Nobuhiro Kawatsuki

Abstract Adhesive layers were prepared using photoreactive polymer liquid crystals, and their application as dismantled adhesives based on the change in thermal properties associated with photoisomerization was investigated. Anisotropy in adhesive strength was successfully achieved by controlling the alignment of the liquid crystal using a polyimide alignment film. Furthermore, anisotropy in the adhesive strength due to photoalignment was investigated using the axis-selective photoreactivity of polymer liquid crystals to linearly polarized light.

Abstract Among the various factors of polarization changes due to surface and internal structures of objects, the phenomenon of depolarization through light scattering is one of the most commonly occurring factors in visualization through polarization imaging. In the present study, depolarization due to surface light scattering over a broad wavelength range from the visible to IR regions was experimentally and theoretically elucidated. Using the polarization-probe-polarization-imaging system, we achieved successful visualization of depolarization based on surface structure differences at the nano-level while observing the instrument dial. The results of electromagnetic field analysis using the finite-difference time-domain method, incorporating surface structures observed through microscopic observation, revealed that Rayleigh scattering on the nano-scale uneven surface extends beyond depolarization.

We constructed a hyperspectral circular polarization (S₃) imaging system in the near-infrared (NIR) region comprising a circularly polarized broadband light source, a polarization grating, and a commercial hyperspectral camera. With this system, we captured hyperspectral S₃ images of plastic samples. We then demonstrated the classification with machine learning and found that the hyperspectral S₃ images showed higher classification precision than the conventional NIR hyperspectral images. This result indicates that the hyperspectral S₃ imaging has potential for object classification even for samples with similar absorption spectra. This hyperspectral S₃ imaging system can be applied in garbage classification in recycling plants.

Abstract Photoalignment of liquid crystalline polymer with N-benzylideneaniline (NBA) side groups and subsequent in situ exchange achieves polarized fluorescence of oriented N-salicylideneaniline (SA) derivatives. In situ hydrolysis of NBA and condensation with 2-hydroxybenzaldehyde derivatives form oriented SA side groups. SA exhibits excited state intramolecular proton transfer (ESIPT), inducing polarized fluorescence with a 2.3 polarization ratio. The emissive wavelength depends on the 2-hydroxybenzaldehyde derivative. Stepwise formation of different oriented SA side groups allows for color tuning of the oriented film.

In this study, we describe the synthesis of a cholesterol-linked cyanostilobazole salt dye and the tuning of its luminescence by physical stimuli such as electricity and grinding. The dyes exhibited liquid-crystalline properties at temperatures above 170 °C. Some of the solutions were transformed into orange luminescent gels upon the addition of poor solvents. When the solvent was evaporated, the resulting solid xerogel exhibited mechanochromism, its color changed, and its luminescent color changed from orange to red. Furthermore, we investigated the construction of functional gels (mechanochromic gels) that can respond to two stimuli, damage detection by abrasive responsiveness, and electrical response using ionic liquid complexes of polymers as dispersing media. This study provides a new strategy for tuning and switching luminescence using non-chemical stimuli in a single-component system using aggregation.

Copolymer films of photoalignable liquid crystalline (LC) copolymethacrylates comprised of a phenyl benzoate mesogen connected with N-benzylideneaniline end moiety (NBA2) and benzoic acid (BA) side groups exhibit a photoinduced reorientation behavior. Significant thermally stimulated molecular reorientation attains a dichroism (D) greater than 0.7 for all copolymer films and a birefringence of 0.113–0.181. In situ thermal hydrolysis of the oriented NBA2 groups decreases the birefringence to 0.111–0.128. However, the oriented structures of the film are maintained, demonstrating a photo-durability, even though the NBA2 side groups photo-react. The hydrolyzed oriented films show higher photo-durability without changing their optical properties.

A polarization-probe polarization-imaging (PPPI) system was developed for the near-infrared (NIR) regime. This system comprises two components operating as a polarization generator and a polarization analyzer to enable polarization image capture under polarized light illumination. The captured polarization images contain considerable object information because the illuminating polarized light beams are affected by many of the Mueller matrix elements. By assembling the polarization camera using two liquid crystal retarders and a polarization grating, the PPPI system offers the potential to measure the Stokes parameters fully with a high extinction ratio, even in the NIR region. The PPPI system’s feasibility was demonstrated experimentally. Its dependence on the state of polarization (SoP) of the illuminating polarized light was discussed. The polarization image acquired by the PPPI system is strongly dependent on the illuminating light’s SoP, so the appropriate SoP must be selected for each object to enhance the polarization image contrast. This PPPI system should expand the range of polarization imaging applications, including LiDAR, product inspection, and bio-imaging.

Featured Application: Optical communication, Optical manipulation, Optical processing. An optical system consisting of a straight-line polarization grating (SPG) and two parabolic-line polarization gratings (PPGs) is presented for the characterization of optical vortices (OVs) with arbitrary states of polarization (SoPs). The PPG is capable of converting an OV with a specific SoP into a bright bar-like spot with 100% efficiency. The number of dark lines and their orientation respectively correspond to the magnitude and sign of topological charge (TC) of the incident OV, thereby enabling characterization of OVs with high efficiency. Furthermore, on combining an SPG with PPGs, the present system can characterize the TC of incident OVs regardless of their SoP. The feasibility of the system was demonstrated in experiments using gratings fabricated by applying the photoalignment method and employing films of a photo-crosslinkable polymer liquid crystal. The experimentally obtained efficiency is 70.2%. We furthermore demonstrate the system’s capability to characterize polarization vortices.

Thermally stimulated photoinduced reorientation of liquid crystalline (LC) polymethacrylate composed of a phenyl benzoate mesogen connected with N-benzylideneaniline (NBA2) end moiety exhibits a significant molecular reorientation (D > 0.7) when the film is exposed to linearly polarized 313 nm light and subsequently annealed in the LC temperature range of the material. Hydrolysis of the NBA2 end moieties yields an oriented mesogen with phenylamine moieties without distorting the oriented structure. In situ condensation of 2-hydroxybenzaldehyde derivatives and phenylamine moieties yields oriented N-salicylideneaniline side groups. The resultant film displays a polarized fluorescence with a polarization ratio up to 3.4.

The liquid crystalline N-benzylideneaniline (NBA) polymer acts as an adhesive, and its bonded pieces detach upon exposure to UV light due to the trans-cis photoisomerization of the NBA moieties and subsequent phase transition of NBA from a liquid crystal polymer to an isotropic polymer. We improved the photoresponsive durability of the polymer and explored the effect of light irradiation on its thermal properties and adhesion stress. We observed that the transition points of the polymer's thermophysical properties (glass transition, liquid crystal-isotropic transition, G" slope, tan δ) shifted to lower temperatures under light irradiation.

Polarized fluorescence of oriented N-salicylideneaniline (SA) derivatives is explored based on the thermally stimulated photoinduced molecular reorientation of liquid crystalline (LC) copolymethacrylate with N-benzylideneaniline derivative (NBA2) and benzoic acid (BA) side groups. The LC copolymer films show significant cooperative molecular reorientation of the NBA2 and BA side groups (D > 0.7). Subsequent thermal hydrolysis of the NBA2 side groups yields free phenylamine moieties. These moieties can form oriented SA derivatives via in situ condensation with 2-hydroxybenzaldehyde derivatives. The excited-state intermolecular proton transfer of the oriented SA molecules induces polarized fluorescence at 510-548 nm with a polarization ratio up to 6.2. Direct in situ exchange from the oriented NBA2 to SA derivatives achieves polarized fluorescence similar to that of the SA side groups.

An in situ exchange of oriented mesogenic side groups containing Schiff base moieties realized birefringence control of photoalignable liquid crystalline polymers between 0.13 and 0.44. Both thermal hydrolysis of the oriented side groups and in situ condensation using phenyl amine derivatives with a high inherent birefringence vary the birefringence and thermal stability of oriented films without distortion of the oriented structure.

Photoresponsive photoalignable liquid crystalline polymers composed of phenyl benzoate terminated with N-benzylideneaniline were evaluated. These polymers are capable of axis-selective photoreaction, photoinduced orientation, and surface relief grating formation. Polarization holography using an He-Cd laser beam at a wavelength of 325 nm demonstrated the formation of a surface relief grating with a molecularly oriented structure based on periodic light-induced reorientation and molecular motion. Electrical switching of diffracted light using an electric field response of twistednematic cell containing a low-molecular-weight liquid crystal in combination was also demonstrated.

Polymethacrylate liquid crystalline polymer having side groups containing N-benzylideneaniline moiety was synthesized, and its photoinduced reorientation behavior was investigated using linearly polarized UV light. Films of these polymer exhibited axis-selective trans-cis-trans photoreorientation of the side groups upon irradiation. Annealing the film in the liquid crystalline temperature range of the polymer after irradiation greatly amplified their otherwise minimal photo-induced optical anisotropy. Due to the extended mesogenic structure of the side groups, the polymer showed significant thermal stability and a high degree of optical anisotropy after annealing.

A random copolymethacrylate containing benzoic acid and benzaldehyde side groups (P1), and a diblock copolymer of P1 and a polymer with heptadecafluorodecyl side groups (P2) were synthesized by means of reversible addition-fragmentation chain transfer (RAFT) polymerization. The linearly polarized (LP) UV light exposure initiated the thermally stimulated photoinduced reorientation of P1(P2)/4-methoxyaniline (AN) films due to the axis-selective photoisomerization of in situ formed N-benzylideneaniline (NBA) groups. A photoinduced birefringent pattern was achieved by sublimation or inkjet coating with AN on the P1(P2) film, which formed NBA moieties in the desired region. Clear birefringent pattern was attained when using P2 because the fluorine-containing block improved the NBA formation at the AN-coated region.

Crosslinked liquid crystal polymers (CLCPs) have recently attracted much attention because they exhibit unique properties in stimuli responsiveness. We have recently focused on the photoinduced deformation behavior of CLCP containing N-benzylideneaniline (NBA) as a photoresponsive moiety. We prepared two types uniaxially aligned NBA-containing crosslinked liquid crystalline polymer film, homogeneous and homeotropic, and evaluated its photomechanical properties. The initial alignment of photoactive mesogens significantly affects the bending behavior of the CLCP films: the homogeneous films bent toward the irradiation direction of the actinic UV light, while the homeotropic films bent away from the light source.

The incident angle dependences of the diffraction properties of orthogonal-circular polarization gratings (OCPGs) fabricated using a polymer liquid crystal exhibiting biaxial anisotropy have been investigated. The investigations were conducted with the plane orthogonal to the surface of the OCPGs as the plane of incidence. It was found that a specific biaxial anisotropy reduces the incident angle dependence of optically thinOCPGs against the wavelength of the incident light, and is less effective on thick OCPGs. In addition, a method also has been proposed to determine the biaxial anisotropic shape that most reduces the incident angle dependence in thin OCPGs.

A novel alkyltolane-terminated cyanostilbene is synthesized. The photoluminescence colour changed depending on the precipitation method, with either a red shift or blue shift in the photoluminescence spectra observed upon mechanical grinding.

A hybrid polarization grating that can spatially separate orthogonal states of a vector beam with the same polarization topological charge is presented. The hybrid polarization grating is assembled using a fork-shaped polarization grating and a quarter-wave polarization grating and acts as a common pass interferometer for right- and left-circular polarization components of incident vector beams. The hybrid polarization grating can separate two vector beam states that have a 90 ° relative polarization azimuth angle difference. The number of detectable vector beams can be increased by replacing the hybrid polarization grating with a crossed-hybrid polarization grating. Device feasibility was demonstrated experimentally using hybrid polarization grating and crossed-hybrid polarization grating fabricated using the photoalignment method for photoreactive liquid crystals. This approach has the potential to demultiplex several vector beams stably and simultaneously using a compact optical system and should be applicable to vector beam division multiplexing and other applications requiring vector beam detection.

Mechanochromic luminescent dyes change their luminescence color upon exposure to external mechanical stimuli. In this study, we synthesized a liquid crystalline mechanochromic luminescent dye containing a terminal cholesterol molecule. The dissolution of the dye in 1,4-dioxane resulted in the formation of a gel. The luminescence of the xerogel obtained from the dioxane solution changed from green to blue upon grinding, indicating mechanochromic luminescence behavior. The anisotropic patterning of short-wavelength-shifted luminescence color change by directional handwriting on surface layer of liquid crystal was successfully demonstrated. Furthermore, blue-shifting mechanoresponsive polymer composite surface was fabricated by using the luminophore.

In this paper, geometric-phase lenses, which can not only correct wavefront aberrations but also magnify an aerial image, have been proposed for use in an aerial display system composed of a retroreflector array, an orthogonal-circular polarization grating, a right-angle prism, and a quarter-wave plate. The geometric-phase lenses were demonstrated both experimentally and computationally by arranging them in the aerial display configuration and observing aerial images. We also investigated the relationship between the magnification and the field of view of an aerial image, and it has been found that there is the trade-off relationship between them.

Hydrogen-bonded liquid crystalline polymer composites containing N-benzylideneaniline (NBA) as a photoresponsive mesogenic unit are prepared. The mesomorphic properties of the composite decrease upon exposure to UV light because of trans-to-cis photoisomerization, which affects the adhesive properties and allows facile exfoliation. In addition, the destabilized phase spontaneously reverts to the initial state when UV light irradiation stops. After exfoliation, the composite can be rebonded to a glass substrate by heating above its melting point. This is the first observation of the phase transition of an NBA liquid crystalline polymer.

A photoalignable liquid crystalline (LC) polymer comprised of phenyl benzoate with a Schiff-base end moiety is synthesized and its thermally stimulated photoinduced reorientation behavior is investigated using linearly polarized (LP) 365nm light. A significant in-plane reorientation (D > 0.8) is achieved. The reoriented film exhibits a high birefringence (Δn = 0.28) and a high thermal stability (∼295 °C). Re-exposure of the reoriented film to LP 365nm light reversely changes the orientation direction. However, the initial reoriented structure is recovered by re-annealing.

Mechanochromic luminescence (MCL) compounds exhibit change of their photoluminescent color in response to mechanical stimuli. The initial molecular alignment of a liquid-crystalline MCL compound was controlled by using an alignment layer. The rubbing method and rubbing direction were successfully determined. Influence of mechanical force on photoluminescent behavior was evaluated.

A polarized laser beam steering system using multiply cascaded rotating polarization gratings (PGs) is presented. The rotating PGs steer incident circularly polarized beams with high optical throughput because the theoretical limitation of the PG diffraction efficiency is 100%. The system also offers more rapid rotation when compared with wedge prism pairs because of the weight of the PGs. The beam steering performance was analyzed theoretically for schemes using two and four rotating PGs. The system’s feasibility was demonstrated experimentally by projecting Lissajous and raster patterns using PGs fabricated from photocrosslinkable polymer liquid crystal films via a photo-alignment technique. The steered beam’s diffraction efficiency and ellipticity were maintained at 83∼89% and 96∼99%, respectively, during PG rotation. This beam steering system will be applicable to light detection and ranging, optical imaging, and laser displays.

The influence of the annealing conditions on the thermally stimulated photoinduced molecular reorientation of a photoinactive liquid crystalline polymethacrylate with phenyl aldehyde and benzoic acid side groups (P1) doped with 4-methoxyaniline, which forms photoalignable 4-methoxy-N-benzylideneaniline (MNBA) side groups in situ, was investigated. Light exposure and subsequent thermal stimulation under a N2 atmosphere realized sufficient cooperative molecular reorientation (D > 0.7), but the simultaneous thermal hydrolysis of the MNBA groups under humid air lowered the molecular reorientation performance. By contrast, subsequent thermal hydrolysis of MNBA after molecular reorientation introduced different aromatic amines into the reoriented P1 film, which regulated the birefringence and photofunctionality of the oriented film.

In this presentation, we propose a scheme for a polarized beam steering system using multiply-cascaded rotating PGs with biaxial anisotropy. Our scheme can steer the polarized beam along both a Lissajous orbit and raster orbit, depending on the synchronization of the rotation frequencies of the PGs. Also, the use of more than two PGs allows us to control the center position of the Lissajous orbit. In addition, by using biaxial anisotropy, the diffraction efficiency and the ellipticity of the steered beam remain almost unchanged during PG rotation. Our beam steering system will apply to LiDAR and laser display.

Thermally stimulated photoinduced molecular reorientation with high dichroism (D>0.6) is explored in liquid crystalline (LC) copolymerthacrylate films comprising of 4-methoxy-N-benzylideneaniline (MNBA) and benzoic acid (BA) side groups. Thermal hydrolysis of MNBA side groups induces free phenyl aldehyde (PA) side groups in the oriented film. Birefringence of the oriented films is adjusted by introducing 2,7-diaminofluorene (FL) to form new imine bonds with free PA side groups. Meanwhile, in situ exchange from MNBA to FL-based imine also controls the birefringence of the oriented film. The initial birefringence of the oriented copolymer film increases up to 0.22 after introducing oriented FL moieties.

The incident angle dependence of the diffraction properties of orthogonal-circular polarization gratings (OCPGs) fabricated using a polymer liquid crystal exhibiting biaxial anisotropy have been investigated. It was found that a specific biaxial anisotropy reduces the incident angle dependence of optically thin OCPGs against the wavelength of an incident light.

An aerial display scheme consisting of an orthogonal circular polarization grating (OCPG), a waveplate, a retroreflector array (RRA), and a right-angle prism (RAP) was developed. Because of the OCPG's functionality, retroreflected light from the RRA is transmitted through the RAP boundary surface by avoiding the total reflection condition. The proposed system can potentially increase optical throughput to 100% by designing the boundary surface incidence angle to be Brewster's angle. The scheme's feasibility was demonstrated experimentally using an OCPG and waveplate that were fabricated from polymer liquid crystal with optical anisotropy using a photoalignment technique. The scheme should be used as a type of aerial display that is compatible with polarization-diffractive elements called Pancharatnam-Berry phase elements.

A novel, to the best of our knowledge, method for circular-polarization imaging with white light is presented. The vital optical elements of the proposed system are two polarization gratings (PGs) and a wedge prism. The chromatic dispersion of the PG diffraction angle is compensated by the difference of the grating period for the two PGs. In addition, the distance between the diffracted lights on the imaging plane is controlled by using a wedge prism. As a result, ${S_3}$S3 of the Stokes parameter, including the wavelength dependence of a scarab beetle, was observed at a time, and the ${S_3}$S3 contribution to each color was determined by using the color filter of the light-receiving element. Because the white-light polarization image includes a lot of information about the illuminated structure, the proposed method should be applied to biomedical sensing and remote sensing.

We have succeeded in forming a polarization grating whose polarization diffraction properties are extremely independent of the incident angle by using polymer liquid crystal exhibiting biaxial optical anisotropy. It is considered that the extension of the optical path length and the decrease in the effective amplitude of optical anisotropy due to oblique incidences are offset by the biaxial optical anisotropy and, as a result, the retardation is compensated. The properties of this developed device have been experimentally demonstrated and theoretically explained.

We present a simple yet versatile and practical polarization camera for imaging full Stokes parameters. The developed system consists of one anisotropic diffraction grating plate and two electro-switchable retarders and obtains in nearly real-time full Stokes images of the light scattered from the objects. The monochromatic S3 image is obtained by physically separating the right and left circularly polarized components with the aid of the anisotropic grating, and S1 and S2 images are obtained by fast electro-switching the retardation of each retarder. The simple polarization imaging system has possible applications in various types of imaging systems and especially should be incorporated into optical microscopes as well as imaging cameras in future work.

Photoinduced macroscopic deformation behavior of crosslinked liquid-crystalline polymer is induced in crosslinked N-benzylideneaniline polymer films. The film is prepared by in situ polymer reaction using polymethacrylate-containing phenylaldehyde side chains and o-tolidine crosslinker. The film exhibits macroscopic bending upon exposure to UV light and then immediately reverts to the initial shape when the light is turned off. The bending behavior is strongly affected by the fabrication conditions the changes in the macroscopic shape and photomechanical properties in response to UV light for the film prepared from a mixture solution are small, while those for the film prepared by top coating the crosslinker solution onto the prepolymer substrate are large.

In situ formation of N-benzylideneaniline (NBA) side groups achieved photoinduced cooperative reorientation of photoinactive copolymers with phenylaldehyde (PA) and benzoic acid (BA) side groups doped with 4-methoxyaniline (AN) molecules. Thermally stimulated molecular reorientation of the side groups was generated due to the axis-selective photoreaction of the NBA moieties. Selective coating with AN on the copolymer film formed NBA moieties in the desired region, resulting in a photoinduced birefringent pattern. Additionally, postannealing at an elevated temperature for a long time attained photoinactivation of the reoriented film, and recoating with AN to form NBA achieved the multiple birefringent patterns and repatterning of the reoriented structures. The slow thermal hydrolysis of NBA, which was 50 times slower than the thermally stimulated self-organization of the side groups due to the presence of BA side groups, contributed to the photodurability of the reoriented film and multiple birefringent patterns.

We propose and demonstrate a photolithography method for fine metal structure fabrication based on laser drawing that uses the interference pattern generated by co-propagating optical vortices. A tiny dark core region of the optical vortex allows us to overcome the diffraction limit for Gaussian beams. This means that the proposed method can be used to fabricate finer structures than those produced by the conventional laser drawing method while using a Gaussian beam, even under low numerical aperture conditions. The feasibility of the proposed method was demonstrated experimentally using a system that included an axially symmetrical polarization element that can generate the co-propagating optical vortices using a common path optical system. Our method has potential to fabricate few tens of nanometer scale metal line structures by increasing numerical aperture conditions and should be applicable to the development of nanometer scale electronic and optical devices and structures, such as integrated circuits and metamaterials, without using electron beam lithography.