Time-asymmetric loop around an exceptional point over the full optical communications band.

Topological operations around exceptional points1-8-time-varying system configurations associated with non-Hermitian singularities-have been proposed as a robust approach to achieving far-reaching open-system dynamics, as demonstrated in highly dissipative microwave transmission3 and cryogenic optomechanical oscillator4 experiments. In stark contrast to conventional systems based on closed-system Hermitian dynamics, environmental interferences at exceptional points are dynamically engaged with their internal coupling properties to create rotational stimuli in fictitious-parameter domains, resulting in chiral systems that exhibit various anomalous physical phenomena9-16. To achieve new wave properties and concomitant device architectures to control them, realizations of such systems in application-abundant technological areas, including communications and signal processing systems, are the next step. However, it is currently unclear whether non-Hermitian interaction schemes can be configured in robust technological platforms for further device engineering. Here we experimentally demonstrate a robust silicon photonic structure with photonic modes that transmit through time-asymmetric loops around an exceptional point in the optical domain. The proposed structure consists of two coupled silicon-channel waveguides and a slab-waveguide leakage-radiation sink that precisely control the required non-Hermitian Hamiltonian experienced by the photonic modes. The fabricated devices generate time-asymmetric light transmission over an extremely broad spectral band covering the entire optical telecommunications window (wavelengths between 1.26 and 1.675 micrometres). Thus, we take a step towards broadband on-chip optical devices based on non-Hermitian topological dynamics by using a semiconductor platform with controllable optoelectronic properties, and towards several potential practical applications, such as on-chip optical isolators and non-reciprocal mode converters. Our results further suggest the technological relevance of non-Hermitian wave dynamics in various other branches of physics, such as acoustics, condensed-matter physics and quantum mechanics.

Past, current, and future of holographic video display [Invited].

Holography has been considered as the ultimate technology for three-dimensional visual experience. Compared to the well-established static holographic technology, holographic video display is still in the research and development stage, before commercial products. This paper reviews various kinds of researches and related systems from the beginning of holographic video display to recent improvements, compares each specification, and describes their features. We discuss the key requirements for holographic display to be commercialized and widely used in everyday life.

Electrically Actuated Varifocal Lens Based on Liquid-Crystal-Embedded Dielectric Metasurfaces.

Compact varifocal lenses are essential to various imaging and vision technologies. However, existing varifocal elements typically rely on mechanically actuated systems with limited tuning speeds and scalability. Here, an ultrathin electrically controlled varifocal lens based on a liquid crystal (LC) encapsulated dielectric metasurface is demonstrated. Enabled by the field-dependent LC anisotropy, applying a voltage bias across the LC cell modifies the local phase response of the silicon meta-atoms, in turn modifying the metalens focal length. In a numerical implementation, a voltage-actuated metalens with continuous zoom and up to 20% total focal shift is demonstrated. The LC-based metalens concept is experimentally verified through the design and fabrication of a bifocal metalens that facilitates high-contrast switching between two discrete focal lengths upon application of a 9.8 Vpp voltage bias. Owing to their ultrathin thickness and adaptable design, LC-driven dielectric metasurfaces open new opportunities for compact varifocal lensing in a diversity of modern imaging applications.

High-throughput phenotyping platform for analyzing drought tolerance in rice.

MAIN CONCLUSION: A new imaging platform was constructed to analyze drought-tolerant traits of rice. Rice was used to quantify drought phenotypes through image-based parameters and analyzing tools. Climate change has increased the frequency and severity of drought, which limits crop production worldwide. Developing new cultivars with increased drought tolerance and short breeding cycles is critical. However, achieving this goal requires phenotyping a large number of breeding populations in a short time and in an accurate manner. Novel cutting-edge technologies such as those based on remote sensors are being applied to solve this problem. In this study, new technologies were applied to obtain and analyze imaging data and establish efficient screening platforms for drought tolerance in rice using the drought-tolerant mutant osphyb. Red-Green-Blue images were used to predict plant area, color, and compactness. Near-infrared imaging was used to determine the water content of rice, infrared was used to assess plant temperature, and fluorescence was used to examine photosynthesis efficiency. DroughtSpotter technology was used to determine water use efficiency, plant water loss rate, and transpiration rate. The results indicate that these methods can detect the difference between tolerant and susceptible plants, suggesting their value as high-throughput phenotyping methods for short breeding cycles as well as for functional genetic studies of tolerance to drought stress.

Large-area liquid crystal beam deflector with wide steering angle.

A slim beam deflector that satisfies both a large steering angle and a large area can be very useful in various applications. However, a smaller electrode pitch for a large steering angle and enlargement of its area are trade-off relations due to the limited number of control channels in an electrically tunable beam deflector system. For a large steering angle in the active area where actual diffraction occurs, an indium tin oxide electrode of 2 µm pitch was implemented through a stepper lithography. The via-hole process was developed to expand the reduced active area due to the small electrode pitch. We developed a beam deflector with 7200 controllable channels in an active area of 14.4mm×14.4mm. The maximum steering angle is 7.643° at a wavelength of 532 nm.

Dual-focal waveguide see-through near-eye display with polarization-dependent lenses.

A waveguide near-eye display (NED) with a dual-focal plane using a polarization-dependent lens device is proposed. The novel optical device is composed of a geometric phase holographic lens, a wave plate, and a circular polarizer, which is operating as a concave lens or a see-through optical window, depending on the polarization state of the input beam. Such property and ultra-thinness of about 1.5 mm can be applied to a combiner-eyepiece lens for augmented reality. This optical device attached to the waveguide provides two depth planes with polarization multiplexing. We have demonstrated that our proof-of-concept system has image planes at infinity and 20 diopters. The devised system can be expected to offer a better immersive experience, compared to a NED system with a single-focal plane.

Hypoxia promotes acquisition of aggressive phenotypes in human malignant mesothelioma.

BACKGROUND: Hypoxia is a hallmark of the solid tumor microenvironment and is associated with poor outcomes in cancer patients. The present study was performed to investigate mechanisms underlying the hypoxia-induced phenotypic changes using human malignant mesothelioma (HMM) cells. METHODS: Hypoxic conditions were achieved by incubating HMM cells in the air chamber. The effect of hypoxia on phenotype changes in HMM cells was investigated by performing in vitro clonogenicity, drug resistance, migration, and invasion assays. Signaling pathways and molecules involved in the more aggressive behaviors of HMM cells under hypoxia were investigated. A two-tailed unpaired Student's t-test or one-way ANOVA with Bonferroni post-test correction was used in this study. RESULTS: Hypoxic conditions upregulated hypoxia-inducible factor 1 alpha (HIF-1α) and HIF-2α in parallel with the upregulation of its target, Glut-1, in HMM cells. In vitro clonogenicity of HMM cells was significantly increased in hypoxic conditions, but the proliferation of cells at a high density in hypoxia was lower than that in normoxic conditions. The expression levels of HIF-2α and Oct4 were increased in hypoxic HMM cells. The percentage of cells with high CD44 expression was significantly higher in HMM cells cultured in hypoxia than those cultured in normoxia. Hypoxia significantly enhanced the resistance of HMM cells to cisplatin, which occurred through cytoprotection against cisplatin-induced apoptosis. While cisplatin treatment decreased the ratio of Bcl-2 to Bax in normoxic condition, hypoxia conversely increased the ratio in HMM cells treated with cisplatin. Hypoxia increased the mobility and invasiveness of HMM cells. Epithelial to mesenchymal transition was promoted, which was indicated by the repression of E-cadherin and the concomitant increase of vimentin in HMM cells. CONCLUSIONS: The data illustrated that hypoxic conditions augmented the aggressive phenotypes of HMM cells at the biological and molecular levels. The present study provides valuable background information beginning to understand aggressiveness of HMM in tumor microenvironments, suggesting that a control measure for tumor hypoxia may be an effective therapeutic strategy to reduce the aggressiveness of cancer cells in HMM patients.

Coherent anti-Stokes Raman scattering under frequency comb excitation.

We study the efficiency of coherent anti-Stokes Raman scattering (CARS) under frequency comb excitation. We calculate the power density of the anti-Stokes signal for two major cases: (1) molecular excitation by frequency comb and cw probe and, (2) both excitation and probing by frequency combs. In the first case, average CARS power varies as an inverse third degree of frequency combs free spectral range (FSR-3); in the second case, it varies as FSR-5. These results were applied to the CARS on blood glucose under frequency comb excitation. It was found that the resulting glucose CARS signal could approach nanowatt (nW) level at FSR=10 GHz.

Electrically tunable transmission-type beam deflector using liquid crystal with high angular resolution.

Highly efficient transmission-type beam deflectors that have high angular resolution have been widely used for various applications. Continuously tunable beam deflectors have also been needed for many purposes. An indium-tin-oxide (ITO), widely used for transparent electrodes, was placed on the upper and lower glass substrate. The ITO layer on the lower substrate was patterned by the contact mask aligner for relatively wide input and output pad compared to main grating ITO patterns in the active area. These input and output pads on the lower substrate are connected to each driving integrated circuit (IC), which has 360 channels for continuous control. A small pixel pitch of grating patterns of 6 µm (the electrode width is 3 µm with a 3 µm spacing) was developed, and the maximum diffraction angle is calculated and measured at 2.541° with a wavelength of 532 nm. A minimal cell gap of 2.5 µm was applied for the full 2π phase modulation by using a high-birefringence liquid crystal. A driving module for continuous beam steering is also developed and applied to the beam deflector system. A diffraction efficiency of about 50.9% is observed at an angle of diffraction about 2.541°.

Visible Wavelength Color Filters Using Dielectric Subwavelength Gratings for Backside-Illuminated CMOS Image Sensor Technologies.

We report transmissive color filters based on subwavelength dielectric gratings that can replace conventional dye-based color filters used in backside-illuminated CMOS image sensor (BSI CIS) technologies. The filters are patterned in an 80 nm-thick poly silicon film on a 115 nm-thick SiO2 spacer layer. They are optimized for operating at the primary RGB colors, exhibit peak transmittance of 60-80%, and have an almost insensitive response over a ± 20° angular range. This technology enables shrinking of the pixel sizes down to near a micrometer.

Slim coherent backlight unit for holographic display using full color holographic optical elements.

The coherent backlight unit (BLU) using a holographic optical element (HOE) for full-color flat-panel holographic display is proposed. The HOE BLU consists of two reflection type HOEs that change the optical beam path and shape by diffraction. The illumination area of backlight is 150 mm x 90 mm and the thickness is 10 mm, which is slim compared to other conventional coherent backlight units for holographic display systems. This backlight unit exhibits a total efficiency of 8.0% at red (660 nm), 7.7% at green (532 nm), and 3.2% at blue (460 nm) using optimized recording conditions for each wavelength. As a result, we could get a bright full color hologram image.

Modulation efficiency of double-phase hologram complex light modulation macro-pixels.

The modulation efficiency of the double-phase hologram macro-pixel that is designed for complex modulation of light waves is defined and analyzed. The scale-down of the double-phase hologram macro-pixel associated with the construction of complex spatial light modulators is discussed.

Can unilateral-approach minimally invasive transforaminal lumbar interbody fusion attain indirect contralateral decompression? A preliminary report of 66 MRI analysis.

PURPOSE: Few studies have measured the amount of indirect decompression at the contralateral neural foramen after unilateral-approach minimally invasive transforaminal lumbar interbody fusion (MITLIF). This study examined the amount of intraoperative indirect decompression at the contralateral neural foramen after a unilateral-approach MITLIF in patients with bilateral foraminal stenosis. METHODS: From February 2009 to October 2012, 66 consecutive patients with bilateral foraminal stenosis underwent unilateral-approach MITLIF and postoperative magnetic resonance imaging (MRI). Direct decompression was performed at the central canal and approach-side neural foramen, while indirect decompression using cage distraction was pursued at the contralateral neural foramen. Qualitative parameters of the central canal (dural sac morphology) and neural foramen (foramen morphology) were analyzed using pre- and post-operative MRI. Quantitative measurement on the central canal (dural sac cross-sectional area) and neural foramen (foramen height and width) were also measured. RESULTS: A total of 69 intervertebral levels in the 66 patients were analyzed. Qualitative parameters of the central canal and contralateral neural foramen improved significantly after unilateral-approach MITLIF (both P < 0.001). The mean dural sac cross-sectional area increased from 51.1 ± 28.8 to 84.8 ± 30.2 mm(2) (P < 0.001). The mean preoperative contralateral foramen height, maximum foramen width, and minimum foramen width were 11.8 ± 2.0, 4.9 ± 1.5, and 1.5 ± 0.7 mm, respectively, and these values increased postoperatively to 14.7 ± 2.5, 6.5 ± 1.8, and 2.4 ± 1.0 mm, respectively (all P < 0.001). CONCLUSION: Quantitative and qualitative parameters of the central canal and contralateral neural foramen increased significantly after unilateral-approach MITLIF.

Direct amplitude-only hologram realized by broken symmetry.

Holographic displays have been a long-standing ambition for decades to realize true-to-life reconstruction. However, their practical adoption is hindered by their subpar image quality compared to two-dimensional displays, which is fundamentally limited by restricted spatial frequency bandwidth and artifacts. We address the limitation by using a symmetry-broken amplitude-only spatial light modulator, demonstrating image quality comparable to that of two-dimensional displays. The broken conjugate symmetry induced by phase noise of modulators eliminates conjugate image that causes issues in amplitude-only holograms and allows direct reconstruction without additional optical elements. The proposed method provides enhanced robustness against artifacts caused by sub-pixel structures of modulators, enabling experimental reconstruction of high-quality holograms. The full bandwidth and the robustness result in a 5-decibel improvement in peak signal-to-noise ratio compared to state-of-the-art holograms. Furthermore, the hologram has 24 times higher optical efficiency and a smaller volume than the traditional amplitude-only holograms while real-time synthesis is enabled by using a neural network.

Resolution enhancement of holographic printer using a hogel overlapping method.

We propose a hogel overlapping method for the holographic printer to enhance the lateral resolution of holographic stereograms. The hogel size is directly related to the lateral resolution of the holographic stereogram. Our analysis by computer simulation shows that there is a limit to decreasing the hogel size while printing holographic stereograms. Instead of reducing the size of hogel, the lateral resolution of holographic stereograms can be enhanced by printing overlapped hogels, which makes it possible to take advantage of multiplexing property of the volume hologram. We built a holographic printer, and recorded two holographic stereograms using the conventional and proposed overlapping methods. The images and movies of the holographic stereograms experimentally captured were compared between the conventional and proposed methods. The experimental results confirm that the proposed hogel overlapping method improves the lateral resolution of holographic stereograms compared to the conventional holographic printing method.

Protein and lipid content estimation in soybeans using Raman hyperspectral imaging.

Unlike standard chemical analysis methods involving time-consuming, labor-intensive, and invasive pretreatment procedures, Raman hyperspectral imaging (HSI) can rapidly and non-destructively detect components without professional supervision. Generally, the Kjeldahl methods and Soxhlet extraction are used to chemically determine the protein and lipid content of soybeans. This study is aimed at developing a high-performance model for estimating soybean protein and lipid content using a non-destructive Raman HSI. Partial least squares regression (PLSR) techniques were used to develop the model using a calibration model based on 70% spectral data, and the remaining 30% of the data were used for validation. The results indicate that the Raman HSI, combined with PLSR, resulted in a protein and lipid model Rp2 of 0.90 and 0.82 with Root Mean Squared Error Prediction (RMSEP) 1.27 and 0.79, respectively. Additionally, this study successfully used the Raman HSI approach to create a prediction image showing the distribution of the targeted components, and could predict protein and lipid based on a single seeds.

Deep learning-based incoherent holographic camera enabling acquisition of real-world holograms for holographic streaming system.

While recent research has shown that holographic displays can represent photorealistic 3D holograms in real time, the difficulty in acquiring high-quality real-world holograms has limited the realization of holographic streaming systems. Incoherent holographic cameras, which record holograms under daylight conditions, are suitable candidates for real-world acquisition, as they prevent the safety issues associated with the use of lasers; however, these cameras are hindered by severe noise due to the optical imperfections of such systems. In this work, we develop a deep learning-based incoherent holographic camera system that can deliver visually enhanced holograms in real time. A neural network filters the noise in the captured holograms, maintaining a complex-valued hologram format throughout the whole process. Enabled by the computational efficiency of the proposed filtering strategy, we demonstrate a holographic streaming system integrating a holographic camera and holographic display, with the aim of developing the ultimate holographic ecosystem of the future.

Optimal synthesis of double-phase computer generated holograms using a phase-only spatial light modulator with grating filter.

We propose an optical system for synthesizing double-phase complex computer-generated holograms using a phase-only spatial light modulator and a phase grating filter. Two separated areas of the phase-only spatial light modulator are optically superposed by 4-f configuration with an optimally designed grating filter to synthesize arbitrary complex optical field distributions. The tolerances related to misalignment factors are analyzed, and the optimal synthesis method of double-phase computer-generated holograms is described.

Optimization of Optical Phase Profile in Beam Deflector with Advanced Simulation Method for High Diffraction Efficiency.

Controlling the phase of light with a high efficiency and precision is essential for applications in imaging, tunable devices, and optical systems. Spatial light modulators (SLMs) based on liquid crystals (LCs) have been regarded as one of the best choices for the generation of phase profiles for the steering of light. The upper glass substrate has an unpatterned electrode for a common electrode, while the lower glass substrate has one-dimensional micro-patterned electrodes for controlling the single pixel level by the applied voltages. By applying different voltages to each electrode to create a sawtooth-shaped phase profile, the collimated input beam is deflected to the desired angle. To maximize the diffraction efficiency (DE) values, an advanced simulation method has been developed to find the optimized phase profile through the analysis of LC director distributions. The resulting diffraction patterns are investigated both computationally and experimentally, with a good agreement between the results obtained. Finally, the beam deflector (BD) system with an advanced driving algorithm has a high 1st order DE, about 60%, 37%, and 7.5% at 1°, 2.5°, and a maximum steering angle of 7.5°, respectively. The LC director distributions in relation to various diffraction angles are simulated and an experimental success in realizing enhanced DE for the beam steering device is presented.

Diffraction-engineered holography: Beyond the depth representation limit of holographic displays.

Holography is one of the most prominent approaches to realize true-to-life reconstructions of objects. However, owing to the limited resolution of spatial light modulators compared to static holograms, reconstructed objects exhibit various coherent properties, such as content-dependent defocus blur and interference-induced noise. The coherent properties severely distort depth perception, the core of holographic displays to realize 3D scenes beyond 2D displays. Here, we propose a hologram that imitates defocus blur of incoherent light by engineering diffracted pattern of coherent light with adopting multi-plane holography, thereby offering real world-like defocus blur and photorealistic reconstruction. The proposed hologram is synthesized by optimizing a wave field to reconstruct numerous varifocal images after propagating the corresponding focal distances where the varifocal images are rendered using a physically-based renderer. Moreover, to reduce the computational costs associated with rendering and optimizing, we also demonstrate a network-based synthetic method that requires only an RGB-D image.