Design and fabrication of a liquid crystal polarization grating for mid- and far-infrared wavelengths.

A lot of research on liquid crystal polarization gratings (LCPGs) that can separate circularly polarized light with 100% diffraction efficiency has been conducted in the visible and near-infrared wavelength regions. In this paper, we tried to design and fabricate the LCPGs that are available for use in the mid- and far-infrared (MIR and FIR) wavelength regions. The materials for making LCPGs were selected in view of low absorption characteristics measured by the use of a Fourier-transform infrared (FT-IR) spectrometer. LCPGs designed for 3.88 µm and 9.5-10.6 µm were fabricated, and we evaluated their diffraction properties experimentally. The MIR and FIR LCPGs should open new application fields of LC technologies including polarimetry, spectroscopy, and beam steering.

Near-infrared hyperspectral circular polarization imaging and object classification with machine learning.

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.

Design of two-dimensional multilevel optical anisotropic diffraction gratings with a generative adversarial network.

This study uses a generative adversarial network to design multilevel optical anisotropic diffraction gratings with specific customizable characteristics. As input, this method uses the far electric field of polarization and intensity in each diffracted light through the gratings to design. Using the finite-difference time-domain method, the designed structures are numerically evaluated, confirming that they can be created with the intended parameters. Multilevel optical anisotropic diffraction gratings created this way can be used in various fields to develop improved optical elements.

Photo-Durable Molecularly Oriented Liquid Crystalline Copolymer Film based on Photoalignment of N-benzylideneaniline.

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.

Polarization-probe polarization-imaging system in near-infrared regime using a polarization grating.

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.

Polarized Fluorescence of N-Salicylideneaniline Derivatives Formed by In Situ Exchange from N-Benzylideneaniline Side Groups in Photoaligned Liquid Crystalline Copolymer Films.

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.

Hybrid polarization grating for mode detection of vector beams.

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.

Magnification and wavefront aberration correction by a geometric-phase lens in a polarized aerial display using a right-angle prism.

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.

Polarized beam steering using multiply-cascaded rotating polarization gratings.

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.

Photoinduced Reorientation and Photofunctional Control of Liquid Crystalline Copolymers with in Situ-Formed N-Benzylideneaniline Derivative Side Groups.

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.

High-efficiency aerial display using a liquid crystal polarization grating, a retroreflector array, and a right-angle prism.

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.

White-light circular-polarization imaging using pairs of polarization gratings and wedge prisms.

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.

Generation of optical vortices using a uniaxially aligned azo-dye-doped liquid crystal cell and space-variant polarization projection system.

A scheme is presented for an optical vortex (OV) generator comprising a uniaxially aligned azo-dye-doped liquid crystal (ADDLC) and a space-variant polarization projection (SVPP) system. The SVPP system consisting of an electro-optic modulator and a micro-electromechanical system projects a time-averaged SVP field equal to a vector beam onto the ADDLC and fabricates a three-dimensional twisted anisotropic structure, which has spatial phase modulation properties from plane to helical shape. The generation of OVs with odd- and even-numbered topological charge is experimentally demonstrated. As a flexible and broadband spatial light modulator, the proposed scheme should be applicable to the research and development of OV applications.

Dynamic control of diffraction angle and separation properties of wavelength and polarization by quaternary liquid crystal grating.

A quaternary liquid crystal (LC) grating simultaneously performs dynamic control of diffraction angle, polarization, and wavelength-separation properties as shown in the following: (1) Diffraction orders in which light waves are diffracted can be selected by applying a voltage, (2) the efficiency of each diffraction order can be controlled depending on a wavelength of an incident beam, and (3) a pair of counter-rotated circular polarizations or linear polarizations with an orthogonal relationship are diffracted simultaneously, and this property can be controlled by applying a voltage. These diffraction properties and LC alignment structures are proposed based on theoretical analyses using Jones calculus, and their properties are now demonstrated experimentally. The quaternary LC grating can be used as an advanced optical element for industrial applications such as tunable polarization beam splitters and dynamic switching of propagation directions of light depending on wavelength.

Diffraction properties of liquid crystal cell with beat structure formed by photoalignment substrates.

The diffraction properties of liquid crystal (LC) cells fabricated by photoalignment substrates with different periodic optical anisotropic distributions are demonstrated. The twist angle distribution structure inside the LC cells formed by the grating period difference between photoalignment substrates is called "beat structure." In addition to diffraction corresponding to the grating periods of photoalignment substrates, beat structure itself also functions as a diffraction grating. The diffraction properties can be controlled by varying the retardation. The fabrication method and diffraction properties of the LC cells with beat structure are explained by both experiment and theoretical analysis.

Incident angle dependence-reduced polarization grating performance by using optically biaxial polymer liquid crystal.

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.

Polarization imaging using an anisotropic diffraction grating and liquid crystal retarders.

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.

Generation of arbitrary axisymmetrically polarized pulses by using the combination of 4-f spatial light modulator and common-path optical system.

We proposed and constructed a system to realize broadband generation of arbitrary axisymmetrically polarized (AP) pulses with spatial complex amplitude modulation. This system employs the combination of a spatial light modulator in the 4-f configuration (4-f SLM), and a space variant wave plate as a common path interferometer. The 4-f SLM and the common path interferometer offer compensation for spatial dispersion with respect to wavelength and stability to perturbation, respectively. We experimentally demonstrated the various AP pulses generation by applying modulations of fundamental and higher-order Laguerre-Gauss modes, whose radial indices were, respectively, p = 0 and 1, with high purity, which showed that we were able to generate arbitral AP pulses with spatial complex amplitude modulation. This technique is expected to be applied in both classical and quantum communications with higher-order modes.

Design and fabrication of a tunable wavelength-selective polarization grating.

Tunable wavelength-selective diffraction with polarization conversion is realized by the design of a liquid crystal (LC) grating containing a twisted nematic alignment structure that is fabricated by an efficient one-step photoalignment method. The diffraction efficiency strongly depends on the wavelength of the incident beam, and this property can be controlled by adjusting the birefringence of the nematic LC using a thermal control. These properties are well described by a theoretical analysis based on the Jones calculus and are experimentally demonstrated using 488, 532, and 633 nm wavelength incident polarized laser beams. The resultant LC grating has potential applications as diffractive optical elements that can simultaneously control the parameters of light such as amplitude, wavelength, and polarization.

Holographic liquid crystal polarization grating with Fabry-Perot structure.

A holographic liquid crystal polarization grating with a Fabry-Perot structure was developed. Because of its resonant structure, the device offers high levels of control of the diffraction properties of incident-polarized light beams, depending on the resonance conditions. The diffracted light beams are emitted in both the reflection and transmission directions, and the device thus works as a multibranch polarization grating with double optical paths, unlike a conventional polarization grating. These device features were experimentally demonstrated and were also explained theoretically.