1,3-cyclopentane) (PMCP) obtained from cyclization polymerization of 1,5-hexadiene (HD) using metallocene catalysts. PMCP clearly shows fluidity and birefringence above the glass transition temperature under the ambient conditions, at around room temperature and normal pressure. First order structures of PMCP, such as cyclization selectivity of polymerized HD units, stereo-regularity of 1,3-cyclopentane units, and molecular weight, affect the liquid crystalline features of PMCP. High cyclization selectivity and trans-rich structure of 1,3-cyclopentane units are necessary to induce the liquid crystalline phase. Polarized optical micrographs of PMCP show Schlieren-like texture, which indicates nematic liquid crystalline phase. Clear diffraction patterns are observed in wide-angle X-ray diffraction analysis below the isotropization temperatures. The isotropization temperature decreases with decreasing of molecular weight of PMCP. PMCP with the higher molecular weight takes the longer time for liquid crystallization from isotropic phase in the cooling process. We have also synthesized an optically active PMCP with an optically active metallocene catalyst in the presence of a chain transfer reagent to control the molecular weights of the resulting polymers. Optically active PMCP with relatively high molecular weight shows finger print texture which indicates cholesteric liquid crystal. On the other hand, decreasing molecular weight the optically active PMCP induces nematic liquid crystalline phase.
Electro-optical effects of oxidized multi-walled carbon nanotube doping on holographic polymer dispersed liquid crystal films
Sameet K. Shriyan, Adam K. Fontecchio
In this work we demonstrate an improved electro-optic response and quantify the effect on transmitted wavefront properties of thiolene based reflection mode holographic polymer dispersed liquid crystals (HPDLC) gratings doped with oxidized multi-walled carbon nanotubes (MWNT). Effect of various doping levels on the reflection efficiencies is evaluated and optical spectrometry results indicate a reduction in the reflection efficiency and an anomalous electrooptic behavior at higher doping levels of MWNT especially in gratings with longer pitch where the diffusion length for liquid crystal (LC) is long. Wavefront analysis based on Shack-Hartmann wavefront sensor show an increase in the transmitted RMS wavefront error in a 633nm wavefront after a critical level of MWNT doping. Polarized optical microscopy results indicate that the MWNT do not participate in the photo polymerization induced phase separation hence acting as physical barriers for the counter diffusing LC at high MWNT concentrations. Reduction in overall size of the LC droplets in the LC rich planes, observed using scanning electron microscopy imaging, leads to faster rise and fall times hence quicker relaxation time. Observation of reduced switching voltage is attributed to the modification of dielectric properties of the medium manifested by an increase in capacitance and decrease in resistivity in presence of MWNT.
Liquid Crystal Lasers: Imaging and Tuning Devices
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Microscopic birefringence imaging by phase-shift interferometry using a liquid crystal phase shifter
T. Nose, Y. Terui, M. Mizumoto, et al.
It is known that the phase-shift intererometry is promising measurement method for the precise optical test. Liquid crystal (LC) phase shifter is very attractive as an electrically tunable phase sifter for the key component of the test system. We adopt here a bend aligned liquid crystal cell for the fast phase shifting, and mount it on the optical system of polarization microscope. A potential application for precise 2D birefringence measurement system is investigated and as high as a few tenths of a wavelength resolution can be obtained by the simple 4-step phase shifting technique. Obtained image data show the phase profile which corresponds to retardation distribution of the prepared sample, and then it becomes possible to perform quantitative analysis of 2D birefringence distribution in planar sample. Since there is basically a setting angle dependency of the test sample, measurement phase data have information of birefringence sign (positive or negative) and we can distinguish between the direction parallel and perpendicular to the anisotropic axis. We observe tiny marine zooplankton as a weakly anisotropic actual sample, and the birefringence of the muscle can be clearly detected. It is also successfully pick up that the elongated direction of muscles show higher index value.
Optical Tuning and Switching: Microwave Application
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New wavelength-tuning method in optical ring resonators with liquid crystal cladding: exploiting the longitudinal E-field
We demonstrate tuning of the resonance wavelength of silicon-on-insulator optical ring resonators. The devices are clad with a layer of nematic liquid crystal. The electrooptic effect of the anisotropic liquid crystal allows us to change the effective index of the TE waveguide mode with an externally applied voltage. The electric field will reorient the liquid crystal director which alters the refractive index of the cladding layer. The evanescent tails of the waveguide mode feel this change. The change in effective index has a direct effect on the resonance wavelength. In our setup, the director tilts from an orientation parallel to the waveguides to an orientation perpendicular to the substrate. This way, it is the longitudinal component of the electric field of the light that experiences the largest change in refractive index. Starting from this principle, we show experimental tuning of the resonance wavelength over 0.6nm towards shorter wavelengths. Theoretical considerations and simulations with a finite element modesolver capable of handling full anisotropy confirm the experimental results and provide insights in the tuning mechanism.
Dynamic control of liquid-crystalline helical structures with the aid of light- and temperature-driven multi-stable chiral materials
Masuki Kawamoto, Natsuki Shiga, Takuya Aoki, et al.
Light- and temperature-driven multistable chiral materials composed of a binaphthyl moiety as a twisting part and an azobenzene moiety as a photoresponsive part in a single component were investigated. It has been found that the materials show reversible change in a dihedral angle of the twisting part at various temperatures. In contrast, no change in the twisting angle of the materials during trans-cis photoisomerization was observed. Furthermore, the chiral compounds were dissolved in nematic (N) liquid crystals (LCs) to produce a chiral N phase with a helical structure. The mixture exhibited a dual molecular response to temperature and light. Helical pitch length of the chiral N LC decreased with increasing temperature because of a molecular twisting motion of the binaphthyl moiety, resulting in a stabilization of the LC helical structure. On the other hand, length of the helical pitch increased upon photoirradiation, and the resulting LC mixture was found to show photoswitching between chiral N and N phases upon trans-cis isomerization of the azobenzene moiety. The photoinduced deformation of the LC helical structure was derived from a bent shape of cis isomer of the azobenzene moiety in the chiral dopant. This dynamic modulation of the self-organizing helical structure was based on dual and selective molecular motions of the guest materials induced by external stimuli.
Photoswitchable gas permeation membranes based on azobenzene-doped liquid crystals
We have fabricated switchable gas permeation membranes in which a photoswitchable low-molecular-weight liquid crystalline (LC) material acts as the active element. Two different LC eutectic mixtures based on cyanobiphenyls and phenyl benzoates, respectively, were doped with mesogenic azo dyes and infused into commercially available tracketched porous polycarbonate membranes with regular cylindrical pores (0.40 to 10.0 μm). Photo-induced isothermal phase changes in the imbibed mesogenic material afforded large, reversible changes in the permeability of the photoswitchable membrane to nitrogen. The membrane imbibed with the photoswitchable cyanobiphenyl LC material demonstrated low permeability in the nematic state, while the photogenerated isotropic state demonstrated a 16×-greater sorption coefficient. Both states obey a high linear sorption behavior in accordance with Henry's Law. The membrane imbibed with the photoswitchable phenyl benzoate LC showed the opposite permeability behavior to the biphenylimbibed membrane, along with nonlinear sorption behavior. Permeability switching response times for the membranes on the order of 5 s were demonstrated using alternating UV and >420-nm radiation at an intensity of 2 mW/cm2. The effect of thermomolecular motion on gas sorption and diffusion over the LC-isotropic phase transitions are, for the first time, evaluated under isothermal conditions. These photoswitchable membranes are the first examples of systems that are capable of rapid and reversible gas permeation switching. Such switchable and/or tunable membranes are in high demand for applications in analytics, screening, and membrane reactors.
Opto-optical Switching: Nonlinear Optics
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Non-linear light propagation and bistability in nematic liquid crystals
Liquid crystals can switch under influence of an electric field or under influence of incident light. In this paper we provide a mathematical description including electrical, optical and elastic torques. Depending on the applied voltage and the incident light, bistability in the director orientation may be possible. Under certain conditions, the sequence of applying incident TM polarized light and a static voltage allows to access different states.
Azobenzene liquid crystals for fast reversible optical switching and enhanced sensitivity for visible wavelengths
Uladzimir Hrozhyk, Svetlana Serak, Nelson Tabiryan, et al.
We study optical switching properties of novel azobenzene liquid crystal (azo LC) material systems based on mesogenic azo dyes distinguished by enhanced absorption in the visible spectrum and a short lifetime of the photoexcited state. Due to their mesogenic nature these azo dyes can be doped at high concentrations in room temperature LCs. This permits one to obtain results using low energy density values required for observation of strong nonlinear optical processes and short spontaneous restoration times of their original state. A photoinduced nematic-isotropic phase transition could be induced with a single nanosecond pulse. Thin material layers of the order of radiation wavelength were used in the study.
Optical Switching, Optoelectronic Devices, Optical Manipulation
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Colloidal structures and interactions in a nematic liquid crystal
The interactions between different types of colloidal particles are measured and analyzed. We use these interactions to build different self-assembled microstructures, such as dimers, chains, wires, crystals and superstructures. In the experiments we have used different size, different symmetry of colloids (elastic dipoles and quadrupoles) and different way of colloidal binding (via localized defects and via entangled defects). We use optical tweezers for directed selfassembly of colloidal particles. Special attention is devoted to the hierarchical superstructures of large and small particles. We show that smaller, submicron colloidal particles are trapped into the topological defect rings or loops, twisting around larger colloidal particles, which are sources of strong nematic deformations. Various possible applications are discussed, especially in photonics and metamaterials.
Fabrications of liquid-crystal polarization converters and their applications
Shih-Wei Ko, Shu-Hao Huang, Andy Y.-G. Fuh, et al.
This study presents a simple but accurate method for measuring the helical twisting power of chiral doped liquid crystals using axially symmetrical photo-alignment in azo dye-doped liquid crystal films. As reported in our previous paper1, the discontinuous twisting effect produces a disclination line in photo-aligned axially symmetrical liquid crystal films, which can be applied for use as a polarization converter. The pitch and helical twisting power can be obtained by measuring the rotation angle of the disclination line in chrial doped liquid crystal. This method is independent of cell gap and provides an error below 0.5%.
Wave Mixing, Slow Light, Holographic Devices, Nanomachine
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Two-wave mixing and slow-light effects in liquid crystal light-valves
Two-wave mixing experiments performed in a liquid crystal light-valve leads to optical amplification in the Raman-Nath regime of diffraction. By exploiting the dispersion properties associated with the two-wave mixing gain, we demonstrate slow and fast light effects, with the output signal pulse either delayed or anticipated with respect to the input light field. Group velocity as slow as a 0.2 mm/s are obtained, and a large tunability of the group delay is achieved thanks to the ability of controlling the liquid crystal nonlinear response through the main experimental parameters. A theoretical model based on the Kerr-like response of the liquid crystal light-valve accounts for the experimental results and allows us to calculate the group delays for the different output orders. We show that the large group delay obtained in the slow light regime can be exploited to enhance the spectral sensitivity of a Mach-Zehnder interferometer and, on the other side, the narrow frequency bandwidth of the two-wave mixing gain can be used to realize an adaptive holographic system that achieves picometer detection.
Rewritable Bragg holograms of azobenzene polymers with fast response
Amorphous copolymers were synthesized with azobenzene methacrylate, cyanobiphenyl methacrylate, and alkyl methacrylate, in which the azobenzene acts as a photoresponsive part to undergo photoisomerization, the cyanobiphenyl is involved in a change in refractive index triggered by the photoisomerization of the azobenzene, and the alkyl methacrylate is to prevent light scattering, respectively. The obtained polymers showed no LC phase in spite of the incorporation of the common mesogenic group. When two writing beams were interfered in the thick polymer films, the diffraction of the probe beam was observed. The first-order diffraction efficiency reached more than 70 % within 500 ms. On the other hand, the intensity of the zeroth-order beam decreased in proportion to the increase in that of the first-order diffracted beam. It is clear that the intensity of the zeroth-order beam is transferred to that of the first-order diffraction with no optical loss due to the formation of the Bragg gratings. When the writing beams were turned off, the diffraction efficiency remained unchanged. When the recorded grating was kept in the dark at room temperature, sufficient diffraction efficiency was generally retained after a year. However, the grating was easily erased when heated up above Tg for several seconds, and rewritability was confirmed more than 100 cycles. No shrinkage was observed through those processes.
Poster Session
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Low voltage adaptive 128×128 element liquid crystal micro lens array with electric tunable focal length
Hui Li, Kan Liu, Xinyu Zhang, et al.
A new LC micro-lens array of 128×128 elements circular hole top electrode is proposed, which is fabricated by both the lithography and HCL etching methods. Some valuable results are got in tests. The focal length of LC micro-lens array is 50μm~400μm under operating voltage from 0.2VRMS to 5.0VRMS. The lowest operating voltage of LC micro-lens array is 0.2VRMS. The size of focus is about 10μm. And the PSF of LC micro-lens array is close to the theory values. The optical multiple images which are got by LC micro-lens array are clear.
Electrochemiluminescence properties of nematic liquid crystal cells doped with rubrene
Michinori Honma, Takao Horiuchi, Masashi Tanimoto, et al.
Light-emitting properties in liquid-crystal (LC) electrochemiluminescent (ECL) cells doped with an organic fluorescent dye (rubrene) were investigated. LC-ECL cells function based on the collision (recombination) of oppositely charged radical ions (anions and cations), which move by an external electric field applied across the cell. In this work, to obtain large values of luminance and current, a heating stir process was introduced in a preparation procedure of rubrene-doped LC. It was found that the heating stir process dramatically improves both of the luminance and current. It is considered that the drastic increase of the luminance and current density is caused by the increase of the actual dye concentration. Next, to obtain polarized emission based on the orientational nature of the nematic LC, emission properties of an LC-ECL cell with an interdigitated electrode were investigated. As a result, the polarized emission from orientating dyes was confirmed although its polarization ratio was not so high.