Polarization Gratings Research Articles

54.3: Invited Paper: High photoinduced ordering and controllable photostability of hydrophilic azobenzene material based on relative humidity and its application in 2D polarization gratings

54.3: <i>Invited Paper:</i> High photoinduced ordering and controllable photostability of hydrophilic azobenzene material based on relative humidity and its application in 2D polarization gratings

Generalization of Pancharatnam-Berry phase interference theory for fabricating phase-integrated liquid crystal optical elements

ABSTRACTIn this paper, we extended the methods for fabricating phase-integrated liquid crystal optical elements by interfering of orthogonal circularly polarised beams with different PB phase and generalised those methods as Pancharatnam-Berry (PB) phase interference theory. The generalised theory exhibits two kinds of interference ways. The first way takes place when various PB optical elements (PBOEs) are sequentially illuminated by a linearly polarised light in half-wave condition. The second way happens when the first PBOE is illuminated by a circularly polarised light in quarter-wave condition while the following PBOEs are in half-wave condition. The fabrication of fork polarisation gratings based on this generalised theory is used to confirm the two different interference ways. The generalised theory can better describe the interference of orthogonal circularly polarised beams with different PB phase and can be widely used for fabricating phase-integrated liquid crystal optical elements using existing PB phase device, which may promote the development and extensive applications of the PBOEs.

Mode demultiplexing of vector beams using crossed-fork-shaped polarization grating fabricated by photoalignment of photo-crosslinkable liquid crystal polymer

A crossed-fork-shaped polarization grating (crossed-FPG) is presented for use in vector beam division demultiplexing. The crossed-FPG was fabricated in a thin photocrosslinkable liquid crystal polymer film by recording multiple polarization holograms between optical vortices and Gaussian beams that have opposite circular polarizations to each other. The recorded holograms work individually to act as FPG with a polarization-dependent topological charge conversion property that corresponds to a recorded pair of vortices. The feasibility of mode demultiplexing was demonstrated experimentally. The proposed crossed-FPG should be used as a mode demultiplexer for optical communications based on vector beam division multiplexing.

High-efficiency large-angle reflective composite polarization grating

ABSTRACT We introduce and demonstrate a new structure of polarisation grating which can substantially enlarge the deflection angle. We derive the polarisation state of a beam passing through the reflective composite polarisation grating by Jones matrix and build a rigorous simulation about the deflection angle of reflective composite polarisation grating using finite-element analysis. The theoretical diffraction efficiency of the reflective composite polarisation grating can reach up to 90%. The experimental result of deflection angle of the reflective composite polarisation grating is consistent with that of simulation. The reflective composite polarisation grating could be expected to be applied in augment reality system, spectroscopy, optical telecommunications, non-mechanical beam steering and so on.

Diffractive gratings with varying period’s shape

Diffractive gratings with varying period’s shape

Device simulation of liquid crystal polarization gratings.

Liquid crystal polarization gratings manifest several unique features, such as high diffraction efficiency, polarization selectivity, and fast switching time. However, few works address the chiral-doped liquid crystal alignment issue in such gratings. Here, we develop an improved relaxation method to analyze the liquid crystal director distribution in chiral-doped polarization gratings. Our simulation result agrees well with experimental data on a polarization volume grating. The criteria for forming planar or slanted polarization grating are discussed.

Design of an augmented reality display based on polarization grating**Project supported by the National Natural Science Foundation of China (Grant No. 11574211).

A new optical system for an augmented reality (AR) display is proposed in this paper. The optical system mainly includes a ray deflector, coupling input grating, optical waveguide, and coupling output grating. Both the ray deflector and the coupling input grating are designed based on the diffraction characteristics of the polarization grating, and the coupling output grating is the Bragg reflection grating. Compared with other AR schemes, this AR optical system not only reduces the number of projections from two to one, but also improves the efficiency of light coupling into the optical waveguides. The energy loss is reduced by utilizing the single-order diffraction characteristics of the polarization grating in its coupling input structure. The light deflector uses the polarization selectivity of the polarization grating and the characteristics of the rotating light of the twisted nematic liquid crystal layer to realize beam deflection. The working principle of the optical system is experimentally and theoretically demonstrated.

Holographic Formation of Non-uniform Diffraction Structures by Arbitrary Polarized Recording Beams in Liquid Crystal-photopolymer Compositions.

In this work, the theoretical model of non-uniform diffraction structures’ holographic formation in liquid crystal-photopolymer (LC-PPM) composite materials with a dye-sensitizer is developed. The model takes into account the arbitrary character of amplitude and phase spatial distributions of recording light field, its arbitrary polarization state and also a non-linearity of the recording process. Two the most common types of liquid crystal-photopolymer composite are investigated: Holographic polymer-dispersed liquid crystals (H-PDLC) and polymer-stabilized liquid crystals (PSLC). Numerical simulations for the most common cases of holographic formation schemes are made. It is shown that due to the photo-induced Freedericksz transition, in the case of arbitrary polarization states of recording light beams, the non-uniform polarization diffraction grating (PDG) is formed in LC-PPM. Numerical simulations’ results show that PDG’s contribution to the change of the dielectric tensor of the media is comparable with the contribution of the photopolymerization-diffusion process.

Diffractive waveplates: introduction

This feature issue of the journal is the first one to bring together ongoing research from many groups developing diffractive waveplate technology. This technology combines the best of three worlds—the thinness of diffraction gratings, the broad bandwidth and high efficiency of conventional refractive optics, and the low-cost fast manufacturing of polymer optics even in large area. The technology is at the intersection of polarization holography and polarization gratings, Pancharatnam–Berry or geometrical phase optics, waveplates, metasurfaces and planar optics. Optics will never be the same again!

Single-step creation of polarization gratings by scanning wave photopolymerization with unpolarized light

Liquid crystalline materials with a cycloidal molecular orientation pattern are attractive for fabricating diffractive waveplates, diffracting incident light regardless of its polarization state into left- and/or right-circularly polarized light only in the +1st and/or &#x2212;1st orders, applicable as next-generation optical devices. However, large-area high-speed processing of such molecular orientation is a challenge, since even state-of-the-art photoalignment methods require a precise spatial modulation of the polarization states of incident light, e.g., polarization holograms. Here, we propose and demonstrate that unpolarized light could easily generate cycloidal molecular orientation patterns over large areas in a single step merely by using our recently developed method of &#x201C;scanning wave photopolymerization&#x201D; with a simple optical setup. Importantly, the processing time for fabricating millimeter-scale films was significantly decreased to less than a few minutes. Detailed investigation revealed that the resultant film showed the desired diffraction behavior with a diffraction efficiency of 50%.

Polarization grating exposure method with easily tunable period via dual rotating polarization grating masks

We introduce a new approach, to the best of our knowledge, to record polarization gratings (PGs) based on dual rotating polarization grating masks. In prior approaches, the linear variation of the orientation angle of the PG pattern was accomplished using discrete holographic optics, which require careful precision alignment, and wherein the relative distances between those optics limit the upper range of PG periods that can be made. Conversely, the setup described and demonstrated here as a single stage is very compact and more robust to vibration compared to other approaches. Moreover, this approach can easily tune the PG period while maintaining the compact size of the setup. This technique enables easy fabrication of arbitrarily large-period PGs. In this work, we discuss general design principles and critically evaluate this fabrication method, as compared to the best of prior approaches.

Highly chromatic retardation via multi-twist liquid crystal films

Here we study birefringent films with highly customizable chromatic retardation spectra, using multi-twist liquid crystal (LC) films. These are made of two or more layers of chiral nematic LC polymer network materials, also known as reactive mesogens, which form a monolithic thin-film wherein the in-plane orientation of subsequent layers is automatically determined by the single alignment layer on the substrate. The multiple layer thicknesses and twists present many degrees of freedom to tailor the retardation. While prior work examined achromatic spectra, here we show how to use Mueller matrix analysis to create highly chromatic spectra. We experimentally demonstrate both a uniformly aligned retarder as a green/magenta color filter and a “hot” polarization grating (PG) that diffracts infrared while passing visible light. The three-twist color filter shows a contrast ratio in transmittance between polarizers as high as 10:1 between the half- and zero-wave retardation bands. The “hot” PG shows an average first-order efficiency of about 90% for 1000–2700 nm and an average zero-order efficiency of about 90% for 500–900 nm. The principles here can be extended to nearly any chromatic retardation spectra, including high/low/bandpass, and to nearly any LC orientation pattern, in general known as geometric-phase holograms.

Fast switching beam steering based on ferroelectric liquid crystal phase shutter and polarisation grating

ABSTRACTWe report on fast-switching and high-efficient optical beam steering based on a polymerisable liquid crystal polarisation grating (PG) in combination with ferroelectric liquid crystal (FLC) phase shutter. The PG was fabricated in a convenient single-step holographic exposure process using photo-sensitive azo-dye material as alignment layer for liquid crystal (LC) director. A binary electro-optical FLC was employed for circular polarisation selection, which enables the electro-tunable steering of the combined system. The efficiency of 95.7% with 82 μs switching time is obtained for 1064 nm laser. This work provides a versatile candidate for non-mechanical beam steering devices.

Numerical analysis of Bragg polarization gratings

Here we study Bragg regime polarization gratings (PGs) using an anisotropic rigorous coupled-wave analysis method. We simulate the most important diffraction properties without paraxial approximation, including the angular, spectral, and polarization responses. We first focus on the angular and spectral bandwidths of the transmissive and reflective Bragg PGs optimized for normal incidence. The effects of material birefringence and average index of refraction were investigated. Second, we examine the nonideal Bragg PGs with nonplanar director profiles and identified degradation in optical performance due to the high tilt of the liquid crystal director. Third, we simulate the polarization response of both types of Bragg PGs and observed complicated angular dependence of the polarization output. Qualitatively, good agreement can be observed between the simulation results and prior experimental work. Finally, we fit the measured angular and polarization data to retrieve actual grating parameters and demonstrated excellent quantitative correspondence, which can be particularly useful in closing the gap between design and fabrication.

Bistable switching of polarization-grating diffractions enabled by a front bistable twisted nematic film.

Bistable electrical switching of high-efficiency grating diffractions is realized by adding a front π-bistable twisted nematic (π-BTN) cell to a passive liquid crystal polarization grating (LCPG). The π-BTN cell can be switched between stable non- and 180°-twisted states, acting as a polarization converter that switches the polarization of a laser beam and thus changes the diffraction behavior of the LCPG. Both states of the π-BTN cell are stable and can be reversibly switched to each other by applying a voltage pulse of different frequencies. We experimentally demonstrate two bistable-switching operations: (i)the BTN-LCPG can either split or deflect the laser beam; and (ii)the BTN-LCPG can selectively diffract the laser beam to the +1st and -1st orders, while maintaining a high diffraction efficiency of ∼90%. With electrical switchability, a simple optical design, and low power consumption, the proposed BTN-LCPG concept is favorable in various applications.

Design of an all-optical logic sequence generator based on polarization holographic gratings

In this Letter, an all-optical logic sequence generator based on two different polarization holographic gratings has been proposed and demonstrated, which has one input port and four output ports. The polarization state of input light signal determines logic output signals. It can produce four kinds of logic sequence output signals: 1000, 0100, 0010, and 0001, corresponding to the input light signal of four different polarization states: the p-linear, s-linear, left-handed circular, and right-handed circular. The two polarization gratings have been fabricated, and the working principle of the logic sequence generator has been proved by diffraction pattern analysis of polarization gratings.

WIRC+Pol: A Low-resolution Near-infrared Spectropolarimeter

WIRC+Pol is a newly commissioned low-resolution (R ∼ 100), near-infrared (J and H bands) spectropolarimetry mode of the Wide-field InfraRed Camera (WIRC) on the 200 inch Hale Telescope at Palomar Observatory. The instrument utilizes a novel polarimeter design based on a quarter-wave plate and a polarization grating (PG), which provides full linear polarization measurements (Stokes I, Q, and U) in one exposure. The PG also has high transmission across the J and H bands. The instrument is situated at the prime focus of an equatorially mounted telescope. As a result, the system only has one reflection in the light path providing minimal telescope induced polarization. A data reduction pipeline has been developed for WIRC+Pol to produce linear polarization measurements from observations. WIRC+Pol has been on-sky since 2017 February. Results from the first year commissioning data show that the instrument has a high dispersion efficiency as expected from the polarization grating. We demonstrate the polarimetric stability of the instrument with rms variation at 0.2% level over 30 minutes for a bright standard star (J = 8.7). While the spectral extraction is photon noise limited, polarization calibration between sources remain limited by systematics, likely related to gravity dependent pointing effects. We discuss instrumental systematics we have uncovered in the data, their potential causes, along with calibrations that are necessary to eliminate them. We describe a modulator upgrade that will eliminate the slowly varying systematics and provide polarimetric accuracy better than 0.1%.

Liquid crystal Pancharatnam-Berry phase lens with spatially separated focuses

ABSTRACTIn this paper, we proposed and fabricated a liquid crystal (LC) lens with spatially separated focuses via liquid crystal photoalignment technology. The novel lens is an integration of the polarisation grating and conventional LC lens. The two focuses of that new lens, one of which is real while the other is virtual, can be spatially separated. When combined with normal convex lens and illuminated by a linearly polarised beam, the two emergent light beams become all convergent and the two focuses are separated in three-dimension space. Moreover, the focal lengths of the lens system can be artificially controlled by adjusting the distance between our new lens and the conventional lens. Our results achieve the potential of LC lens’ application in imaging devices such as multifocal cameras, bifocal eyeglasses and so on.

Effect of Yttrium Oxide Nanoparticles on the Diffraction Efficiency of Holographic Polymer-liquid Crystal Gratings

Electro-optical properties of the holographic and polarization holographic diffraction gratings formed in polymer-liquid crystal composites doped with Y2O3 nanoparticles are considered. The effect of impurities on the diffraction efficiency and operated electric field is shown. The diffraction efficiency of doped holographic diffraction gratings decreases, whereas the diffraction efficiency of polarization diffraction gratings increases. Doping with nanoparticles leads to a decrease in the operated electric field.

Circular Polarization and Wavelength Selective Gratings Based on Holographic Cholesteric Liquid Crystal Templates

Refilling cholesteric liquid crystal (CLC) template gratings with circular polarization, wavelength selectivity, and dual operation mode are first demonstrated. The gratings with template and nontemplate regions are obtained through the two-beam interfering photo-polymerization, washing-out, and refilling procedure. The refilling CLC has different chiral handedness (left or right) and reflection band (at green and red regions). When the wavelength of a probe beam is within the refilled CLC reflection band of the nontemplate region, the device works as an amplitude grating since the sample in the template and nontemplate regions reflects distinct colour region. In addition, the diffraction intensity of the grating can be electrically controlled. Therefore, this CLC-based device can be potentially used in controllable diffraction elements in optics.