We have developed reverse- and normal-mode thermoresponsive polymer network liquid crystals (PNLCs) that are applicable to switchable windows and other devices. Our nonuniform photoirradiation is an easy technique to produce meso- (submicron- to micron-) scale phase separation of orientation-ordered liquid crystals (LCs) with (an)isotropicpolymerized reactive mesogens (RMs) and achieve various thermoresponsive light attenuators. The PNLCs were fabricated in a simple self-assembling process through photopolymerization induced phase separation (PPIPS) by nonuniform irradiation. The size of phase-separation domains and the orientation order of LC molecules in the domains are intrinsic factors to produce high transparence at low temperatures and intense light scattering at high temperatures. Their factors are controlled by size of laser speckle patterns projected on the samples during PPIPS in nonuniform irradiations. Optical polarization property of the transmittance spectrum is also controlled by speckle patterns projected on the sample in the nonuniform irradiation. Different microscopic structures of PNLCs were formed depending on the size of speckle patterns but not simply in proportion to the size. The results suggest that the dynamics of domain formation through PPIPS is connected to the competition between intrinsic nature of phase separation and extrinsic influence by nonuniform light intensity in the irradiated area.
The effects of the combination of cholesteric liquid crystal (CLC) cells on selective wavelength are investigated by the spectroscopic analysis under thermal modulation. The several kinds of CLC cells are formed by using the chiral dopants with different magnitudes and signs of helical twisting power (HTP). The combination of CLC cells with different temperature dependence shows that the infrared light range longer than 700nm is widely reflected and the visible light is little modulated when the temperature increase from 25 °C to 45 °C. The results demonstrate that the thermal modulation of selective transmittance spectra is controllable by the combination of CLC cells with different temperature dependence.
The cholesteric liquid crystal (CLC) cells are fabricated by varying the concentration of various chiral dopants and liquid crystal (LC) diacrylate monomers. The wavelength and bandwidth of selective reflection spectrum in CLC cells are measured by a spectroscopic technique. The variation of the selective reflection spectrum in CLC cells is investigated by doping the different kinds of liquid crystal (LC) diacrylate monomers which stabilize a helical twisting structure by photopolymerization. The effects of the selective reflection spectrum on the visible and infrared lights in spectral solar irradiance are explained by the performance for a solar-ray controller based on the spectral solar irradiance for air mass 1.5 and the standard luminous efficiency function for photopic vision.
Compact and easy-manufacturable wavelength selectors are desired for various types of optical equipments. Our optical wavelength selectors, which possess holographic structure consisting of liquid crystal (LC) and polymer phases, or socalled holographic polymer dispersed liquid crystals (HPDLCs), are switchable in Bragg diffraction wavelength due to polarization states. In the HPDLCs, LC submicrometer droplets are spatially periodically dispersed in polymer matrix with highly-orientational order of LC molecules in the droplets. Such structure is not difficult to be fabricated, that is, it is formed by photo-polymerization and consequent phase separation due to interferometric exposure. Furthermore, the orientation of LC is self-organized during the phase separation process.
A microperiodic structure composed of polymer and liquid crystal (LC) phases, called holographic polymer dispersed liquid crystal (HPDLC), was fabricated based on a photo-induced phase separation technique by laser interferometric exposure. The diffraction wavelength of HPDLC gratings formed by different LC composites and grating structures was experimentally investigated by spectroscopic measurements as a function of temperature at around 30 °C. The HPDLC gratings composed of nematic LC having low nematic to isotropic temperature (TNI) and film thickness of 25 μm showed the switch of diffraction wavelength between visible and infrared lights by the change of temperature. The optical characteristics achieved in HPDLC gratings are expected to be applicable for the basis of diffractive type of thermodriven light controller which can supply visibility constantly for solar-ray control windows.
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.
A new family of organic-inorganic hybrid materials will be introduced as an undoubted candidate for advanced
photonics applications. Alternating oxo-copolymers modified with organic functional groups were prepared through
solventless and catalyst-free process. A variety of optical functional centers such as rare-earth ion, organic dye and
metal/semiconductor nano-particles can be introduced into the systems with an excellent dispersivity to attain efficient
optical activities. In the present paper, doping of rhodamine 6G dye or/and Au particles, etc., was demonstrated.
Reversible photorefractive effect was observed in dye-doped hybrid thin film. Photothermal processing was used to
attain such a reversible effect in combination with the slow dynamics of glassy nature of the photorefractive material.
Rewritable holographic memory with a novel operating mechanism was studied.
In silica glass frozen-in temperature of glass structure, so-called the fictive temperature, is strongly related to optical properties such as the Rayleigh scattering intensity and location of ultraviolet absorption edge. We investigated these optical properties with respect to the fictive temperature of samples with various fluorine concentrations. Furthermore, the structural relaxation during glass forming process was systematically examined. We have found that doping of fluorine strongly encourages the structural relaxation and lowers the fictive temperature. We have also found that relations between the fictive temperature and the optical properties vary sensitively to fluorine concentration.
The Rayleigh scattering intensity in fluorine-doped silica glass was found to decrease very effectively with lowering frozen-in temperature of the glass, so-called the fictive temperature, while such reduction becomes less in samples with higher fluorine concentrations. The present result implies that the Rayleigh scattering in fluorine-doped silica glass is affected by concentration fluctuation besides density fluctuation. From the result, we established a simulated annealing effect of the glass on reduction of the Rayleigh scattering intensity.
From infrared absorption spectra, the present authors elucidated behaviors of the structural relaxation in silica glass doped with various elements, and claim that sub-relaxation process exists besides the main-relaxation given by the viscous flow. Light scattering measurement also suggested the existence of such an additional relaxation channel. The sub- and main-structural relaxations are in general greatly accelerated by a proper dopant. And, the sub-relaxation, which extends below the glass transition temperature, gives a breakthrough to bring silica glass to a more stable structure even below the glass transition temperature. The sub-relaxation then reduces the Rayleigh scattering, a predominant component of the optical loss in the glass. Fluorine and chlorine seem to greatly accelerate the relaxations in silica glass.
Recent studies on silica glass as a photonic material will be described. With regard to the structural disorder, investigations have been made to improve transparency and to shift the optical absorption edge in the ultraviolet towards shorter wavelengths. Remarkable advances have been achieved in the understanding of both light scattering, which is a dominant factor in the optical losses in silica fibers, and the absorption edge. Freezing of the structural disorder was observed, and structural relaxations are found to be important for improving the transparency, whereas for the absorption edge thermal vibration effects seem to be more predominant than the structural disorder. From the results, the present authors have tried to control the structural relaxation for developing silica glass with an ultimate optical transparency, finding that a very tiny amount of the proper impurity species gives rise to structural subrelaxations, which are effective in reducing the Rayleigh scattering. The scattering was reduced by 13% by addition of only 10 wt.ppm Na2O, for example.
Accurate film thickness controls are indispensable for manufacturing defect-free semiconductor devices. Moreover, recent high integration requires simultaneous measurement of each film thickness and optical constants in multi-layers. This paper explains a microspectroscopic film thickness measurement system that measures film thickness at a very small spot (several micrometers in diameter) in several angstrom increments. This system also enables the user to measure film thickness on bulk wafers and SOI wafers, and determine optical constants of unknown films.
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