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.

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.

Tunable liquid crystal grating based holographic 3D display system with wide viewing angle and large size.

As one of the most ideal display approaches, holographic 3-dimensional (3D) display has always been a research hotspot since the holographic images reproduced in such system are very similar to what humans see the actual environment. However, current holographic 3D displays suffer from critical bottlenecks of narrow viewing angle and small size. Here, we propose a tunable liquid crystal grating-based holographic 3D display system with wide viewing angle and large size. Our tunable liquid crystal grating, providing an adjustable period and the secondary diffraction of the reconstructed image, enables to simultaneously implement two different hologram generation methods in achieving wide viewing angle and enlarged size, respectively. By using the secondary diffraction mechanism of the tunable liquid crystal grating, the proposed system breaks through the limitations of narrow viewing angle and small size of holographic 3D display. The proposed system shows a viewing angle of 57.4°, which is nearly 7 times of the conventional case with a single spatial light modulator, and the size of the reconstructed image is enlarged by about 4.2. The proposed system will have wide applications in medical diagnosis, advertising, education and entertainment and other fields.

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.

Enhancement of Charge Injection in Organic Field-Effect Transistors Through Semiconducting Organic Buffer Layer.

We investigate the effect of a semiconducting organic buffer layer (SOBL) on the injection and transport of charges in organic field-effect transistors (OFETs). Here, two different injection barriers at the source/organic semiconductor interface are respectively studied with the aid of a numerical simulation: one is intermediate (0.4 eV), and the other is large energy barriers (0.6 eV). The introduction of nanostructure buffer layer, or SOBL, exhibits the decrease of potential loss at the contact interfaces, improving the electrical performance of the OFETs. It is also found that the energy level as well as the mobility of the SOBL plays an important role in determining the injection properties at the metal/organic hetero-interfaces and thus improving the device performance. Our systematic investigation on the injection barrier by the introduction of the nanostructure buffer layer will provide a useful guideline for the fabrication of high-performance FETs with molecular semiconductors.

Realization of Biomimetic Synaptic Functions in a One-Cell Organic Resistive Switching Device Using the Diffusive Parameter of Conductive Filaments.

Toward the successful development of artificial intelligence, artificial synapses based on resistive switching devices are essential ingredients to perform information processing in spiking neural networks. In neural processes, synaptic plasticity related to the history of neuron activity plays a critical role during learning. In resistive switching devices, it is barely possible to emulate both short-term plasticity and long-term plasticity due to the uncontrollable dynamics of the conductive filaments (CFs). Despite extensive effort to realize synaptic plasticity in such devices, it is still challenging to achieve reliable synaptic functions due to the overgrowth of CFs in a random fashion. Herein, we propose an organic resistive switching device with bio-realistic synaptic functions by adjusting the CF diffusive parameter. In the proposed device, complete synaptic plasticity provides the history-dependent change in the conductance. Moreover, the homeostatic feedback, which resembles the biological process, regulates CF growth in our device, which enhances the reliability of synaptic plasticity. This novel concept for realizing synaptic functions in organic resistive switching devices may provide a physical platform to advance the fundamental understanding of learning and memory mechanisms and develop a variety of neural circuits and neuromorphic systems that can be linked to artificial intelligence and next-generation computing paradigm.

Precise capture and dynamic relocation of nanoparticulate biomolecules through dielectrophoretic enhancement by vertical nanogap architectures.

Toward the development of surface-sensitive analytical techniques for biosensors and diagnostic biochip assays, a local integration of low-concentration target materials into the sensing region of interest is essential to improve the sensitivity and reliability of the devices. As a result, the dynamic process of sorting and accurate positioning the nanoparticulate biomolecules within pre-defined micro/nanostructures is critical, however, it remains a huge hurdle for the realization of practical surface-sensitive biosensors and biochips. A scalable, massive, and non-destructive trapping methodology based on dielectrophoretic forces is highly demanded for assembling nanoparticles and biosensing tools. Herein, we propose a vertical nanogap architecture with an electrode-insulator-electrode stack structure, facilitating the generation of strong dielectrophoretic forces at low voltages, to precisely capture and spatiotemporally manipulate nanoparticles and molecular assemblies, including lipid vesicles and amyloid-beta protofibrils/oligomers. Our vertical nanogap platform, allowing low-voltage nanoparticle captures on optical metasurface designs, provides new opportunities for constructing advanced surface-sensitive optoelectronic 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.

High Resolution Micro-patterning of Stretchable Polymer Electrodes through Directed Wetting Localization.

A microarray of conducting polymer electrodes with high resolution and high pattern-fidelity is developed on a stretchable substrate through the directed wetting localization (DWL) by the differential hydrophobicity. The large difference in the surface energy between the wetting and dewetting regions serves as the major determinant of the pattern resolution and the pattern-fidelity, yielding the full surface coverage in the stretchable electrode array (SEA) with 30 µm in width. The electrical characteristics of the SEA are well preserved under different types of elastic deformations. All-solution-processed polymer light-emitting diodes (except for the cathode) based on our patterned stretchable electrodes show no appreciable degradation of the performance under stretching. The DWL provides a simple and effective way of building up diverse stretchable electrical and optoelectronic devices in advanced wearable and bio-integrated electronics.

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.

Kinetics of lipid raft formation at lipid monolayer-bilayer junction probed by surface plasmon resonance.

A label-free, non-dispruptive, and real-time analytical device to monitor the dynamic features of biomolecules and their interactions with neighboring molecules is an essential prerequisite for biochip- and diagonostic assays. To explore one of the central questions on the lipid-lipid interactions in the course of the liquid-ordered (lo) domain formation, called rafts, we developed a method of reconstituting continuous but spatially heterogeneous lipid membrane platforms with molayer-bilayer juntions (MBJs) that enable to form the lo domains in a spatiotemporally controlled manner. This allows us to detect the time-lapse dynamics of the lipid-lipid interactions during raft formation and resultant membrane phase changes together with the raft-associated receptor-ligand binding through the surface plasmon resonance (SPR). For cross-validation, using epifluorescence microscopy, we demonstrated the underlying mechanisms for raft formations that the infiltration of cholesterols into the sphingolipid-enriched domains plays a crucial roles in the membrane phase-separation. Our membrane platform, being capable of monitoring dynamic interactions among lipids and performing the systematic optical analysis, will unveil physiological roles of cholesterols in a variety of biological events.

Interfacial Triggering of Conductive Filament Growth in Organic Flexible Memristor for High Reliability and Uniformity.

We demonstrate the physical pictures of the localization of the conductive filaments (CFs) growth in flexible electrochemical metallization (ECM) memristors through an interfacial triggering (IT) into the polymer electrolyte. The IT sites (ITSs), capable of controlling the pathways of the CF growth, are formed at the electrode-polymer interfaces via the Ostwald ripening at low temperatures (below 230 °C). The injection and migration of metal ions and the resultant CF growth are found to be effectively controlled through the ITSs with the local electric field enhancement. The reliability, uniformity, and switching voltage of the device are much improved by the presence of the ITSs. Our flexible ECM memristor exhibits a high mechanical flexibility and a stable memory performance under repeated bending deformations.

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.

Highly Sensitive Color Tunablility by Scalable Nanomorphology of a Dielectric Layer in Liquid-Permeable Metal-Insulator-Metal Structure.

A liquid-permeable concept in a metal-insulator-metal (MIM) structure is proposed to achieve highly sensitive color-tuning property through the change of the effective refractive index of the dielectric insulator layer. A semicontinuous top metal film with nanoapertures, adopted as a transreflective layer for MIM resonator, allows to tailor the nanomorphology of a dielectric layer through selective etching of the underneath insulator layer, resulting in nanopillars and hollow voids in the insulator layer. By allowing outer mediums to enter into the hollow voids of the dielectric layer, such liquid-permeable MIM architecture enables to achieve the wavelength shift as large as 323.5 nm/RIU in the visible range, which is the largest wavelength shift reported so far. Our liquid-permeable approaches indeed provide dramatic color tunablility, a real-time sensing scheme, long-term durability, and reproducibility in a simple and scalable manner.

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.

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.

Electrowetting-on-Dielectric Device Controlled by Embedded Undulating Electrode for Liquid Transport.

We demonstrated a new architecture of an electrowetting-on-dielectric (EWOD) device to transport a liquid droplet by the spatial modulation of an electric field produced using an embedded undulating electrode. The undulating electrode was constructed on an array of dielectric microstructures with different periods in region by region to generate a gradually varying lateral electric field. The contact angle of a droplet of water on the EWOD surface was found to decrease monotonically from 120 degrees to about 50 degrees with increasing the strength of the electric field. The transport of the water droplet was driven by the surface wettability gradient produced by means of the amplitude modulation of the electric field in space but not in time. Our EWOD configuration allows the flexibility in design, the simplicity in driving scheme, and the high accuracy in position for the liquid transport.

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.

Continuity of Monolayer-Bilayer Junctions for Localization of Lipid Raft Microdomains in Model Membranes.

We show that the selective localization of cholesterol-rich domains and associated ganglioside receptors prefer to occur in the monolayer across continuous monolayer-bilayer junctions (MBJs) in supported lipid membranes. For the MBJs, glass substrates were patterned with poly(dimethylsiloxane) (PDMS) oligomers by thermally-assisted contact printing, leaving behind 3 nm-thick PDMS patterns. The hydrophobicity of the transferred PDMS patterns was precisely tuned by the stamping temperature. Lipid monolayers were formed on the PDMS patterned surface while lipid bilayers were on the bare glass surface. Due to the continuity of the lipid membranes over the MBJs, essentially free diffusion of lipids was allowed between the monolayer on the PDMS surface and the upper leaflet of the bilayer on the glass substrate. The preferential localization of sphingomyelin, ganglioside GM1 and cholesterol in the monolayer region enabled to develop raft microdomains through coarsening of nanorafts. Our methodology provides a simple and effective scheme of non-disruptive manipulation of the chemical landscape associated with lipid phase separations, which leads to more sophisticated applications in biosensors and as cell culture substrates.