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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.
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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.
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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.
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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.
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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.
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Optical Tuning and Switching: Microwave Application
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.
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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.
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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.
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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.
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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.
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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.
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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%.
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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.
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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.
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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.
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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.
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