Equalization of RGB coupling efficiencies of metasurface waveguide coupler by adjusting imaginary part of refractive index.

Optical metasurfaces offer high-efficiency and flexible wavefront shaping for near-eye displays, especially in wideband waveguide couplers accommodating RGB primary colors. By leveraging the resonance characteristics of sub-wavelength periodic nanostructures, metasurfaces surpass the limitations of traditional optics that rely on multiple components and mediums. In this study, we propose adjustment of the imaginary parts of the material refractive indices as a new method to achieve balanced first-order diffraction efficiencies among RGB colors over a wide field of view (FOV) in an in-coupling metasurface waveguide coupler. Physical mechanism is investigated deeply and systematically in theory. It is found that nanostructure resonances deflect the wavefront and Poynting vector, significantly enhancing first-order diffraction efficiency, while resonance-enhanced absorption plays a crucial role in balancing the diffraction efficiency of RGB primary colors. First experimental demonstration well confirms the practical feasibility of this method and a uniform first-order diffraction efficiency of approximately 20% is achieved among RGB colors across a FOV as large as ∼30° over a single-piece glass substrate. This research provides insights into the design and mechanisms of metasurface waveguide couplers, advancing our understanding of metasurface-based RGB displays and facilitating further advancements in this field.

Alternative deep learning method for fast spatial-frequency shift imaging microscopy.

Spatial-frequency shift (SFS) imaging microscopy can break the diffraction limit of fluorescently labeled and label-free samples by transferring the high spatial-frequency information into the passband of microscope. However, the resolution improvement is at the cost of decreasing temporal resolution since dozens of raw SFS images are needed to expand the frequency spectrum. Although some deep learning methods have been proposed to solve this problem, no neural network that is compatible to both labeled and label-free SFS imaging has been proposed. Here, we propose the joint spatial-Fourier channel attention network (JSFCAN), which learns the general connection between the spatial domain and Fourier frequency domain from complex samples. We demonstrate that JSFCAN can achieve a resolution similar to the traditional algorithm using nearly 1/4 raw images and increase the reconstruction speed by two orders of magnitude. Subsequently, we prove that JSFCAN can be applied to both fluorescently labeled and label-free samples without architecture changes. We also demonstrate that compared with the typical spatial domain optimization network U-net, JSFCAN is more robust to deal with deep-SFS images and noisy images. The proposed JSFCAN provides an alternative route for fast SFS imaging reconstruction, enabling future applications for real-time living cell research.

Optical metasurfaces for waveguide couplers with uniform efficiencies at RGB wavelengths.

Optical metasurfaces hold great potential for near-eye display applications with high optical efficiency, light-weight and compactness. Taking advantage of optical resonances in subwavelength features, metasurfaces can diffract optical beams at RGB primary colors efficiently, forming superimposed virtual images over a real-world scene with merely a single glass substrate in augmented realities (AR) applications. We report for the first time a metasurface with double or triple nano-beams in each period for high angle diffraction with a uniform efficiency at RGB wavelengths. An efficiency as high as 30-40% of the first diffraction order is obtained across field of view, allowing a single piece of AR glass for light in-coupling to deliver image uniformly. The underlying physics is investigated through systematic full-vector numerical simulations. It is found that strong resonances inside the nano-beams with different sizes are the main reason for the deflection of wavefront as well as the Poynting vectors, leading to an efficient coupling of the incident light into the first-order diffraction. The resonances also manipulate the light absorption among the RGB colors for uniform efficiency. This work provides a new understanding of optimal metasurface structure for waveguide couplers using multiple nano-beams.

Oxidizing Hexagonal Boron Nitride into Fluorescent Structures by Photodissociated Directional Oxygen Radical.

Modifying the wide band gap semiconductor hexagonal boron nitride (hBN) can bring new chances in photonics. By virtue of the solvothermal/hydrothermal oxidation or functionalization, hBN can be converted into fluorescent nanodots. Until now, it has been a big challenge to drily oxidize hBN and turn it into bright fluorescent structures due to its superior chemical stability. Here, we report the oxidation of multilayer hBN into fluorescent structures by ultraviolet (UV, λ = 172 nm) photodissociated directional oxygen radical [O(3P)] in a gradient magnetic field. The paramagnetic O(3P), produced in a low-pressure O2 atmosphere, drifts toward hBN and then converts it into boron nitride oxide (BNO) micro/nanometer structures constituted by BO, BO2, and O-doped hBN. For a properly oxidized BNO substance, bright and photostable wide-band photoluminescence is realized with nanosecond-scaled lifetimes under the excitation of UV and visible lights.

Patterning Graphene Films by H2O-Based Magnetic-Assisted UV Photolysis.

Properly cutting graphene into certain high-quality micro-/nanoscale structures in a cost-effective way has a critical role. Here, we report a novel approach to pattern graphene films by H2O-based magnetic-assisted ultraviolet (UV) photolysis under irradiation at 184.9 nm. By virtue of the paramagnetic characteristic, the photo-dissociated hydroxyl [OH(X2Π)] radicals are magnetized and have their oxidation capability highly enhanced through converting into an accelerated directional motion. Meanwhile, the precursor of H2O(X̃1A1) molecules distributes uniformly thanks to its weak diamagnetic characteristic, and there exists no instable diamagnetic intermediate to cause lateral oxidation. Possessing these unique traits, the H2O-based magnetic-assisted UV photolysis has the capability of making graphene microscale patterns with the linewidth down to 8.5 µm under a copper grid shadow mask. Furthermore, it is feasible to pattern graphene films into 40 nm-wide ribbons under ZnO nanowires and realize hybrid graphene/ZnO nanoribbon field-effect transistors with a hole mobility up to 7200 cm2·V-1·s-1. The X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses reveal that OH(X2Π) radicals act as a strong oxidant and that another product of H(12S) adsorbs weakly on graphene.

UV Rewritable Hybrid Graphene/Phosphor p-n Junction Photodiode.

Graphene-based p-n junction photodiodes have a potential application prospect in photodetection due to their broadband spectral response, large operating bandwidth, and mechanical flexibility. Here, we report an ultraviolet (UV) rewritable p-n junction photodiode in a configuration of graphene coated with an amorphous phosphor of 4-bromo-1,8-naphthalic anhydride derivative polymer (poly-BrNpA). Under moderate UV irradiation, occurrence of photoisomerization reaction in the poly-BrNpA film leads to its drastically modified optical characteristics and a concurrent n-type doping in the underneath graphene. Meanwhile, the poly-BrNpA film, highly sensitive to water molecules, has a capability of restoring graphene to its initial p-type doping status by means of water adsorption. Based on these findings, a lateral graphene/poly-BrNpA p-n junction photodiode, responsive to visible light at the junction interface, can be written by UV irradiation and then erased via water adsorption. The p-n junction photodiode is rewritable upon such repetitive loops showing repeatable optoelectronic properties. This study provides a new scheme and perspective of making graphene-based rewritable p-n junction photodiodes in a flexible and controllable way, and it may contribute to expanding new families of optoelectronic devices based on two-dimensional materials.