Angle-based wavefront sensing enabled by the near fields of flat optics

Nonlinear Optical Information Encoding with Grayscale Lithography Enabled Metasurfaces

Optical information encoding is promising for many applications in sensing, data storage, and computing. Recently, various strategies have been suggested to encode optical information in planar devices. Among these, optical metasurfaces represent a flexible platform for manipulating multiple degrees of freedom of light with subwavelength scale meta-atoms. However, to realize both amplitude and phase control of light with metasurfaces, usually multiple meta-atoms per unit cell are required, so information density will be greatly reduced. Here, we develop a novel approach of nonlinear optical information encoding with grayscale lithography enabled hybrid metasurfaces composed of gold plasmonic meta-atoms deposited on an epsilon-near-zero material. By controlling the spacer layer thickness with electron beam grayscale lithography and varying orientation angles of the meta-atoms, we can control at the single-pixel level both the amplitude and phase of the generated second-harmonic waves. The proposed method opens new avenues for developing advanced nonlinear nanophotonic sources.

Filterless vector light field photodetector based on photonic-electronic co-designed non-Hermitian silicon nanostructures

Recent advances in near-field interference detection, inspired by the non-Hermitian coupling-induced directional sensing of Ormia ochracea, have demonstrated the potential of paired semiconductor nanowires for compact light field detection without optical filters. However, practical implementation faces significant challenges including limited active area, architectural scaling constraints, and incomplete characterization of angular and polarization information. Here, we demonstrate a filterless vector light field photodetector, leveraging the angle- and polarization-sensitive near-field interference of non-Hermitian semiconductor nanostructures. Our design unit comprises four devices, each containing identical silicon nanowires but varying in orientation and electric connection configuration, of which the four-dimensional photoconductive output can be uniquely mapped to key vector light field parameters: intensity, polar angle, azimuth angle, and the linear polarization difference (Stokes parameter, S 1). Optimization of the geometry and doping concentration of these optoelectronic nanostructures yields a theoretical polar angle detectivity of 4 × 10-5 °/Hz0.5. This work establishes a paradigm for multi-output photodetectors with full-rank response matrices for multi-dimensional light field characterization, paving the way for integrated vector light field sensing systems.

Optoelectronic metadevices

Metasurfaces have introduced new opportunities in photonic design by offering unprecedented, nanoscale control over optical wavefronts. These artificially structured layers have largely been used to passively manipulate the flow of light by controlling its phase, amplitude, and polarization. However, they can also dynamically modulate these quantities and manipulate fundamental light absorption and emission processes. These valuable traits can extend their application domain to chipscale optoelectronics and conceptually new optical sources, displays, spatial light modulators, photodetectors, solar cells, and imaging systems. New opportunities and challenges have also emerged in the materials and device integration with existing technologies. This Review aims to consolidate the current research landscape and provide perspectives on metasurface capabilities specific to optoelectronic devices, giving new direction to future research and development efforts in academia and industry.

Linearized wavefront sensing model for aberration retrieval from low-frequency Fourier coefficients

This paper proposes and demonstrates a linearized model for phase diversity wavefront sensing, facilitating real-time processing and much less data required for training. Specifically, we find that the low-frequency Fourier coefficients of point spread function images are linearly proportional to pupil aberration coefficients under certain conditions. Simulation and experimental results show that the model can greatly reduce the processing time to several milliseconds by merely requiring hundreds of training samples while maintaining a comparatively high accuracy with state-of-the-art methods.

3D Light-Direction Sensor Based on Segmented Concentric Nanorings Combined with Deep Learning

High-precision, ultra-thin angular detectable imaging upon a single pixel holds significant promise for light-field detection and reconstruction, thereby catalyzing advancements in machine vision and interaction technology. Traditional light-direction angle sensors relying on optical components like gratings and lenses face inherent constraints from diffraction limits in achieving device miniaturization. Recently, angle sensors via coupled double nanowires have demonstrated prowess in attaining high-precision angle perception of incident light at sub-wavelength device scales, which may herald a novel design paradigm for ultra-compact angle sensors. However, the current approach to measuring the three-dimensional (3D) incident light direction is unstable. In this paper, we propose a sensor concept capable of discerning the 3D light-direction based on a segmented concentric nanoring structure that is sensitive to both elevation angle (θ) and azimuth angle (ϕ) at a micrometer device scale and is validated through simulations. Through deep learning (DL) analysis and prediction, our simulations reveal that for angle scanning with a step size of 1°, the device can still achieve a detection range of 0∼360° for ϕ and 45°∼90° for θ, with an average accuracy of 0.19°, and DL can further solve some data aliasing problems to expand the sensing range. Our design broadens the angle sensing dimension based on mutual resonance coupling among nanoring segments, and through waveguide implementation or sensor array arrangements, the detection range can be flexibly adjusted to accommodate diverse application scenarios.

Meta Shack-Hartmann wavefront sensor with large sampling density and large angular field of view: phase imaging of complex objects

Shack-Hartmann wavefront sensors measure the local slopes of an incoming wavefront based on the displacement of focal spots created by a lenslet array, serving as key components for adaptive optics for astronomical and biomedical imaging. Traditionally, the challenges in increasing the density and the curvature of the lenslet have limited the use of such wavefront sensors in characterizing slowly varying wavefront structures. Here, we develop a metasurface-enhanced Shack-Hartmann wavefront sensor (meta SHWFS) to break this limit, considering the interplay between the lenslet parameters and the performance of SHWFS. We experimentally validate the meta SHWFS with a sampling density of 5963 per mm2 and a maximum acceptance angle of 8° which outperforms the traditional SFWFS by an order of magnitude. Furthermore, to the best of our knowledge, we demonstrate the first use of a wavefront sensing scheme in single-shot phase imaging of highly complex patterns, including biological tissue patterns. The proposed approach opens up new opportunities in incorporating exceptional light manipulation capabilities of the metasurface platform in complex wavefront characterization.

Performance analysis of free space optical communications with FOA-WFS

Adaptive optics (AO) technology can correct wavefront distortion in coherent free space optical communication (FSOC), with wavefront sensors playing a vital role in this process. However, traditional wavefront sensors are large and expensive. Therefore, we propose using the inexpensive and easy-to-deploy flat optics angle-based wavefront sensor (FOA-WFS) to measure the wavefront aberration. It aims to meet the needs of various FSOC applications. We first establish the relationship between the energy ratio and the Zernike coefficient through theoretical studies and analyze the feasibility of applying the FOA-WFS to the FSOC. We then generate experimental datasets based on the relevant principles. Through numerical simulation, we verify that it can reconstruct wavefront aberration accurately and improve system performance. Finally, we analyze the mixing efficiency and bit error rate based on the collected aberration data by the experimental platform. The results indicate that the AO system based on the FOA-WFS can efficiently improve the performance of the FSOC. This study provides a novel wavefront aberration detection method for designing the AO systems in the FSOC.

Characterizing Biphoton Spatial Wave Function Dynamics with Quantum Wavefront Sensing

With an extremely high dimensionality, the spatial degree of freedom of entangled photons is a key tool for quantum foundation and applied quantum techniques. To fully utilize the feature, the essential task is to experimentally characterize the multiphoton spatial wave function including the entangled amplitude and phase information at different evolutionary stages. However, there is no effective method to measure it. Quantum state tomography is costly, and quantum holography requires additional references. Here, we introduce quantum Shack-Hartmann wavefront sensing to perform efficient and reference-free measurement of the biphoton spatial wave function. The joint probability distribution of photon pairs at the back focal plane of a microlens array is measured and used for amplitude extraction and phase reconstruction. In the experiment, we observe that the biphoton amplitude correlation becomes weak while phase correlation shows up during free-space propagation. Our work is a crucial step in quantum physical and adaptive optics and paves the way for characterizing quantum optical fields with high-order correlations or topological patterns.

Gradient-metasurface directional photodetectors

Angle-sensitive photodetectors are a promising device technology for many advanced imaging functionalities, including lensless compound-eye vision, lightfield sensing, optical spatial filtering, and phase imaging. Here we demonstrate the use of plasmonic gradient metasurfaces to tailor the angular response of generic planar photodetectors. The resulting devices rely on the phase-matched coupling of light incident at select geometrically tunable angles into guided plasmonic modes, which are then scattered and absorbed in the underlying photodetector active layer. This approach naturally introduces sharp peaks in the angular response, with smaller footprint and reduced guided-mode radiative losses (and therefore improved spatial resolution and sensitivity) compared to analogous devices based on diffractive coupling. More broadly, these results highlight a promising new application space of flat optics, where gradient metasurfaces are integrated within image sensors to enable unconventional capabilities with enhanced system miniaturization and design flexibility.

Single-shot deterministic complex amplitude imaging with a single-layer metalens

Conventional imaging systems can only capture light intensity. Meanwhile, the lost phase information may be critical for a variety of applications such as label-free microscopy and optical metrology. Existing phase retrieval techniques typically require a bulky setup, multiframe measurements, or prior information of the target scene. Here, we proposed an extremely compact system for complex amplitude imaging, leveraging the extreme versatility of a single-layer metalens to generate spatially multiplexed and polarization phase-shifted point spread functions. Combining the metalens with a polarization camera, the system can simultaneously record four polarization shearing interference patterns along both in-plane directions, thus allowing the deterministic reconstruction of the complex amplitude light field in a single shot. Using an incoherent light-emitting diode as the illumination, we experimentally demonstrated speckle-noise-free complex amplitude imaging for both static and moving objects with tailored magnification ratio and field of view. The miniaturized and robust system may open the door for complex amplitude imaging in portable devices for point-of-care applications.

Asymmetric metasurface photodetectors for single-shot quantitative phase imaging

The visualization of pure phase objects by wavefront sensing has important applications ranging from surface profiling to biomedical microscopy, and generally requires bulky and complicated setups involving optical spatial filtering, interferometry, or structured illumination. Here we introduce a new type of image sensors that are uniquely sensitive to the local direction of light propagation, based on standard photodetectors coated with a specially designed plasmonic metasurface that creates an asymmetric dependence of responsivity on angle of incidence around the surface normal. The metasurface design, fabrication, and angle-sensitive operation are demonstrated using a simple photoconductive detector platform. The measurement results, combined with computational imaging calculations, are then used to show that a standard camera or microscope based on these metasurface pixels can directly visualize phase objects without any additional optical elements, with state-of-the-art minimum detectable phase contrasts below 10 mrad. Furthermore, the combination of sensors with equal and opposite angular response on the same pixel array can be used to perform quantitative phase imaging in a single shot, with a customized reconstruction algorithm which is also developed in this work. By virtue of its system miniaturization and measurement simplicity, the phase imaging approach enabled by these devices is particularly significant for applications involving space-constrained and portable setups (such as point-of-care imaging and endoscopy) and measurements involving freely moving objects.

Optical spatial filtering with plasmonic directional image sensors

Photonics provides a promising approach for image processing by spatial filtering, with the advantage of faster speeds and lower power consumption compared to electronic digital solutions. However, traditional optical spatial filters suffer from bulky form factors that limit their portability. Here we present a new approach based on pixel arrays of plasmonic directional image sensors, designed to selectively detect light incident along a small, geometrically tunable set of directions. The resulting imaging systems can function as optical spatial filters without any external filtering elements, leading to extreme size miniaturization. Furthermore, they offer the distinct capability to perform multiple filtering operations at the same time, through the use of sensor arrays partitioned into blocks of adjacent pixels with different angular responses. To establish the image processing capabilities of these devices, we present a rigorous theoretical model of their filter transfer function under both coherent and incoherent illumination. Next, we use the measured angle-resolved responsivity of prototype devices to demonstrate two examples of relevant functionalities: (1) the visualization of otherwise invisible phase objects and (2) spatial differentiation with incoherent light. These results are significant for a multitude of imaging applications ranging from microscopy in biomedicine to object recognition for computer vision.