Liquid Crystal Polarization Gratings Research Articles

Performance and compensation method of first-order liquid crystal beam-steering system under continuous wave laser irradiation

The effects of laser irradiation thermal deposition on the performance degradation of liquid crystal (LC) devices in high-power laser systems are extremely vital. In this study, the first-order beam-steering system (FBS) was developed using a passive liquid crystal polarization grating (LCPG) and liquid crystal adjustable wave-plate (LCAW). The LCPG was prepared by holographic exposure technology, and the LCAW was prepared using the standard LC cell preparation process. We investigated the influences of diffraction efficiency of FBS under a 1070-nm continuous-wave (CW) laser irradiation. The energy is concentrated in the −1st order at first. When the half-wave retardation was reached, all the energy is concentrated in the +1st order (∼97 %), and the beam is deflected. Once the phase retardation decreases below the half-wave retardation, the energy gradually reconcentrates in the −1st order. When the LCAW fails, the diffraction the energy is refocused to the −1st order by the LCPG. The results indicated that both thermal deposition and driving voltage reduce the phase retardation of the LCAW, generating a variation of energy between in ±1st orders. By reducing the driving voltage, this half-wave retardation can be recovered under a certain half-wave voltage and laser power density, and the diffraction efficiency of the +1st order can be compensated to ∼97 %. If the reduced phase retardation is not less than half-wave, the FBS can still be modulated in high-power continuous lasers. Our research provides a reference for the development of beam-steering technology based on LC materials in high-power laser systems.

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 3.88µm and 9.5-10.6µm 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.

Orbit design and attitude control of diffractive sail and validation with solar polar mission

Liquid crystal diffractive sails can generate controllable tangential radiation pressures, which have advantages in the Solar Polar Imager mission (SPI). This paper proposes a composite diffractive sail design consisting of an ideal passive diffraction grating sail and liquid crystal polarization gratings (LCPGs). The main diffractive grating sail generates thrust for orbital transfer with a sun-facing attitude. An optimal 3.8-year transfer trajectory is obtained by an improved direct method. Small-area LCPGs distributed around the edge are used for three-axis attitude control. The LCPGs can continuously modulate the force and torque by controlling the applied voltage based on the electro-optical characteristic of liquid crystal materials. A control strategy, including an adaptive attitude-tracking PD controller and a control allocation method, is designed to track the attitude profile generated by the transfer trajectory. Finally, the attitude-orbit coupling dynamics are used to simulate the whole process, and the results show that orbital insertion errors are minor.

Electrically-switched differential microscopy based on computing liquid-crystal platforms

Abstract Detection of transparent phase specimens especially biological cells with desired contrasts is of great importance in visual display and medical diagnosis. Due to the pure-phase nature, conventional detection approaches may damage samples or require complex operations. Computing liquid crystal (LC) is a thin and flat optical element with excellent capability in optical field modulation, which gives a feasible way to this issue from the perspective of analog optical computing. We here propose and experimentally implement an electrically switched two-dimensional (2D) differential microscopy based on computing LC platforms. The Pancharatnam–Berry phase LC polarization grating induces light’s spin separation to promote the 2D differential operation. Using the electrically tunable LC plate as the system phase retardance provider, the detecting mode can be flexibly switched from bright-field images to edge-enhanced images with desired contrasts. Remarkably, owing to the wavelength-independent feature closely related to the geometric phases, our proposed scheme is demonstrated to be applicable to the multi-wavelength microscopy imaging. These results open avenues to form real-time all-optical image processing and may facilitate multifunctional differential microscopy.

Optical properties and nanosecond laser damage characterization of liquid crystal polarization gratings

Liquid crystal polarization gratings (LCPGs) possess unique optical properties and therefore are widely used in laser systems. Therefore, developing our understanding of the laser damage resistance of LCPGs is critical for future high-intensity laser applications. In this study, the LCPGs (Λ = 8 μm) were fabricated from liquid crystal polymer (LCP) films with photo-alignment technology, which had a small-angle (∼7.51°) diffraction order with high diffraction efficiency (∼92 %) under chiral polarized incidence at 1064 nm. The diffraction efficiency and polarization of each diffraction order were determined by the polarization of the incident light. The damage characteristics of the LCPGs were investigated using a chiral polarized nanosecond laser at 1064 nm in a one-on-one test. The laser-induced damage thresholds (LIDTs) of left-handed circular polarization (LHCP) and right-handed circular polarization (RHCP) were 8.0 J/cm2 and 8.5 J/cm2, respectively. For comparison, the LIDTs of S-polarization and P-polarization were 7.8 J/cm2 and 8.4 J/cm2, respectively. All the damage occurred at the interface between the LCP layer and the substrate when the laser fluence was near the LIDT, and the damage morphology was observed with a scanning electron microscope (SEM), including flat pits, cracks, and bumps at the edge of the pits. The results indicate that the LIDTs and damage morphology of the LCPGs were insensitive to the polarization of the irradiation. The electrical effects of differently polarized irradiation on the LCPGs were simulated using the finite element method (FEM). The electric field distribution was symmetrical under irradiation by LHCP and RHCP lasers. However, the distribution of the electric field under different linearly polarized irradiation conditions were distinct. Based on these findings, we believe that the contribution of chiral polarized irradiation to the damage was consistent and that the LIDTs were similar. This study has important implications for the application and development of LCPGs in high-intensity laser systems.

Research on one-to-two-point FSO system based on liquid crystal variable retarder cascade polarization grating.

We experimentally demonstrated a one-to-two-point free-space optical communication (FSO) system based on non-mechanical beam servo device in the laboratory. After the initial pointing, two sets of liquid crystal variable retarder cascaded polarization gratings perform non-mechanical beam servo and realized switching or working simultaneously of two communication links. The non-mechanical beam steerer had four diffraction fields; each can achieve beam steering with a 3.72° field and 30.77 µrad resolution, and the system emission efficiency was higher than 77%. The corresponding switching times of links at 2, 4, and 10 Hz were 46.7, 43.8, and 42.1 ms, respectively. In the quasistatic condition, the sensitivities of the two links under the data rate of 10.3125 Gbps were -23.18 and -23.01 dBm, respectively, indicating the service transmission capability of the multi-node beam control system.

Large aperture and defect-free liquid crystal planar optics enabled by high-throughput pulsed polarization patterning.

The liquid crystal (LC) geometrical phase optics, which is realized by the high-resolution control of the optical axis orientation in transparent micrometer-thin polymer films, is emerging as a next generation of planar optics. It features pronounced optical properties and stimuli-responsive behaviors, which could introduce appealing and new possibilities for photonic purposes. The development of fabrication techniques producing elements with large aperture sizes and arbitrarily varying molecular orientation is of significance in terms of practical utility. Here, we propose the pulsed polarization patterning technique to create large-aperture and defect-free LC geometrical phase elements. We investigated the capability of the azo photo-alignment material responding to nanosecond laser pulses and the corresponding anchoring behaviors to LCs. The threshold was reduced to one fourth of that under the continuous wave recording. The patterning resolution was found to be enhanced to around 0.71 µm, due to the ultra-fast interaction nature of the photo-alignment material with the polarized light field. We proposed the flying exposure mode to deliver high frequency modulated polarized laser pulses (8 kHz), with the precision stage moving in a uniform velocity for light-field stitching and the servo auto-focusing in the sample normal, enabling the stable and reliable polarization patterning for large aperture sizes. We further report on representative fabrication of LC polarization gratings with an aperture of 4 inch and 99.2% average diffraction efficiency.

Photoalignment of sub-micrometer periodic liquid crystal polarization grating by using the optical imprinting method.

Photoalignment of liquid crystal polarization grating based on optical imprinting is a promising technique for polarization grating mass production. However, when the period of the optical imprinting grating is in the sub-micrometer level, the zero-order energy from the master grating will become high, and it will strongly affect the photoalignment quality. This paper proposes a double-twisted polarization grating structure to eliminate the zero-order disturbance of master grating and gives the design method. Based on the designed results, a master grating was prepared, and the optically imprinted photoalignment of polarization grating with a period of 0.5μm was fabricated. This method has the advantages of high efficiency and significantly greater environmental tolerance than the traditional polarization holographic photoalignment methods. It has the potential to be used for large-area polarization holographic gratings production.

Optical imprinting subwavelength-period liquid crystal polarization gratings with dual-twist templates.

In this Letter, we report a dual-twist template imprinting method to fabricate subwavelength-period liquid crystal polarization gratings (LCPGs). In other words, the period of the template must be reduced to 800 nm-2 µm, or even smaller. To overcome the inherent problem that the diffraction efficiency shrinks as the period decreases, the dual-twist templates were optimized by rigorous coupled-wave analysis (RCWA). With the help of the rotating Jones matrix to measure the twist angle and thickness of the LC film, the optimized templates were fabricated eventually, and the diffraction efficiencies were up to 95%. Therefore, subwavelength-period LCPGs with a period of 400-800 nm were imprinted experimentally. Our proposed dual-twist template provides the possibility for fast, low-cost, and mass fabrication of large-angle deflectors and diffractive optical waveguides for near-eye displays.

P‐12.1: Two‐dimensional Edge Detection Implementation of Liquid Crystal Computing Metasurfaces

Computing metasurfaces have received widespread attention since they were proposed, and the ability to realize mathematical operations has enabled real time image processing with new breakthroughs, such as optical edge detection. In this paper, a new method for two-dimensional edge detection is presented, and the two-dimensional liquid crystal polarization grating is designed from the theoretical derivation. The gratings with different periods are fabricated by photoalignment technology. The one-dimensional and two-dimensional imaged edge detection are experimental realized in this work. By measuring the displacement of edge detection, the experimental results with different periods are in line with theoretical formula. The low cost and easily fabricated photoaligned liquid crystal polarization gratings achieved the real time optical computation with broadband and adjustable, which can be further applied in biological image edge and other optical display related fields.

Optimal Control of Passive Cascaded Liquid Crystal Polarization Gratings

The incidence angle of a beam entering a liquid crystal variable retarder (LCVR) and the liquid crystal polarization grating (LCPG) is greater when the LCPG is utilized in the quadrature cascade, which reduces the diffraction efficiency. Therefore, this study investigates the best method for controlling a cascaded LCPG. By optimizing the control coefficient and the method for controlling the voltage of the LCVR, this study reduces the angle between the incident LCVR and the polarization grating, and increases the diffraction efficiency of the grating. It was confirmed that this strategy can increase the diffraction effectiveness of the quadrature cascaded LCPG through the design of the experimental setup.

Development of a compact broadband circular dichroism spectropolarimeter for circular polarizer applications

A compact circular dichroism (CD) spectropolarimeter is demonstrated in this study. The system utilizes liquid crystal polarization gratings (LCPGs) and mini spectrometers to analyze the ellipticity spectra of polarized light. The residual spectra of the LCPG from the opposite diffraction order are subtracted to improve measurement accuracy by almost one order of magnitude. The novel CD spectropolarimeter is applied to inspect the quality of a commercial circular polarizer (CP), and the results are compared with those obtained with a commercial CD spectropolarimeter. The measurement accuracy of our system is determined to be around 0.3°. The results show that our design is feasible to produce a compact and low-cost CP inspection apparatus.

Optimization method of diffraction efficiency of cascade liquid crystal polarization gratings based on polarization compensation

Optimization method of diffraction efficiency of cascade liquid crystal polarization gratings based on polarization compensation

Full Stokes Polarization Imaging Based on Broadband Liquid Crystal Polarization Gratings

A method for full Stokes polarization imaging based on broadband liquid crystal polarization grating (LCPG) is presented. Firstly, the properties of the LCPG-based polarization detection module, which consists of a phase retarder and a broadband polarization grating in series, were investigated by means of Stokes calculus. The relationship between the polarization state of the incidence and the efficiencies of the diffractions was derived. Then, the azimuth parameters of the polarization detection module were optimized by using an equally weighted variance (EWV) evaluation criterion. Finally, the detection of the linearly and arbitrarily polarized laser beam and a linearly polarized broadband object were experimentally verified. The proposed method provides the design of polarization imaging systems based on LCPGs and has the advantages of compact structure, and broad wavelength coverage, therefore offering potential applications for image polarization information acquisition.

Establishment and verification of formulas of target tracking based on dual liquid crystal polarization gratings.

In this paper, the relationship between miss distance, target position, and rotation angle of dual liquid crystal polarization gratings (LCPGs) is proposed for the first time, a target tracking system based on the rotating dual LCPGs is established, and a closed-loop target tracking process based on the formula is described. We establish the optical structure of the dual LCPGs, and the simulation compares the difference between the altitude and azimuth angles of the target position from the formula and the ZEMAX simulation. In the simulation validation, we also compare and analyze the proposed formula with the formula for vertical incidence to illustrate the necessity to introduce miss distance. We assemble a target coarse tracking device and perform the tracking experiment using the proposed formula to test the coarse tracking accuracy of the system. The coarse tracking accuracy and the angle information we obtained from simulation and experiment meet our proposed technical specification of no more than 0.45mrad, showing that the proposed formula provides a good basis for stable target coarse tracking and is instructive for engineering applications.

Self-interference polarisation pantographer method to fabricate Pancharatnam-Berry phase optical elements

ABSTRACT This study proposed a novel method based on self-interference polarisation pantograph (SiPPer) for fabricating Pancharatnam-Berry (PB) devices. The SiPPer method was composed of a standard Cassegrainian system and a polarization patterned template. Based on the self-interference effect between the ordinary and extraordinary components after the birefringence template, all the polarization information were losslessly transferred to the focal plane on which a photosensitive polarization sample was placed. All PB devices are compatible with this method, including polarization grating, q-plate, q-lens, etc. The SiPPer method indeed lowers the requirement on the exposing source, particularly on the parameters of coherence length and linewidth. To verify and evaluate the properties of the SiPPer method, two typical examples, liquid crystal polarization grating and q-plate, were designed and fabricated according to the standard processing. And the experimental results showed fabulous agreement with the theoretical simulation results.

P‐12.10: Fast Switchable Depth‐ranging System Based on Combination of Liquid Crystal Polarization Grating and Ferroelectric Liquid Crystal Dammann Grating

P‐12.10: Fast Switchable Depth‐ranging System Based on Combination of Liquid Crystal Polarization Grating and Ferroelectric Liquid Crystal Dammann Grating

Optically imprinted polarization gratings with a twisted nematic liquid crystal as a photomask

Liquid crystal Pancharatnam–Berry phase optical elements (LCPBOEs) are becoming increasingly attractive elements to be used to enhance near-eye displays or manipulate light beams, owing to their flatness and circular polarization-dependent behavior. The optical imprinting technique that is one of the commonly used methods for fabricating LCPBOEs is considered to be the most promising technique for mass production at a low cost, but the design of the photomask must be improved. In this work, a photomask that is based on a twisted nematic liquid crystal polarization grating (TNLCPG) cell, acting as a broadband linear polarization rotator, is designed. This TNLCPG cell, with a predefined alignment profile of a polarization grating, can convert linearly polarized light into a light field with spatially varying linear polarization and exhibits broadband operation over the entire visible spectrum. The designed TNLCPG photomask is used to demonstrate a liquid crystal polarization grating (LCPG) film using a white flashlight as a writing source. A high diffraction efficiency of greater than 95% is obtained. With its simple design and fabrication, the proposed TNLC-based photomask can potentially be used in the fabrication of LCPBOEs.

High-accuracy circular dichroism measurement using a liquid crystal polarization grating

A significant improvement in the accuracy of a circular dichroism (CD) measurement system using a liquid crystal polarization grating (LCPG) for broadband applications can be achieved. The influences of the diffraction efficiency and residue from the opposite diffraction order for an LCPG are studied theoretically and experimentally. The results show that the measurement accuracy is highly dependent on the residue. The diffraction efficiency only affects the measurement resolution. We also find that when an LCPG is operated at a wavelength far from the design wavelength, the LCPG has a high residue, which will generate a significant measurement error. Five lasers with different wavelengths ranging from visible to infrared are applied to investigate the accuracy of our proposed system for broadband applications. The measurement error can be reduced by one order of magnitude for wavelengths far from the design wavelength of an LCPG by subtracting the residue effect. The final measurement accuracy of our CD apparatus is determined to be a few mdeg as shown in a comparison of the measurement results for a vitamin B12 solution with those obtained using a commercial CD spectropolarimeter.

Flexible and wide-range tuning of liquid crystal polarization grating period based on single and interference-free exposure.

We propose a period control method of liquid crystal polarization grating (LCPG) based on an nterference-free and single exposure process. By adjusting three parameters of exposure setup, including incident angle of exposure beam, wedge angle of birefringent prism and tilt angle of the sample, polarization distribution of the exposure beam is changed. The spatially variant polarization of the exposure beam is transferred to liquid crystal (LC) molecules by an azo-dye photo-sensitive layer. Consequently, the LCPG with the target period is obtained. The proposed method has high flexibility and a wide range of period adjustment covering several microns to more than thousands of microns according to calculated results. Experimental results fit well with calculations. The LCPGs with different values of period from 4.5µm to more than 200µm have been realized experimentally. The proposed interference-free method would accelerate the application of LCPGs with a robust and simple fabrication process.