Aberration Theory of a Flat, Aplanatic Metalens Doublet and the Design of a Meta-Microscope Objective Lens.

A theoretical approach for reducing multiple monochromatic aberrations using a flat metalens doublet is proposed and verified through ray tracing simulations. The theoretical relation between the Abbe sine condition and the generalized Snell's law is revealed in the doublet system. Starting from the Abbe aplanat design, minimization conditions of astigmatism and field curvature are derived. Based on the theory, a metalens doublet is semi-analytically optimized as a compact, practical-level meta-microscope objective lens working for a target wavelength. The proposed approach also reveals how to reduce lateral chromatism for an additional wavelength. The design degree of freedom and fundamental limits of the system are both rigorously analyzed in theory and verified through ray tracing simulations. It is expected that the proposed method will provide unprecedented practical opportunities for the design of advanced compact microscopic imaging or sensing systems.

Preparation of Cellulose-Based Activated Carbon Fibers with Improved Yield and Their Methylene Chloride Adsorption Evaluation.

The recent rapid growth of the battery industry has led to a rapid increase in methylene chloride emissions. Methylene chloride causes health and social problems in humans. In this study, cellulose-based activated carbon fibers (CACFs) with improved yield were prepared for the removal of methylene chloride. The concentration of ammonium phosphate in the pretreatment controlled the crosslink density of cellulose fibers and improved the yield. From the results, the specific surface area and total pore volume of cellulose-based activated carbon fibers pretreated with ammonium phosphate (AP-CACFs) were determined to be 1920-2060 m2/g and 0.83-1.02 cm3/g, respectively, and the total yield improved by 6.78-11.59% compared to that of CACFs (4.97%). In particular, a correlation between the textural properties of CACFs and methylene chloride adsorption/desorption behavior was obtained. This correlation can be used to develop efficient adsorbents for methylene chloride removal.

Analysis on image quality of a holographic lens with a non-converging signal wave for compact near-eye displays.

We analyze an image quality of a holographic lens (HL) in order to implement compact near-eye displays using a flat-panel-type micro-display panel. The proposed method utilizes a non-converging signal wave in a fabrication process of the HL, so that it provides affordable eye-box size with minimizing the aberration due to rays in the off-Bragg condition. For analyzing and optimizing the HL based on the non-converging signal wave, we introduce a comprehensive analysis model for an assessment of the image quality in the HL. The analysis model, inspired from the conventional lens design strategy for near-eye displays, evaluates the focal spot quality for incident rays forming each pixel with considering the on- and off-Bragg diffraction. The theoretical analysis is validated by simulation results using a volume hologram model in Zemax OpticStudio. As experimental verifications, we realize a prototype system using photopolymer-based HLs in a green color with the high transmittance of 89.3%. The image quality of the HLs is analyzed, which coincides well with the proposed analysis and assessment metric. By building a compact experimental setup employing the HL and a micro-organic light emitting diode display, we present see-through images with 8.0 mm of eye-box with reduced aberrations.

Projection-type see-through near-to-eye display with a passively enlarged eye-box by combining a holographic lens and diffuser.

We propose a projection-type see-through near-to-eye display by combining two holographic optical elements (HOEs), a holographic lens with the on-axis projection configuration and a holographic diffuser. The proposed method provides an enlarged eye-box by virtue of diffusing properties of an HOE diffuser (HOED) without any temporal multiplexing methods. In this paper, a thorough analysis on imaging characteristics of an HOE lens (HOEL) according to the projection configuration is provided, so that we optimize the recording geometry of the HOEL with the passively enlarged eye-box. The theoretical analysis is validated by simulation results using a volume hologram model in OpticStudio. As experimental verifications, we realize a prototype of the proposed method using the photopolymer-based HOEs in a single color. The fabricated HOEL and HOED show high transmittance of 84.9% and 62.2%, respectively. By using the fabricated HOED with a diffusing angle over 20 ° and an angular selectivity of 8.7 °, the prototype successfully provides see-through images with the eye-box larger than 5 mm in width.

Dielectric Metalens: Properties and Three-Dimensional Imaging Applications.

Recently, optical dielectric metasurfaces, ultrathin optical skins with densely arranged dielectric nanoantennas, have arisen as next-generation technologies with merits for miniaturization and functional improvement of conventional optical components. In particular, dielectric metalenses capable of optical focusing and imaging have attracted enormous attention from academic and industrial communities of optics. They can offer cutting-edge lensing functions owing to arbitrary wavefront encoding, polarization tunability, high efficiency, large diffraction angle, strong dispersion, and novel ultracompact integration methods. Based on the properties, dielectric metalenses have been applied to numerous three-dimensional imaging applications including wearable augmented or virtual reality displays with depth information, and optical sensing of three-dimensional position of object and various light properties. In this paper, we introduce the properties of optical dielectric metalenses, and review the working principles and recent advances in three-dimensional imaging applications based on them. The authors envision that the dielectric metalens and metasurface technologies could make breakthroughs for a wide range of compact optical systems for three-dimensional display and sensing.

Deep neural network for multi-depth hologram generation and its training strategy.

We present a deep neural network for generating a multi-depth hologram and its training strategy. The proposed network takes multiple images of different depths as inputs and calculates the complex hologram as an output, which reconstructs each input image at the corresponding depth. We design a structure of the proposed network and develop the dataset compositing method to train the network effectively. The dataset consists of multiple input intensity profiles and their propagated holograms. Rather than simply training random speckle images and their propagated holograms, we generate the training dataset by adjusting the density of the random dots or combining basic shapes to the dataset such as a circle. The proposed dataset composition method improves the quality of reconstructed images by the holograms generated by the network, called deep learning holograms (DLHs). To verify the proposed method, we numerically and optically reconstruct the DLHs. The results confirmed that the DLHs can reconstruct clear images at multiple depths similar to conventional multi-depth computer-generated holograms. To evaluate the performance of the DLH quantitatively, we compute the peak signal-to-noise ratio of the reconstructed images and analyze the reconstructed intensity patterns with various methods.

Wide-angle speckleless DMD holographic display using structured illumination with temporal multiplexing.

We propose a digital micromirror device (DMD) holographic display, where speckleless holograms can be observed in the expanded viewing zone. Structured illumination (SI) is applied to expand the small diffraction angle of the DMD using a laser diode (LD) array. To eliminate diffraction noise from SI, we utilize an active filter array for the Fourier filter and synchronize it with the LD array. The speckle noise is reduced via temporal multiplexing, where the proposed system supports a dynamic video of 60 Hz using the DMD's fast operation property. The proposed system is verified and evaluated with experimental results.

Holographically customized optical combiner for eye-box extended near-eye display.

We propose a customizing method of a holographic optical element (HOE) by using a holographic printer, which extends the eye-box with high field of view (FOV) for a holographic augmented reality near-eye display (AR NED). The holographic printer setup to manufacture HOE is presented and a prototype of the AR NED is implemented. To make a simple AR NED system, we propose a total internal reflection holographic printing method using an index-matching optical frame. As a result, the eye-box of the AR NED is extended in both vertical and horizontal directions and FOV of 50° is achieved at the center of the eye-box. Through the simulations and the experimental results, the feasibility of the proposed method is verified.

Fast recovery process of carbon fibers from waste carbon fibers-reinforced thermoset plastics.

In this work, we report a fast recycling process for carbon fiber-reinforced thermosetting resin matrix composites, to obtain recycled carbon fibers. Steam (H2O) was selected as an oxidant to decompose the resin of the composites. The recycling reaction temperature and time were set in the range of 600-800 °C and 60 min, respectively. The recovery yield, surface morphologies, and mechanical properties including tensile strength and modulus of the recovered fibers were measured to evaluate the recycling efficiency. Microstructural properties of the recycled fiber were observed by X-ray studies, and the correlation of mechanical properties of the fibers with crystallite size and distribution was also evaluated. In conclusion, the carbon fibers were successfully recycled, while retaining 65% and 100% of the fibers' original tensile strength and modulus, respectively. 100% recovery yield was achieved in 60 min of decomposition time and 140 min of total process time.

Broadband efficient modulation of light transmission with high contrast using reconfigurable VO2 diffraction grating.

Ultra-compact dynamically reconfigurable modulation of optical transmission has been widely studied by using subwavelength-spaced resonant metasurface structures containing reconfigurable optical materials. However, it has been difficult to achieve high transmissivity, large modulation depth, and broad bandwidth simultaneously with the conventional resonance-based metasurface schemes. Here, we propose a reconfigurable phase-transition diffractive grating, made of thick VO2 ridge waveguides, for achieving the above-mentioned three goals simultaneously in the near-infrared range. Based on the large dielectric-to-plasmonic transition characteristic of VO2 in the near-infrared range, diffraction directivity of dual-VO2 ridge waveguide is designed to be tuned by thermally driven phase transition of VO2 for transverse electrically polarized illumination. Then, the diffractive VO2 ridge waveguide grating composed of the periodically arranged dual VO2 ridge waveguides is designed with on-state efficiency around 0.3 and minimum modulation depth about 0.35 over a broad bandwidth of 550 nm (1100-1650 nm). The working principle and excellent modulation performance are thoroughly verified through numerical and experimental studies.

Viewing angle enhancement of an integral imaging display using Bragg mismatched reconstruction of holographic optical elements.

Holographic-optical-element (HOE)-based integral imaging display can be applied to augmented reality. However, a narrow viewing angle is a bottleneck for commercialization. Here, we propose a method to enhance the viewing angle of the integral imaging display using Bragg mismatched reconstruction of HOEs. The viewing angle of the integral imaging display can be enlarged with two probe waves, which form two different viewing zones. The effect of Bragg mismatched reconstruction is analyzed with simulation and experiment. In order to show feasibility of the proposed method, a display experiment is demonstrated.

Recent progress in see-through three-dimensional displays using holographic optical elements [Invited].

The principles and characteristics of see-through 3D displays are presented. We especially focus on the integral-imaging display system using a holographic optical element (IDHOE), which is able to display 3D images and satisfy the see-through property at the same time. The technique has the advantage of the high transparency and capability of displaying autostereoscopic 3D images. We have analyzed optical properties of IDHOE for both recording and displaying stages. Furthermore, various studies of new applications and system improvements for IDHOE are introduced. Thanks to the characteristics of holographic volume grating, it is possible to implement a full-color lens-array holographic optical element and conjugated reconstruction as well as 2D/3D convertible IDHOE. Studies on the improvements of viewing characteristics including a viewing angle, fill factor, and resolution are also presented. Lastly, essential issues and their possible solutions are discussed as future work.

Space bandwidth product enhancement of holographic display using high-order diffraction guided by holographic optical element.

A space bandwidth product (SBP) enhancement method for holographic display using high-order diffraction of a spatial light modulator (SLM) is proposed. Among numerous high order diffraction terms, the plus-minus first and the zeroth are adopted and guided by holographic optical elements (HOEs) to an identical direction with the same intensity. By using a set of electro-shutters synchronized with corresponding order component, the system acts as if three SLMs are tiled in the horizontal direction. To confirm the feasibility of using HOE as the guiding optics for the system, several optical characteristics of the recording material are measured before using them. Furthermore, a computer generated hologram algorithm is proposed for compensating the wavefront distortion caused by use of the HOE. The demonstrated system achieves a three-fold increase in SBP of a single SLM. The results are verified experimentally.

Three-dimensional/two-dimensional convertible projection screen using see-through integral imaging based on holographic optical element.

We propose a 3D/2D convertible screen using a holographic optical element and angular multiplexing method of volume hologram. The proposed screen, named a multiplexed holographic optical element screen (MHOES), is composed of passive optical components, and displaying modes between 3D and 2D modes are converted according to projection directions. In a recording process, the angular multiplexing method by using two reference waves with different incidence angles enables the functions of 3D and 2D screens to be recorded in a single holographic material. Also, in order to avoid the bulky experimental setup due to adopting different projectors for the 3D and 2D modes, the projection part is realized based on a prism. The designed projection part enables the single projector to present 3D on 2D mode, where the 3D and 2D contents are simultaneously displayed in one scene, without active components. The optical characteristics of MHOES are experimentally analyzed, and displaying experiments with a full-color MHOES are presented in order to verify the 3D/2D convertibility and see-through properties.

Reflection-type integral imaging system using a diffuser holographic optical element.

A reflection-type integral imaging (InIm) system using a diffuser holographic optical element (DHOE) is proposed for improving the fill factor of displayed three-dimensional images. The DHOE performs an optical function similar to that for a conventional diffuser only for Bragg matched light, while Bragg mismatched light passes through the DHOE. Elemental images projected under Bragg matching condition are scattered by the DHOE. Meanwhile, light reflected by a concave mirror-array becomes Bragg mismatched light, and is integrated into three-dimensional images without the fill factor problem. The optical characteristics of the DHOE are examined by measuring diffraction efficiencies, and the feasibility of the fill-factor-improved InIm is verified by a concave mirror-array and DHOE.

Projection-type dual-view three-dimensional display system based on integral imaging.

A dual-view display system provides two different images in different directions. Most of them only present two-dimensional images for observers. In this paper, we propose a projection-type dual-view three-dimensional (3D) display system based on integral imaging. To assign directivities to the images, a projection-type display and dual-view screen with lenticular lenses are implemented. The lenticular lenses split the collimated image from the projection device into two different directions. The separated images are integrated by a single lens array in front of the screen, and full-parallax 3D images are observed in two different viewing regions. The visibility of the reconstructed 3D images can be improved by using high-density lenticular lenses and a high numerical aperture lens array. We explain the principle of the proposed method and verify the feasibility of the proposed system with simulations and experimental results.

An innovations-based noise cancelling technique on inverse kepstrum whitening filter and adaptive FIR filter in beamforming structure.

This paper presents an acoustic noise cancelling technique using an inverse kepstrum system as an innovations-based whitening application for an adaptive finite impulse response (FIR) filter in beamforming structure. The inverse kepstrum method uses an innovations-whitened form from one acoustic path transfer function between a reference microphone sensor and a noise source so that the rear-end reference signal will then be a whitened sequence to a cascaded adaptive FIR filter in the beamforming structure. By using an inverse kepstrum filter as a whitening filter with the use of a delay filter, the cascaded adaptive FIR filter estimates only the numerator of the polynomial part from the ratio of overall combined transfer functions. The test results have shown that the adaptive FIR filter is more effective in beamforming structure than an adaptive noise cancelling (ANC) structure in terms of signal distortion in the desired signal and noise reduction in noise with nonminimum phase components. In addition, the inverse kepstrum method shows almost the same convergence level in estimate of noise statistics with the use of a smaller amount of adaptive FIR filter weights than the kepstrum method, hence it could provide better computational simplicity in processing. Furthermore, the rear-end inverse kepstrum method in beamforming structure has shown less signal distortion in the desired signal than the front-end kepstrum method and the front-end inverse kepstrum method in beamforming structure.

See-through optical combiner for augmented reality head-mounted display: index-matched anisotropic crystal lens.

A novel see-through optical device to combine the real world and the virtual image is proposed which is called an index-matched anisotropic crystal lens (IMACL). The convex lens made of anisotropic crystal is enveloped with the isotropic material having same refractive index with the extraordinary refractive index of the anisotropic crystal. This optical device functions as the transparent glass or lens according to the polarization state of the incident light. With the novel optical property, IMACL can be utilized in the see-through near eye display, or head-mounted display for augmented reality. The optical property of the proposed optical device is analyzed and aberration by the anisotropic property of the index-matched anisotropic crystal lens is described with the simulation. The concept of the head-mounted display using IMACL is introduced and various optical performances such as field of view, form factor and transmittance are analyzed. The prototype is implemented to verify the proposed system and experimental results show the mixture between the virtual image and real world scene.