A review of dielectric optical metasurfaces for spatial differentiation and edge detection

Optical spatial differentiation enabled layer sensing of two-dimensional atomic crystals

Zero-thickness model and slab model are two important models in the description of optical behaviors in two-dimensional atomic crystals. The predicted difference in optical behaviors between the two models is very small, which is difficult to distinguish by established measurement methods. Here, we present an optical spatial differentiation method to examine the difference in edge images of different graphene layers. The theoretical results show that the edge imaging is significantly different between the two different models. When the beam reflection is at the Brewster angle, different graphene layers are used to adjust the spatial differentiation. It is shown that the slab model is more sensitive to the number of graphene layers. The zero-thickness model is more suitable for one-dimensional optical differential operation. Moreover, the spatial differentiation plays the role of a band-pass filter. The high-frequency edge information components will pass through the filter, thus realizing layer-sensitive edge-enhanced imaging. In addition, we do not focus on the verification of the exact model, but only provide an alternative method to characterize the number of graphene layers based on two models, and also provide possibilities for achieving imaging edge detection by graphene differential operators. This study may provide a possible method for the optical characterization of two-dimensional atomic crystals.

Broadband and large-aperture metasurface edge encoders for incoherent infrared radiation

The prevalence of computer vision systems necessitates hardware-based approaches to relieve the high computational demand of deep neural networks in resource-limited applications. One solution would be to off-load low-level image feature extraction, such as edge detection, from the digital network to the analog imaging system. To that end, this work demonstrates incoherent, broadband, low-noise optical edge detection of real-world scenes by combining the wavefront shaping of a 24-mm aperture metasurface with a refractive lens. An inverse design approach is used to optimize the metasurface for Laplacian-based edge detection across the 7.5- to 13.5-μm LWIR imaging band, allowing for facile integration with uncooled microbolometer-based LWIR imagers to encode edge information. A polarization multiplexed approach leveraging a birefringent metasurface is also demonstrated as a single-aperture implementation. This work could be applied to improve computer vision capabilities of resource-constrained systems by leveraging optical preprocessing to alleviate the computational requirements for high-accuracy image segmentation and classification.

Metalens for Accelerated Optoelectronic Edge Detection under Ambient Illumination

Analog systems may allow image processing, such as edge detection, with low computational power. However, most demonstrated analog systems, based on either conventional 4-f imaging systems or nanophotonic structures, rely on coherent laser sources for illumination, which significantly restricts their use in routine imaging tasks with ambient, incoherent illumination. Here, we demonstrated a metalens-assisted imaging system that can allow optoelectronic edge detection under ambient illumination conditions. The metalens was designed to generate polarization-dependent optical transfer functions (OTFs), resulting in a synthetic OTF with an isotropic high-pass frequency response after digital subtraction. We integrated the polarization-multiplexed metalens with a polarization camera and experimentally demonstrated single-shot edge detection of indoor and outdoor scenes, including a flying airplane, under ambient sunlight illumination. The proposed system showcased the potential of using polarization multiplexing for the construction of complex optical convolution kernels toward accelerated machine vision tasks such as object detection and classification under ambient illumination.

Intelligent Phase Contrast Meta-Microscope System

Phase contrast imaging techniques enable the visualization of disparities in the refractive index among various materials. However, these techniques usually come with a cost: the need for bulky, inflexible, and complicated configurations. Here, we propose and experimentally demonstrate an ultracompact meta-microscope, a novel imaging platform designed to accomplish both optical and digital phase contrast imaging. The optical phase contrast imaging system is composed of a pair of metalenses and an intermediate spiral phase metasurface located at the Fourier plane. The performance of the system in generating edge-enhanced images is validated by imaging a variety of human cells, including lung cell lines BEAS-2B, CLY1, and H1299 and other types. Additionally, we integrate the ResNet deep learning model into the meta-microscope to transform bright-field images into edge-enhanced images with high contrast accuracy. This technology promises to aid in the development of innovative miniature optical systems for biomedical and clinical applications.

All-optical image edge detection based on the two-dimensional photonic spin Hall effect in anisotropic metamaterial

Optical image processing based on the photonic spin Hall effect (SHE) has been gaining significant attention as a convenient and an accurate way for image edge detection. However, the recent edge imaging techniques depending on optical differentiation is mainly achieved by modulation of one-dimensional photonic SHE. Here, we theoretically predict the two-dimensional photonic SHE in the anisotropic metamaterial, and find that its longitudinal and transverse displacements exhibit spin-dependent property at filling factors with increasing incidence angle. As the transverse and in-plane displacements induced by the photonic SHE can be controlled by the filling factor of the crystal structure, the optical axis angle, and the incident angle, this intrinsical effect can be used to realize a tunable edge imaging. Interestingly, by changing the optical axis of the anisotropic metamaterial, the in-plane displacements are equal to the transverse displacements for a certain filling factor and the corresponding incident angle. Therefore, we propose a two-dimensional image edge detection method based on the photonic SHE in anisotropic metamaterial. Further numerical results validate the theoretical proposal.

Electric-Driven Polarization Meta-Optics for Tunable Edge-Enhanced Images

In this study, we demonstrate an electrically driven, polarization-controlled metadevice to achieve tunable edge-enhanced images. The metadevice was elaborately designed by integrating single-layer metalens with a liquid-crystal plate to control the incident polarization. By modulating electric-driven voltages applied on the liquid-crystal plate, the metalens can provide two polarization-dependent phase profiles (hyperbolic phase and focusing spiral phase). Therefore, the metalens can perform two-dimensional focusing and spatial differential operation on an incident optical field, allowing dynamic switching between the bright-field imaging and the edge-enhanced imaging. Capitalizing on the compactness and dynamic tuning of the proposed metadevice, our scheme carves a promising path to image processing and biomedical imaging technology.

High-Performance Asymmetric Optical Transmission Based on a Dielectric-Metal Metasurface

Asymmetric optical transmission plays a key role in many optical systems. In this work, we propose and numerically demonstrate a dielectric-metal metasurface that can achieve high-performance asymmetric transmission for linearly polarized light in the near-infrared region. Most notably, it supports a forward transmittance peak (with a transmittance of 0.70) and a backward transmittance dip (with a transmittance of 0.07) at the same wavelength of 922 nm, which significantly enhances operation bandwidth and the contrast ratio between forward and backward transmittances. Mechanism analyses reveal that the forward transmittance peak is caused by the unidirectional excitation of surface plasmon polaritons and the first Kerker condition, whereas the backward transmittance dip is due to reflection from the metal film and a strong toroidal dipole response. Our work provides an alternative and simple way to obtain high-performance asymmetric transmission devices.

Optical spatiotemporal differentiator using a bilayer plasmonic grating

As a key element in wave-based analog computation, optical differentiators have been implemented to directly perform information processing, such as edge detection and pulse shaping, in both spatial and temporal domains. Here, we propose an optical spatiotemporal differentiator, which simultaneously performs first-order spatial and temporal differentiation in transmission by breaking the mirror symmetry of a subwavelength bilayer metal grating. The spatial and temporal performance of the plasmonic differentiator is evaluated numerically using the output field profiles of an optical beam and pulse envelope, showing resolutions of ∼2µm and ∼50fs, respectively. Moreover, the function of spatiotemporal differentiation is demonstrated with input flat-top pulse fields. The proposed optical differentiator has potential applications in ultra-compact real-time optical multifunctional computing systems and parallel signal processing.

Bifocal focusing and polarization demultiplexing by a guided wave-driven metasurface

Metasurfaces have shown extraordinary light-manipulation abilities, however, most of them deal with free-space waves. It is highly desirable to develop a guided wave-driven metasurface which can extract the in-plane guided modes in the waveguide and mold it into the desired out-of-plane free-space modes. In this paper, an all-dielectric guided wave-driven metasurface, composed of an array of silicon meta-atoms on top of a silicon nitride waveguide, is proposed and simulatively demonstrated. When directly driven by fundamental transverse electric (TE₀₀) and fundamental transverse magnetic (TM₀₀) guided modes at operation wavelength 1.55 µm, the guided wave-driven metasurface converts them into y-polarized and x-polarized free-space light, respectively, and focuses them at different focal points, with polarization extinction ratio over 27 dB, thus simultaneously realizing triple functions of coupling guided modes to free-space waves, bifocal metalens and polarization demultiplexing. Our work offers an alternate way to control light across photonic integrated devices and free-space platforms.