Introduction to the feature issue on augmented/virtual reality: optics & photonics.

In recent years, augmented/virtual reality (AR/VR) has been attracting attention and investment in both the tech and academic communities, kickstarting a new wave of innovations. In the wake of this momentum, this feature issue was launched to cover the latest advances in this burgeoning field that pertains to optics and photonics. Alongside the 31 research articles being published, this introduction is appended to share with readers the behind-the-issue stories, submission statistics, reading guides, author biographies, and editors' perspectives.

Development of a Photonic Switch via Electro-Capillarity-Induced Water Penetration Across a 10-nm Gap.

With narrow and dense nanoarchitectures increasingly adopted to improve optical functionality, achieving the complete wetting of photonic devices is required when aiming at underwater molecule detection over the water-repellent optical materials. Despite continuous advances in photonic applications, real-time monitoring of nanoscale wetting transitions across nanostructures with 10-nm gaps, the distance at which photonic performance is maximized, remains a chronic hurdle when attempting to quantify the water influx and molecules therein. For this reason, the present study develops a photonic switch that transforms the wetting transition into perceivable color changes using a liquid-permeable Fabry-Perot resonator. Electro-capillary-induced Cassie-to-Wenzel transitions produce an optical memory effect in the photonic switch, as confirmed by surface-energy analysis, simulations, and an experimental demonstration. The results show that controlling the wetting behavior using the proposed photonic switch is a promising strategy for the integration of aqueous media with photonic hotspots in plasmonic nanostructures such as biochemical sensors.

Full-coloration based on metallic nanostructures through phase discontinuity in Fabry-Perot resonators.

We demonstrate a flexible full-color plate using Fabry-Perot (FP) resonators with two different types of silver nanostructures, a uniform nanofilm and a layer of nanoislands, for transmissive color elements. Two different nanostructures with deep-subwavelength features are selectively generated according to the layer thickness during vacuum deposition with no patterning process. In the nanofilm case, the primary optical mode accountable for generating the color shifts to blue from the original FP resonance while in the nanoislands case, it shifts to red so that a wide spectrum in the visible range is available through the phase discontinuity in the FP resonators. The peaks in the FP resonance shifted toward the opposite directions are attributed to the opposite signs of the phase retardations by a nanofilm and nanoislands. This approach paves a new way of constructing full-color elements for a variety of display devices and image storage systems.

Concept of chiral image storage and selection based on liquid crystals by circular polarization.

We demonstrate a liquid crystal (LC)-based optical device with the polarization switching capability, which can store two different chiral images to be selected according to the polarization state of the viewing polarizer. The chiral dual-image device consists of chiral surface patterns for image storage and the LC layer as a tunable phase retarder. Each chiral surface pattern behaves as a helical photonic crystal that reflects circularly polarized light at a specific wavelength. Depending on the applied voltage across the LC layer, either a right-handed or a left-handed circular polarization image appears, and thus one of the two stored images can be selectively read by the polarization state. Our concept of the LC-based chiral image storage and selection provides simplicity in fabrication, flexibility in design, and high optical efficiency. It will be directly applicable for reflective-type 3D displays, color filters, and anti-counterfeiting devices.

Selective photonic printing based on anisotropic Fabry-Perot resonators for dual-image holography and anti-counterfeiting.

We present the photonic printing that can display different color images depending on the optical polarization of incident light. The dynamic selection among different images becomes possible by using anisotropic Fabry-Perot resonators that incorporate a layer of liquid crystal molecules aligned by directional molecular registration (DMR) as polarization-dependent color pixels. Using the new device platform, we demonstrate a prototype of an anticounterfeiting label with inherent anti-replicability that results from the molecular-level origin of security images. In addition, this concept is extended to polarization-selective holography. Our molecular-level approach enables to develop a new class of security labels and holographic storage media.

Generation of intensity-tunable structural color from helical photonic crystals for full color reflective-type display.

A new concept of intensity-tunable structural coloration is proposed on the basis of a helical photonic crystal (HPC). The HPCs are constructed from a mixture of chiral reactive mesogens by spin-coating, followed by the photo-polymerization. A liquid crystal (LC) layer, being homogeneously aligned, is prepared on the HPCs to serve as a tunable waveplate. The electrical modulation of the phase retardation through the LC layer directly leads to the intensity-tunable Bragg reflection from the HPCs upon the incidence of the polarized light. The bandwidths of the structural colors are found to be well preserved regardless of the applied voltage. A prototype of a full color reflective-type display, incorporated with three primary color units, is demonstrated. Our concept of decoupling two mutually independent functions, the intensity modulation by the tunable waveplate and the color reflection by the HPCs provides a simple and powerful way of producing a full color reflective-type display which possesses high color purity, high optical efficiency, the cycling durability, and the design flexibility.

Concept of active parallax barrier on polarizing interlayer for near-viewing autostereoscopic displays.

We proposed a concept of an active parallax barrier using a liquid crystal-on-polarizing interlayer (LPI) for near-viewing autostereoscopic displays. In contrast to a conventional two-panel configuration where two independent panels are stacked together for displaying and parallaxing purposes, a monolithic one-panel architecture was demonstrated with the help of the LPI. The LPI was constructed using a polarizer sheet, one side of which provided the support for the active parallax barrier and the other served as the substrate for the image panel. For the active parallax barrier, an array of periodically patterned indium-tin-oxide electrodes was first prepared on the LPI and bi-level structures were subsequently fabricated for the cell gap and the liquid crystal alignment. Our monolithic one-panel architecture allows the near-viewing distance property which is essential for mobile applications.

F-number matching method in light field microscopy using an elastic micro lens array.

In light field microscopy (LFM), the F-number of the micro lens array (MLA) should be matched with the image-side F-number of the objective lens to utilize full resolution of an image sensor. We propose a new F-number matching method that can be applied to multiple objective lenses by using an elastic MLA. We fabricate an elastic MLA with polydimethylsiloxane (PDMS) using a micro contact printing method and address the strain for the F-number variation. The strain response is analyzed, and the LFM system with the elastic MLA is demonstrated. Our proposed system can increase the F-number up to 27.3% and can be applied to multiple objective lenses.

Dynamic Manipulation of Charged Lipids in Model Membrane for Bio-Microarrays.

We describe the dynamic manipulation of the charged lipids in a confined geometry where two dispersive factors arising from the random diffusion-based Brownian motion and the field-induced drift of target lipids compete with each other. It is found that the lateral distribution of the target lipids is well controlled through a combined effect of an external electric field and the geometric restrictions by the confinement. The dynamic manipulation scheme for the charged lipids in two-dimension would be useful for understanding the spatial organization of membrane components in a supported lipid membrane mimicking a real cell membrane and for producing membrane-based microarrays.

Lipid Membrane Deformation Accompanied by Disk-to-Ring Shape Transition of Cholesterol-Rich Domains.

During vesicle budding or endocytosis, biomembranes undergo a series of lipid- and protein-mediated deformations involving cholesterol-enriched lipid rafts. If lipid rafts of high bending rigidities become confined to the incipient curved membrane topology such as a bud-neck interface, they can be expected to reform as ring-shaped rafts. Here, we report on the observation of a disk-to-ring shape morpho-chemical transition of a model membrane in the absence of geometric constraints. The raft shape transition is triggered by lateral compositional heterogeneity and is accompanied by membrane deformation in the vertical direction, which is detected by height-sensitive fluorescence interference contrast microscopy. Our results suggest that a flat membrane can become curved simply by dynamic changes in local chemical composition and shape transformation of cholesterol-rich domains.

Submicro-pillars and holes from the depth-wise Talbot images of a conical phase mask.

We construct two-dimensional arrays of submicro-pillars and holes from the depth-wise Talbot images of a conical phase mask in a photoactive layer prepared on a quartz substrate. In contrast to the conventional Talbot lithography employing only one image in the photoactive layer, two images of the phase mask are produced in a depth-wise manner such that the pillar patterns are in the primary image plane while the hole patterns in the secondary image plane according to the penetration depth of the exposure energy. The conical symmetry plays a critical role in producing the covariant patterns of the phase mask in the photoactive layer through the suppression of higher orders of diffraction. Our two image-type approach is simple and versatile for producing different kinds of periodic structures for photonic applications and surface engineering on a micrometer-to-nanometer scale.

Optically switchable grating based on dye-doped ferroelectric liquid crystal with high efficiency.

We demonstrate an all-optically switchable ferroelectric liquid crystal (FLC) grating constructed in an alternating binary configuration with different optical properties from domain to domain. A dye-doped FLC is uniformly aligned in one type of domains whereas it is infiltrated into the photo-polymerized networks of reactive mesogens in the other. Compared to conventional nematic LC cases, our FLC grating allows more efficient all-optical modulation and faster diffraction switching between the 0th and the 1st orders in subsecond since the optical response associated with the dye molecules in the layered state is less hindered than in the orientationally ordered state. Our dye-doped FLC grating with periodically infiltrated structures will be useful for designing a new class of all-optically switching systems.

Self-organized wrinkling patterns of a liquid crystalline polymer in surface wetting confinement.

Self-organized wrinkling patterns of a liquid crystalline polymer, dictated by the chemico-physically anisotropic nature of surface wettability, are demonstrated in confined geometries. The symmetry of the geometrical constraints of the confinement primarily governs the periodic wrinkling patterns of such a polymer in the wetting region. In a circular geometry, the number of the radial domains with multi-fold symmetries is linearly proportional to the radius of the confinement. The physical origin of the wrinkling process comes from the periodic bend-splay distortions through the relaxation of the curvature elasticity.

Liquid crystal-based square lens array with tunable focal length.

We demonstrate a liquid crystal (LC)-based square lens array with two focusing modes according to the polarization state of the input light. The homogeneously aligned LC layer is placed on an array of static square lenses fabricated using a photo-curable polymer whose refractive index is matched with the refractive index of the LC. For the input beam polarized parallel to the easy axis of the LC, the focal length is varied with the applied voltage from a few meters to 21 mm which corresponds to the focal length of the static lens. For the perpendicularly polarized input beam, the focal length is independent of the applied voltage and remains constant. The two focusing effects with high optical performance over fully activated areas are useful for polarization-dependent imaging systems and three-dimensional displays in projection and integral imaging.

Combinatorial color arrays based on optical micro-resonators in monolithic architecture.

We demonstrate two types of combinatorial color arrays based on the Fabry-Perot (FP) micro-resonators in monolithic architecture. Optical micro-resonators corresponding to color elements are constructed using a soluble dielectric material between two transreflective layers by transfer-printing in either a pattern-by-pattern or a pattern-on-pattern fashion. The color palette depends primarily on the thickness and the refractive index of a dielectric material embedded in the micro-resonator. A self-defined lateral gap between two adjacent color elements provides the functionality of light-blocking by the underlying background layer. A prototype of a liquid crystal display incorporated with our combinatorial color array is also demonstrated. This monolithic integration of different FP micro-resonators leads to a versatile platform to build up a new class of color arrays for a variety of visual applications including displays and coloration devices.

Quasi-surface emission in vertical organic light-emitting transistors with network electrode.

We demonstrate a vertical-type organic light-emitting transistor (VOLET) with a network electrode of closed topology for quasi-surface emission. In our VOLET, the spatial distribution of the surface emission depends primarily on the relative scale of the aperture in the network electrode to the characteristic length for the charge carrier recombination. Due to the closed topology in the network of the source electrode, the charge transport and the resultant carrier recombination are substantially extended from individual network boundaries toward the corresponding aperture centers in the source electrode. The luminance was found to be well-controlled by the gate voltage through an organic semiconducting layer over the network source electrode.

Biocompatible patterning of proteins on wettability gradient surface by thermo-transfer printing.

We develop a simple and biocompatible method of patterning proteins on a wettability gradient surface by thermo-transfer printing. The wettability gradient is produced on a poly(dimethylsiloxane) (PDMS)-modified glass substrate through the temperature gradient during thermo-transfer printing. The water contact angle on the PDMS-modified surface is found to gradually increase along the direction of the temperature gradient from a low to a high temperature region. Based on the wettability gradient, the gradual change in the adsorption and immobilization of proteins (cholera toxin B subunit) is achieved in a microfluidic cell with the PDMS-modified surface.

Color liquid crystal grating based color holographic 3D display system with large viewing angle.

Holographic 3D display is highly desirable for numerous applications ranging from medical treatments to military affairs. However, it is challenging to simultaneously achieve large viewing angle and high-fidelity color reconstruction due to the intractable constraints of existing technology. Here, we conceptually propose and experimentally demonstrate a simple and feasible pathway of using a well-designed color liquid crystal grating to overcome the inevitable chromatic aberration and enlarge the holographic viewing angle, thus enabling large-viewing-angle and color holographic 3D display. The use of color liquid crystal grating allows performing secondary diffraction modulation on red, green and blue reproduced images simultaneously and extending the viewing angle in the holographic 3D display system. In principle, a chromatic aberration-free hologram generation mechanism in combination with the color liquid crystal grating is proposed to pave the way for on such a superior holographic 3D display. The proposed system shows a color viewing angle of ~50.12°, which is about 7 times that of the traditional system with a single spatial light modulator. This work presents a paradigm for achieving desirable holographic 3D display, and is expected to provide a new way for the wide application of holographic display.

Tunable binary retarder using self-aligned liquid crystal on anisotropic polymer film by photo-assisted imprinting.

We demonstrate an electrically tunable binary retarder (ETBR) with a self-aligned liquid crystal (LC) on an anisotropic polymer film produced by photo-assisted imprinting. The ETBR has two parts: a tunable optical layer of an LC and a static optical layer of an imprinted anisotropic polymer film possessing two different in-plane optic axes. The anisotropic polymer film was produced using reactive mesogens spontaneously aligned along the topographic microgrooves by imprinting under the exposure of ultraviolet light. An electrically tunable hybrid wave plate, whose phase retardation varies from a quarter to a half-wave, is constructed using the self-aligned LC layer on the imprinted polymer film that behaves as a quarter wave plate with two alternating optic axes. This approach can be used to design a new class of tunable optical devices with multiple in-plane optic axes.

Fully continuous liquid crystal diffraction grating with alternating semi-circular alignment by imprinting.

We demonstrate a fully continuous liquid crystal (LC) grating device with the alternating semi-circular alignment which exhibits the switching effect between the diffraction orders independent of the thickness of the LC cell. The continuous phase modulation in the LC grating with the rotational symmetry was achieved on a micro-imprinted surface where the semi-circular alignment of the LC was spontaneously produced. Our LC grating device in the hybrid geometry exhibited the perfect continuity of the phase retardation and the switchable diffraction with the diffraction efficiency of 44% at ±1st orders as a function of an applied voltage. It was found that the symmetry of the input polarization direction with respect to the grating patterns results in the interchange between two symmetric grating configurations.