Liquid Crystal Diffraction Research Articles

The Kinetics of the Formation of Multiplexed Chirped Multilayer Photopolymer–Liquid Crystal Diffraction Structures

The Kinetics of the Formation of Multiplexed Chirped Multilayer Photopolymer–Liquid Crystal Diffraction Structures

Achromatic diffractive liquid-crystal optics for virtual reality displays

Diffractive liquid-crystal optics is a promising optical element for virtual reality (VR) and mixed reality as it provides an ultrathin formfactor and lightweight for human factors and ergonomics. However, its severe chromatic aberrations impose a big challenge for full-color display applications. In this study, we demonstrate an achromatic diffractive liquid-crystal device to overcome this longstanding chromatic aberration issue. The proposed device consists of three stacked diffractive liquid crystal optical elements with specifically designed spectral response and polarization selectivity. The concept is validated by both simulations and experiments. Our experimental results show a significant improvement in imaging performance with two types of light engines: a laser projector and an organic light-emitting diode display panel. In addition, our simulation results indicate that the lateral color shift is reduced by ~100 times in comparison with conventional broadband diffractive liquid-crystal lens. Potential applications for VR-enabled metaverse, spatial computing, and digital twins that have found widespread applications in smart tourism, smart education, smart healthcare, smart manufacturing, and smart construction are foreseeable.

Highly dispersive liquid crystal diffraction gratings with continuously varying periodicity

By using well-designed anchoring patterns in a liquid crystal (LC) device, novel electro-optic components with enhanced functionality can be designed. We present two types of nematic LC diffraction gratings with varying periodicity, manufactured with patterned photoalignment at the confining substrates. By varying the surface anchoring periodicity, the incident light can be redirected into many different diffraction orders, giving rise to a scattering appearance of the grating. In Type I gratings the anchoring patterns at the top and bottom substrates are aligned in the same direction, whereas in the Type II grating the anchoring patterns at the top and bottom substrates have a relative rotation of 90°. The structural and optical properties of the gratings can be tuned by applying an external voltage. The director configuration in the bulk is revealed with the use of polarizing optical microscopy images and numerical simulations. Experimental results of the diffraction spectra and intensities of different diffraction orders are supported by finite difference time domain (FDTD) simulations. The results specifically aim at tunable LC diffraction gratings which are highly dispersive and have weak zeroth order diffraction. In general, this work is a contribution to the field of LC flat optical components with electric tunability.

Transmission functions of transmissive multilayer inhomogeneous holographic photopolymer liquid crystal diffraction structures

A new analytical model of diffraction of quasimonochromatic light beams on spatially inhomogeneous multilayer diffraction structures formed in photopolymer material with nematic liquid crystals having smooth optical inhomogeneity in the thickness of layers is presented. It is shown that when an applied electric field isapplied to diffraction layers containing a photopolymer composition with a high proportion of liquid crystalline component, a transformation of the selective response with a significant shift in angular selectivity occurs

A simulation of diffractive liquid crystal smart window for privacy application

Using a single substrate, we demonstrate a simple two-dimensional (2-D) phase grating cell with an octothorp electrode. Owing to the large spatial phase difference in any direction, the proposed grating cell has a high haze value in the opaque state (76.7%); Moreover, it has the advantages of a one-dimensional (1-D) phase grating cell, such as high fabricability, fast response time, and low operating voltage. Furthermore, the proposed grating cell has a faster response time than the 2-D grating cell (comparable to a 1-D grating cell). All the electro-optic parameters have been calculated using a commercial modeling tool. Consequently, we expect our proposed grating cell to find applications in virtual reality (VR)/augmented reality (AR) systems or window displays with fast response times.

6‐1: Invited Paper: Tutorial on Diffractive Liquid‐Crystal Devices for AR/VR Displays

Present AR and VR displays are facing technical challenges in inadequate angular resolution, limited field‐of‐view, vergence‐accommodation conflict, chromatic aberrations, low optical efficiency, compact size, and lightweight. In this review paper, using novel diffractive liquid crystal devices, including deflectors and lenses, to mitigate these issues will be presented.

Reconfigurable liquid crystal diffraction grating based on flexoelectric effect

Reconfigurable optical diffraction grating is of great importance for monochromator, spectrometer, optical fiber telecommunication, laser protection, antenna and more. We report a liquid crystal diffraction grating based on flexoelectric effect. When no voltage is applied, the liquid crystal is unidirectionally aligned and does not diffract light. When a DC voltage is applied, the liquid crystal is switched to a 1-dimensional periodic structure due to flexoelectric interaction and becomes diffracting. The periodicity and refractive index undulation of the periodic structure are determined by the elastic energy, due to the spatial variation of the liquid crystal director, and flexoelectric interaction energy. The diffraction angle and efficiency can be controlled by the applied voltage. The liquid crystal diffraction grating has the advantages of low driving voltage, high diffraction efficiency and independency of the polarization of incident light.

Gaze‐Matched Pupil Steering Maxwellian‐View Augmented Reality Display with Large Angle Diffractive Liquid Crystal Lenses

Maxwellian‐view structure exhibits several advantages in augmented reality (AR) displays, such as high efficiency, always‐in‐focus (no vergence‐accommodation conflict (VAC)), and simple optical structure. However, the bottleneck of Maxwellian‐view is its tiny eyebox. Extensive efforts have been devoted to enlarge the eyebox of the Maxwellian‐view system based on pupil duplication or pupil steering by creating multiple viewing points, however, the important gaze matching is neglected. Once the virtual image center deviates from the user's eye gaze, it will bring an unnatural viewing experience. Herein, a gaze‐matching Maxwellian‐view AR system with an enlarged eyebox is demonstrated. In the meantime, this AR system also maintains the properties of aberration‐free, high efficiency, highly transparent for ambient light, and relatively large field of view. Moreover, two layers of off‐axis cholesteric liquid crystal (CLC) lenses are applied as the optical combiner in the system, which is lightweight and compact. The off‐axis angle of such a CLC lens is as large as 60 degrees, which plays a vital role in future Maxwellian‐view AR headsets.

Light-Driven Fabrication of a Chiral Photonic Lattice of the Helical Nanofilament Liquid Crystal Phase.

A photonic lattice is an efficient platform for optically exploring quantum phenomena. However, its fabrication requires high costs and complex procedures when conventional materials, such as silicon or metals, are used. Here, we demonstrate a simple and cost-effective fabrication method for a reconfigurable chiral photonic lattice of the helical nanofilament (HNF) liquid crystal (LC) phase and diffraction grating showing wavelength-dependent diffraction with a rotated polarization state. Furthermore, the UV-exposed areas of the HNF film having chiral characteristics act as optical building blocks that induce resonant intensity modulation in the reflectance and transmittance modes and the optical rotation of the linear polarization. Our photonic lattice of the HNF can be an efficient platform for a chirality-embedded photonic lattice at a low cost.

Effects of a radiation dose in gamma-ray irradiation fields on holographic gratings formed by liquid crystal composites

The radiation resistance of liquid crystal diffraction gratings was investigated using a cobalt 60 gamma radiation source until the radiation reached a total dose of 1000 Mrad, corresponding to a very severe amount of radiation. The optical properties of gratings exposed to gamma-ray radiation were discussed with internal observations such as polarization optical microscopy and scanning electron microscopy. The radiation resistance of liquid crystal diffraction gratings for gamma-ray irradiation has been explained by associating the optical characteristic measurements for liquid crystal composite materials, including a glass substrate, by analyzing the internal volume grating structure. The radiation resistance for anisotropic diffraction in the HPDLC grating has been explained by the modulation of the LC orientation in the polymer network configuration affected by the gamma-ray irradiation.

Diffraction and Polarization Properties of Electrically-Tunable Nematic Liquid Crystal Grating.

This work demonstrates an electrically-tunable nematic liquid crystal (NLC) diffraction grating with a periodic electrode structure, and discusses the polarization properties of its diffraction. The efficiency of the first-order diffraction can be gradually controlled by applying external electric fields cross the NLC, and the maximum diffraction efficiency of the first-order diffraction that can be obtained is around 12.5% under the applied voltage of 5.0 V. In addition to the applied electric field, the efficiency of the first-order diffraction can also vary by changing the polarized state of the incident beam. Antisymmetric polarization states with symmetrical intensities in the diffractions corresponding to the +1 and −1 order diffraction signals are also demonstrated.

Polymer‐Stabilized Monodomain Blue Phase Diffraction Grating

AbstractThe structural beauty of liquid crystalline blue phases is attested not only by their colorful optical micrographic textures but also by optical isotropy. More importantly, the homogeneity of the structures influences greatly to the electro‐optic properties. In this work, how the lattice orientation in polymer‐stabilized blue phase liquid crystal (PS‐BPLC) diffraction grating affects phase‐modulated diffraction efficiency is elaborated. A well‐arranged monodomain structure of blue phase is achieved and its diffraction capability is compared with multidomain PS‐BPLC. The diffraction efficiency in the zeroth order of monodomain PS‐BPLC is improved by 9% compared to that of multidomain structure. In addition, both driving and threshold voltages in monodomain PS‐BPLC are decreased by 29% and 30%, respectively, compared to those in multidomain PS‐BPLC owing to the regular arrangement of cubic lattices that give rise to increase in the coherence length and unified response in molecular reorientation under electric fields. Furthermore, the monodomain PS‐BPLC shows an efficient polarization conversion of linear polarization into circular polarization. It is believed that the device exhibits great potential to be used as a polarization converter or beam steering devices.

Limits of applicability of the direct ray approximation in modeling optical properties of liquid-crystal diffraction gratings

Using computer modeling, we estimate limits of applicability of the direct ray approximation in modeling the optical properties of liquid-crystal diffraction gratings with continuous spatial modulation of the local optic axis orientation in a liquid crystal layer. The data presented concerning the influence of the spatial frequency and character of modulation of the local optic axis, as well as the magnitude of birefringence of the medium, on the accuracy of the results obtained in this approximation are also useful in considering birefringent layers with an aperiodic variation of the local optic axis.

Photopatterned azo poly(amide imide) layers as aligning substrates of holographic liquid crystal diffraction gratings for beam steering applications

Custom synthesized “T-type” azobenzene-functionalized poly(amide imide) allows for effective fabrication of a tunable liquid crystal photonic device for light beam steering.

Optimal triplicator design applied to a geometric phase vortex grating.

In this work, a geometric phase liquid-crystal diffraction grating based on the optimal triplicator design is realized, i.e., a phase-only profile that generates three diffraction orders with equal intensity and maximum diffraction efficiency. We analyze the polarization properties of this special diffraction grating and then use embedded spiral phases to design geometric phase vortex diffraction gratings. Finally, the fabrication of a two-dimensional version of such a design using a micro-patterned half-wave retarder is demonstrated, where the phase distribution is encoded as the orientation of the fast axis of the retarder. This proof-of-concept element is made of liquid crystal on BK7 substrate where the orientation of the LC is controlled via photoalignment, using a commercially available fabrication facility. Experimental results demonstrate the parallel generation of vortex beams with different topological charge and different states of polarization.

Ultra-fast switching blue phase liquid crystals diffraction grating stabilized by chiral monomer

We have demonstrated an ultra-fast switching and efficient polymer stabilized blue phase liquid crystal (PS-BPLC) diffraction grating utilizing a chiral monomer. We have obtained a 0.5 ms response time by a novel polymer stabilization method which is three times faster than conventional PS-BPLC. In addition, the diffraction efficiency was improved 2% with a much wider phase range and the driving voltage to switch the device is reduced. The polarization properties of the diffracted beam are unaffected by this novel polymer stabilization. This device can be useful for future photonic applications.

Fast response and transparent optically isotropic liquid crystal diffraction grating.

We have demonstrated an electrically tunable less polarization sensitive and fast response nanostructured polymer dispersed liquid crystal (nano-PDLC) diffraction grating. Fabricated nano-PDLC is optically transparent in visible wavelength regime. The optical isotropic nature was increased by minimizing the liquid crystal droplet size below visible wavelength thereby eliminated scattering. Diffraction properties of in-plane switching (IPS) and fringe-field switching (FFS) cells were measured and compared with one another up to four orders. We have obtained a pore-type polymer network constructed by highly interlinked polymer beads at which the response time is improved by strong interaction of liquid crystal molecules with polymer beads at interface. The diffraction pattern obtained by transparent nano-PDLC film has several interesting properties such as less polarization dependence and fast response. This device can be used as transparent tunable diffractor along with other photonic application.

Two-dimensional switchable blue phase gratings manufactured by nanosphere lithography.

Switchable two dimensional liquid crystal diffraction gratings are promising candidates in beam steering devices, multiplexers and holographic displays. For these areas of applications a high degree of integration in optical systems is much sought-after. In the context of diffraction gratings this means that the angle of diffraction should be rather high, which typically poses a problem as the fabrication of small grating periods is challenging. In this paper, we propose the use of nanosphere lithography (NSL) for the fabrication of two-dimensionally structured electrodes with a periodicity of a few micrometers. NSL is based on the self-assembly of micro- or nanometer sized spheres into monolayers. It allows for easy substrate structuring on wafer scale. The manufactured electrode is combined with a liquid crystalline polymer-stabilized blue phase, which facilitates sub-millisecond electrical switching of the diffraction efficiency at a diffraction angle of 21.4°.

Fast Control of Haze Value Using Electrically Switchable Diffraction in a Fringe-Field Switching Liquid Crystal Device

In this paper, we present a diffractive liquid crystal (LC) device capable of rapid switching between the transparent and translucent states for window display applications. In contrast to previously reported LC light shutters based on light scattering, the proposed LC device relies on diffraction of white incident light by an electric field-induced periodic continuous LC profile. It can be switched between the transparent and translucent states without a complicated driving scheme or a polymer structure. This device exhibits outstanding features for window display applications, such as a high transparency and a wide viewing angle in the transparent state, a low operating voltage, and a short response time.

A novel method of measuring the interaction matrix for diffractive liquid crystal wavefront correctors

The interaction matrix significantly affects the correction accuracy of wavefront correctors. For diffractive liquid crystal wavefront correctors (LCWFCs), the diffraction wavefront error must be taken into account to improve the accuracy of the interaction matrix. In this paper, a tunable Zernike polynomial coefficient method is demonstrated that decreases the effect of the diffraction wavefront error on the interaction matrix. Moreover, to eliminate the effect of the random error, a least squares method is combined with the tunable coefficient method. Experimental results show that, with the combined method, the error of the interaction matrix is decreased by a factor of 2.5 compared to that when the least squares method is used alone. Furthermore, an open loop adaptive correction experiment was performed and a considerable improvement of the image resolution was obtained by the novel method. Therefore, this method is very useful for liquid crystal adaptive optics systems to acquire the high correction accuracy.