Volume Gratings Research Articles

Alignment of polarization volume gratings using semi-circular arc patterns via nanofabrication technology

Polarization volume gratings have the potential for future applications such as AR due to their high single-order diffraction efficiency and large angular and wavelength response range. However, the preparation method of photo alignment, which is currently employed, presents challenges to the application of PVGs due to issues of poor uniformity and difficulty in preparing large sizes. To promote the application, a scheme for fabricating the alignment layer of PVGs by nanofabrication technology is proposed. The alignment structure, where the basic unit is a semicircular arc, was designed. We prepared the structure using EBL and spin-coated a layer of chiral liquid crystals (CLC) on top of it. The dispersion effect in white light indicates that the designed alignment structure imparts lateral periodicity to the CLC. The device could efficiently reflect specific right-polarized light and efficiently transmit specific left-polarized light. Therefore, the diffraction performance of the device can be considered to be consistent with the photo-alignment PVGs within the error allowance. This work provides what we believe to be a novel scheme for the alignment of PVGs, creating a new platform for mass production and multifunctional integrated design of PVGs.

Polarization conversion effect in cholesteric liquid crystal-based polarization volume gratings

Following the recent experimental discovery of a new polarization conversion phenomenon in polarization volume gratings (PVGs), in this paper, we investigate its underlying physical mechanisms in cholesteric liquid crystal (CLC) reflectors and PVGs. We examine the transition of eigenstates from circular to linear polarization as the incident angle deviates from the helical axis, which originates such an anomalous polarization conversion effect. While this transition enables PVGs to double the in-coupling efficiency and brightness uniformity in waveguide-based augmented reality (AR) displays, it also degrades the polarization selectivity and alters the transmitted polarization state in both CLC reflectors and PVGs, which in turn narrows down their spectral and angular bandwidth in the multi-layer design. These findings help not only deepen the understanding of polarization behaviors in CLC-based optical elements but also provide valuable insights to optimize their performances for emerging AR smart glasses.

Numerical Estimation of Bending in Holographic Volume Gratings by Means of RCWA and Deep Learning

In this paper, we introduce a novel approach to model bending phenomena on holographic volume gratings based on Rigorous Coupled Wave Analysis (RCWA), in which the bending as a phase in the dielectric permittivity expansion is introduced, and the Shooting Method (SM) is employed to solve the resulting system of equations. Further validation of our model is conducted by comparing its predictions to those obtained from reference Finite-Difference Time-Domain (FDTD) simulations and Coupled Wave Theory (CWT, referring to Kubota’s model that includes the bending phenomenon). Furthermore, we propose a methodology for estimating the bending from the diffraction efficiency curves in transmission volume gratings based on deep learning models, with a subsequent study of their accuracy and applicability.

Three-layer polarisation volume grating with enhanced diffraction efficiency and FOV in AR and VR applications

ABSTRACT In the rapidly evolving fields of Augmented Reality (AR) and Virtual Reality (VR), achieving high diffraction efficiency and a wide field of view (FOV) is critical for enhancing visual experience. This paper presents a three-layer Polarisation Volume Grating (TL-PVG) structure designed to address the limitations of single-layer PVGs in AR/VR applications. By stacking multiple PVG layers with different tilt angles, TL-PVG achieves an expanded angular bandwidth and improved diffraction efficiency, enabling brighter and more immersive displays. The proposed TL-PVG structure incorporates polarisation multiplexing to optimise the optical diffraction efficiency by approximately 75%, thereby enhancing image brightness and sharpness. The experimental results demonstrate that TL-PVG significantly outperforms traditional single-layer PVGs, offering a promising solution for waveguide elements in next-generation AR/VR displays. This study provides a comprehensive analysis of the fabrication process, optical properties, and applications of TL-PVGs, highlighting their potential to revolutionise the AR/VR technology.

Simultaneous monitoring of humidity and temperature by polarization volume gratings utilizing responsive cholesteric liquid crystals

Optical sensing devices are utilized for noncontact operation in harsh environments due to their advantage of local separation between the measurement and detector systems. Cholesteric liquid crystals (CLCs) with vivid structural colors are integrated into optical sensors, attracting significant interest for naked-eye detection. To enable vision-based, real-time remote monitoring, we propose a cost-effective method to convert the stimuli-responsive reflection change of CLCs into recordable diffraction patterns using polarization volume gratings (PVGs). This enables real-time remote readout of environmental parameters. PVGs are constructed through holographic polarization interference and photoalignment technologies. The diffraction efficiency of PVGs varies with relative humidity (RH). To improve sensing accuracy and extend the monitoring range of RH, we employ two methods: an orthogonal grating structure and dual-wavelength detection optical path. In addition, we demonstrate the simultaneous measurement of humidity and temperature by cascading independently responsive PVGs. We believe that this work will provide a broader perspective and expand the application of light dimensions in LC-based optical sensing.

Breaking the in-coupling efficiency limit in waveguide-based AR displays with polarization volume gratings

Augmented reality (AR) displays, heralded as the next-generation platform for spatial computing, metaverse, and digital twins, empower users to perceive digital images overlaid with real-world environment, fostering a deeper level of human-digital interactions. With the rapid evolution of couplers, waveguide-based AR displays have streamlined the entire system, boasting a slim form factor and high optical performance. However, challenges persist in the waveguide combiner, including low optical efficiency and poor image uniformity, significantly hindering the long-term usage and user experience. In this paper, we first analyze the root causes of the low optical efficiency and poor uniformity in waveguide-based AR displays. We then discover and elucidate an anomalous polarization conversion phenomenon inherent to polarization volume gratings (PVGs) when the incident light direction does not satisfy the Bragg condition. This new property is effectively leveraged to circumvent the tradeoff between in-coupling efficiency and eyebox uniformity. Through feasibility demonstration experiments, we measure the light leakage in multiple PVGs with varying thicknesses using a laser source and a liquid-crystal-on-silicon light engine. The experiment corroborates the polarization conversion phenomenon, and the results align with simulation well. To explore the potential of such a polarization conversion phenomenon further, we design and simulate a waveguide display with a 50° field of view. Through achieving first-order polarization conversion in a PVG, the in-coupling efficiency and uniformity are improved by 2 times and 2.3 times, respectively, compared to conventional couplers. This groundbreaking discovery holds immense potential for revolutionizing next-generation waveguide-based AR displays, promising a higher efficiency and superior image uniformity.

Large-size PVG-based waveguide simulation and uniformity optimization for AR-HUD.

Polarization volume grating (PVG), a kind of diffractive optical element, is applied widely for augmented reality (AR) near-eye display (NED) lately. However, PVG-based AR head-up display (AR-HUD) requires a large-size exit pupil and uniform efficiency, and there is presently no systematic simulation method for this type of application. Here, we introduce a unique simulation analysis method via the large-size PVG-based waveguide technology for AR-HUD. Through the self-built particle swarm optimization (PSO) algorithm, on the waveguide structure of 290 mm × 160 mm × 3 mm, with an eyerelief distance of 600 mm, the binocular field of view uniformity reaches 35.37% at an eye box of 100 mm × 50 mm, and the monocular uniformity can reach 31.65% and 32.48% respectively. The design scheme in this paper provides guidance for the large-size diffractive waveguide display for AR-HUD.

3‐5: Distinguished Paper: Full‐color, Wide FoV Single‐layer Waveguide for AR Displays

We analyze the field‐of‐view (FoV) limitations in a single‐layer, full‐color waveguide‐based augmented reality display, revealing key influences from the waveguide's refractive index, exit pupil expansion (EPE) scheme, and combiner's angular response. Based on these analyses, we propose an optimized butterfly EPE scheme with gradient‐pitch polarization volume gratings (PVGs), achieving a theoretical diagonal FoV of 54.06° with a 16:10 aspect ratio.

P‐45: A simulation method for crossed‐type exit pupil based on polarization volume grating

We propose a simulation method for two‐dimensional (2D) crossed‐type exit pupil expansion based on polarization volume grating (PVG) applied in diffractive waveguide holographic display. The functions of turning and decoupling of the PVG are realized through two continuous Bragg regions, and the spatial director distribution of liquid crystal is combined with RCWA to model its diffractive characteristics and ray tracing simulation.

15‐3: Reflective Liquid Crystal Polarization Volume Grating for SWIR with High Diffraction Efficiency and Large Diffraction Angle and Sensor Application

We have developed a reflective polarization volume grating using liquid crystal polymer for SWIR with high efficiency and a large diffraction angle. By using this grating and a lightguide system, we demonstrated a sensor system that can capture identical images simultaneously in both visible and SWIR light.

Full‐color, wide field‐of‐view single‐layer waveguide for augmented reality displays

AbstractIn the quest for more compact and efficient augmented reality (AR) displays, the standard approach often necessitates the use of multiple layers to facilitate a large full‐color field of view (FoV). Here, we delve into the constraints of FoV in single‐layer, full‐color waveguide‐based AR displays, uncovering the critical roles played by the waveguide's refractive index, the exit pupil expansion (EPE) scheme, and the combiner's angular response in dictating these limitations. Through detailed analysis, we introduce an innovative approach, featuring an optimized butterfly EPE scheme coupled with gradient‐pitch polarization volume gratings (PVGs). This novel configuration successfully achieves a theoretical diagonal FoV of 54.06° while maintaining a 16:10 aspect ratio.

28‐1: Study of the diffraction properties of gradient period polarization volume gratings

28‐1: Study of the diffraction properties of gradient period polarization volume gratings

P‐4.19: Tuning the Diffraction Efficiency of Polarization Volume Grating by UV Erosion

P‐4.19: Tuning the Diffraction Efficiency of Polarization Volume Grating by UV Erosion

P‐4.25: AR Waveguide Display with Enlarged Eyebox Based on Polarization Volume Gratings

P‐4.25: AR Waveguide Display with Enlarged Eyebox Based on Polarization Volume Gratings

14‐2: Research Progresses and Challenges for Polarization Volume Gratings Based Waveguide Display

14‐2: Research Progresses and Challenges for Polarization Volume Gratings Based Waveguide Display

Liquid-Crystal Polarization Volume Gratings Technology and its Applications in Diffractive Optical Waveguides Systems for Augmented Reality

Liquid-Crystal Polarization Volume Gratings Technology and its Applications in Diffractive Optical Waveguides Systems for Augmented Reality

Volume holograms with linear diffraction efficiency relation by (3 + 1)D printing.

We demonstrate the fabrication of volume holograms using two-photon polymerization with dynamic control of light exposure. We refer to our method as (3 + 1)D printing. Volume holograms that are recorded by interfering reference and signal beams have a diffraction efficiency relation that is inversely proportional to the square of the number of superimposed holograms. By using (3 + 1)D printing for fabrication, the refractive index of each voxel is created independently and thus, by digitally filtering the undesired interference terms, the diffraction efficiency is now inversely proportional to the number of multiplexed gratings. We experimentally demonstrated this linear dependence by recording M = 50 volume gratings. To the best of our knowledge, this is the first experimental demonstration of distributed volume holograms that overcome the 1/M2 limit.

Liquid crystal polarization volume gratings and their applications in augmented reality waveguide displays

Liquid crystal polarization volume gratings and their applications in augmented reality waveguide displays

Transversely graded polarization volume gratings fabricated by freeform holographic photoalignment.

Polarization volume gratings (PVGs) based on chiral nematic liquid crystals offer a great potential as polarization-dependent holographic optical elements, but it is not easy to fabricate PVGs with varying pattern periods in the transverse plane. Here, we fabricate a PVG with an in-plane gradient of the pattern period by performing two-beam interference photoalignment on a flexible polyimide substrate. The pattern period varies depending on the local interference angle, which is controlled by the bent shape of the flexible substrate. We demonstrate fabrication of a PVG with a linearly graded sub-micrometer period, showing the potential of the proposed method to fabricate designer PVGs.

Study on Two-Dimensional Exit Pupil Expansion for Diffractive Waveguide Based on Holographic Volume Grating

Diffraction gratings are becoming a preferred option for waveguide head-mounted in–out coupling devices due to their flexible optical properties and small size and light weight. At present, diffraction waveguide coupling devices for AR head-mounted displays (HMD) have difficulties such as a long development cycle and complicated processing. In this paper, we first establish a set of two-dimensional (2D) grating ray tracing models, based on which we determine the initial architecture of the dual-region two-dimensional exit pupil expansion (2D-EPE) AR-HMD holographic waveguide diffraction system. Second, we propose a honeycomb coupled grating array and optimize the optical energy utilization and brightness uniformity of the holographic waveguide and use a custom dynamic linked library (DLL) function to implement the ray tracing of the 2D grating and add a probabilistic splitting function to the DLL, which reduces the single simulation time from 11.853 min to 1.77 min. We also propose a holographic lithography device composed of holographic optical elements (HOEs) and a method for preparing HOEs. Finally, in order to obtain the diffraction efficiency preoptimized by the above DLL for the uniformity of the exit pupil brightness and light energy utilization, we inverse design with the preparation process parameters as the optimization variables and develop the adaptable electromagnetic calculation program Holo-RCWA. Using Holo-RCWA with nondominated sorting genetic algorithm II (NSGA-II), we inverse design to obtain the process parameters satisfying the diffraction efficiency distribution, and the optimization time of the whole system is reduced from 2–3 days to 10 h. This work is of great significance for AR/VR applications.