Phase-change metasurface for switchable terahertz edge detection and bright-field imaging based on quasi-bound states in the continuum.

Combining bright-field and edge-enhanced imaging affords an effective avenue for extracting complex morphological information from objects, which is particularly beneficial for biological imaging. Multiplexing meta-lenses present promising candidates for achieving this functionality. However, current multiplexing meta-lenses lack spectral modulation, and crosstalk between different wavelengths hampers the imaging quality, especially for biological samples requiring precise wavelength specificity. Here, we experimentally demonstrate the nonlocal Huygens’ meta-lens for high-quality-factor spin-multiplexing imaging. Quasi-bound states in the continuum (q-BICs) are excited to provide a high quality factor of 90 and incident-angle dependence. The generalized Kerker condition, driven by Fano-like interactions between q-BIC and in-plane Mie resonances, breaks the radiation symmetry, resulting in a transmission peak with a geometric phase for polarization-converted light, while unconverted light exhibits a transmission dip without a geometric phase. Enhanced polarization conversion efficiency of 65% is achieved, accompanied by a minimal unconverted value, surpassing the theoretical limit of traditional thin nonlocal metasurfaces. Leveraging these effects, the output polarization-converted state exhibits an efficient wavelength-selective focusing phase profile. The unconverted counterpart serves as an effective spatial frequency filter based on incident-angular dispersion, passing high-frequency edge details. Bright-field imaging and edge detection are thus presented under two output spin states. This work provides a versatile framework for nonlocal metasurfaces, boosting biomedical imaging and sensing applications.

Nanostructured thin film absorbers embedded with phase-change thermochromic material can provide a large level of absorption tunability in the near-infrared region. Vanadium dioxide was employed as the phase-change material in the designed structures. The optical absorption properties of the designed structures with respect to the geometric and material parameters were systematically investigated using finite-difference time-domain computations. Absorption level of the resonance wavelength in the near-IR region was tuned from the perfect absorption level to a low level (17%) with a high positive dynamic range of near-infrared absorption intensity tunability (83%). Due to the phase transition of vanadium dioxide, the resonance at the near-infrared region is being turned on and turned off actively and reversibly under the thermal bias, thereby rendering these nanostructures suitable for infrared camouflage, emitters, and sensors.

Directly performing optical analog computations and image processing in space, such as optical differential operations and image edge detection, is a burgeoning area. To avoid the bulkiness and low efficiency of traditional 4 f filtering systems, one can utilize Green's function and metasurfaces for advanced wavefront control. However, some metasurface differentiators can be hindered by issues like polarization sensitivity, restricted bandwidth, low resolution, and the need for additional polarization devices or digital post‐processing, potentially degrading their performance and operation efficiency. In this work, a dual‐polarization Laplace differentiator is engineered to address these issues based on nonlocal hollow metasurface. The optical transfer function (OTF) required by the Laplace operation can be obtained by exciting different quasi‐bound states in the continuum (Q‐BIC) modes with distinct angular dispersion capabilities under p ‐ and s ‐polarized illumination, respectively. This Laplace differentiator not only directly realizes 2D second‐order edge detection in a dual‐polarization channel but also features a numerical aperture (NA) with an upper limit close to 0.42 and a broadband range reaching 165 nm. Such an efficient, high‐quality dual‐polarization and bandwidth image edge detection approach offers powerful imaging techniques for applications in machine vision, microscopic imaging, and image processing.

Optical metasurfaces offer significant advantages in enhancing the speed, efficiency, and miniaturization of imaging systems. However, most existing metasurfaces are limited to static functionalities and lack reconfigurability, which is a key feature for practical applications in dynamic environments. In this work, we demonstrate a reconfigurable optical metasurface capable of switching between two distinct imaging functions (edge detection and bright-field imaging) within the visible spectrum. This reconfigurability is achieved by tuning the phase transition of antimony sulfide (Sb2S3), which controls the angular dependence of the magnetic dipole resonance. The phase transition of Sb2S3 from the amorphous phase to the crystalline phase enables different optical transfer functions, achieving high-performance imaging with a numerical aperture of 0.42, isotropic second-order differentiation, and high-resolution imaging, respectively. This approach allows for functional switching on a single surface, opening up possibilities for applications in medical imaging, optical sensing, and microscopy.

Terahertz (THz) wave is an electromagnetic wave with frequency in a range of 0.1–10 THz, which possesses excellent photonic and electronic properties. THz wave has higher penetration and lower photon energy to non-polar materials, which makes it possess great academic value in medical, non-destructive testing and other related fields. In addition, the features such as wide bandwidth and large communication capacity of THz wave allow it to be widely used in communication, radar detection and other applications. Despite its rapid development in recent years, THz technology is used still mainly in free space currently and it is difficult to control the transmission direction of THz wave over a long distance in free space. What is more, the transmission of THz waves in free space is affected usually by the dust and water vapor. For achieving the efficient transmission of THz waves, researchers have proposed a variety of THz waveguides, including plastic fiber, Bragg fiber, photonic crystal fiber and anti-resonant fiber (ARF). The ARF confines the incident beam within the air hole of fiber center by the anti-resonance effect, which has aroused great interest because of its simple structure, low transmission loss, high damage threshold, low dispersion, and high transmission bandwidth. At present, adjustable THz fiber devices based on ARF are still reported rarely. In the near-infrared band, researchers have combined ARF with vanadium dioxide (VO<sub>2</sub>) to realize the exceptional modulation effects. The VO<sub>2</sub> is a metal oxide with insulator-metal phase transition when the ambient temperature is near 68 ℃, in which its electrical conductivity, dielectric constant and other properties will change drastically. In this paper, the VO<sub>2</sub> is coated on the inner wall of the THz ARF cladding tubes, and the effect of the phase transition of VO<sub>2</sub> on the propagation characteristics of the ARF is studied. Simulation results indicate that in the THz band, the phase transition of VO<sub>2</sub> will cause the anti-resonance period of the ARF to change greatly, in which the confinement effect of the ARF cladding tubes on the incident beam is converted from anti-resonant state to resonant state. Without changing the structure of the ARF, the effective modulation on the THz wave in the core of the ARF can be achieved only by controlling the phase transition of VO<sub>2</sub>, which has a wide application prospect in the field of THz adjustable devices. In this paper, a THz optical switch and a polarization controller based on VO<sub>2</sub>-coated ARF are proposed. With the optical switch being on and off, the corresponding losses are 0.5 dB/m and 110 dB/m respectively at 120 μm. If phase transition of VO<sub>2</sub> is induced by the excitation laser, it is expected to realize a fast-optical switch. Regarding the polarization controller, the polarization state and polarization direction of the THz wave in the core of the ARF can be controlled, and the birefringence coefficient of the ARF in the polarization state is more than 1.4 × 10<sup>–4</sup>.

Edge detection is an unsolved problem in that, so far, there is no general optimal solution. However, edge detection provides rich information about the scene being observed. This is particularly true in range images, where 3D information is explicit. Many researchers have been taking advantage of edge detection information to improve the segmentation of range images by integrating edge detection with other different segmentation techniques. This paper presents a methodology to perform edge detection in range images in order to provide a reliable and meaningful edge map, which helps to guide and improve range image segmentation by clustering techniques. The obtained edge map leads to three important improvements: (1) the definition of the ideal number of regions to initialize the clustering algorithm; (2) the selection of suitable initial cluster centers; and (3) the successful identification of distinct regions with similar features. Experimental results that substantiate the effectiveness of this work are presented.

Edge detection is one of the most principal techniques for detecting discontinuities in the gray levels of image pixels. The Modulation Transfer Function (MTF) is one of the main criteria for assessing imaging quality and is a parameter frequently used for measuring the sharpness of an imaging system. In order to determine the MTF, it is essential to determine the best edge from the target image so that an edge profile can be developed and then the line spread function and hence the MTF, can be computed accordingly. For regular image sizes, the human visual system is adept enough to identify suitable edges from the image. But considering huge image datasets, such as those obtained from satellites, the image size may range in few gigabytes and in such a case, manual inspection of images for determination of the best suitable edge is not plausible and hence, edge profiling tasks have to be automated. This paper presents a novel, yet simple, algorithm for edge ranking and detection from image data-sets for MTF computation, which is ideal for automation on vectorised graphical processing units.

The combination of phase change materials and metasurfaces has enabled myriads of versatile platforms for dynamic wave control, especially for various applications in integrated photonics and optoelectronics. In this paper, an electrically reconfigurable metasurface is demonstrated to work as a terahertz (THz) broadband digital switch by integration of vanadium dioxide (VO2). In such integrated optoelectronic frameworks, active and bistable digital control of optical responses is enabled by applying electrical stimuli to vertically cascaded metasurfaces with broadband behaviors for Joule heating that causes phase change and state switching. Before and after phase change, a high contrast ratio of transmittance is demonstrated within a broad THz range up to 400 GHz. Essentially, fast switching can be guaranteed compared with current devices based on thin film phase change materials, due to the local electrical heating that proves inherently efficient and faster to trigger the phase transition process. As a result, such active optoelectronic framework based on phase change materials may pave a new way for emerging integrated devices such as photoelectric switch, photonic memory, signal processing and so on

In the optical satellite on-orbit imaging quality estimation system, the calculation of Modulation Transfer Function (MTF) is not fully standardized, and the influence of atmosphere is often simplified, making it difficult to obtain completely consistent on-orbit MTF measurements and comparisons. This study investigates the effects of various factors—such as edge angle, edge detection methods, oversampling rate, and interpolation techniques—on the accuracy of MTF calculations in the commonly used slanted-edge method for on-orbit MTF assessment, informed by simulation experiments. A relatively optimal MTF calculation process is proposed, which employs the Gaussian fitting method for edge detection, the adaptive oversampling rate, and the Lanczos (a = 3) interpolation method, minimizing the absolute deviation in the MTF results. A method to quantitatively analyze the atmospheric scattering and absorption MTF is proposed that employs a radiative transfer model. Based on the edge images of GF-2 satellite, images with various atmospheric conditions and imaging parameters are simulated, and their atmospheric scattering and absorption MTF is obtained through comparing the MTFs of the ground and top atmosphere radiance. The findings reveal that aerosol optical depth (AOD), viewing zenith angle (VZA), and altitude (ALT) are the primary factors influencing the accuracy of GF-2 satellite on-orbit MTF measurements in complex scenarios. The on-orbit MTF decreases with the increase in AOD and VZA and increases with the increase in ALT. Furthermore, a collaborative analysis of the main influencing factors of atmospheric scattering and absorption MTF indicates that, taking the PAN band of the GF-2 satellite as an example, the atmospheric MTF values are consistently below 0.7905. Among these, 90% of the data are less than 0.7520, with corresponding AOD conditions ranging from 0 to 0.08, a VZA ranging from 0 to 50°, and an ALT ranging from 0 to 5 km. The results can provide directional guidance for the selection of meteorological conditions, satellite attitude, and geographical location during satellite on-orbit testing, thereby enhancing the ability to accurately measure satellite MTF.

This paper present the bifunctional design of a broadband terahertz (THz) meta-device based on a sandwich structure through the insulator-to-metal phase transition of vanadium dioxide (VO2). When VO2 is in the metallic state, the proposed design behaves as a broadband polarization converter. The metasurface consists of mutually orthogonal VO2 films, bilayer-aligned polyimide spacers, and a gold split-ring resonator (GSRR). The calculation results show that the designed system can convert the incident x-polarized wave to its orthogonal state in 0.4-1.2 THz. By changing the conductivity of VO2 in the simulation, the amplitude of the PCR spectrum can be effectively regulated. By adjusting the geometric parameters and selecting a suitable unit cell, the proposed design can achieve full phase coverage of 360° with a step of 45°, thereby effectively controlling the propagation direction of the transmitted wave. The designed system could provide a new pathway to develop bi-functional THz devices in phase change materials.

We demonstrate that the resonances of infrared plasmonic antennas can be tuned or switched on/off by taking advantage of the thermally driven insulator-to-metal phase transition in vanadium dioxide (VO(2)). Y-shaped antennas were fabricated on a 180 nm film of VO(2) deposited on a sapphire substrate, and their resonances were shown to depend on the temperature of the VO(2) film in proximity of its phase transition, in good agreement with full-wave simulations. We achieved tunability of the resonance wavelength of approximately 10% (>1 μm at λ~10 μm).

We present the bifunctional design of a broadband absorber and a broadband polarization converter based on a switchable metasurface through the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is metal, the designed switchable metasurface behaves as a broadband absorber. This absorber is composed of a vanadium dioxide square, silica spacer, and vanadium dioxide film. Calculated results show that in the frequency range of 0.52-1.2 THz, the designed system can absorb more than 90% of the energy, and the bandwidth ratio is 79%. It is insensitive to polarization due to the symmetry, and can still work well even at large incident angles. When vanadium dioxide is an insulator, a terahertz polarizer is realized by a simple anisotropic metasurface. Numerical calculation shows that efficient conversion between two orthogonal linear polarizations can be achieved. Reflectance of a cross-polarized wave can reach 90% from 0.42 THz to 1.04 THz, and the corresponding bandwidth ratio is 85%. This cross-polarized converter has the advantages of wide angle, broad bandwidth, and high efficiency. So our design can realize bifunctionality of broadband absorption and polarization conversion between 0.52 THz and 1.04 THz. This architecture could provide one new way to develop switchable photonic devices and functional components in phase change materials.

A terahertz switchable metamaterial is presented with bifunctional properties of absorption and electromagnetically induced transparency based on phase transition of vanadium dioxide. When vanadium dioxide is metal, the designed configuration acts as a wideband absorber consisting of a vanadium dioxide square-shaped array, a dielectric spacer, and a vanadium dioxide continuous film. The working frequency of absorptance larger than 90% ranges from 0.271 THz to 0.592 THz with bandwidth ratio 74.4%. The absorber can work well over a wide range of incident angle for transverse electric polarization and transverse magnetic polarization. When vanadium dioxide is insulator, the designed configuration acts as an analog of electromagnetically induced transparency in the terahertz frequency. The interaction between metallic cross and split ring resonator gives rise to a transparency peak in the transmission spectrum. The performance of electromagnetically induced transparency is robust against polarization and incident angle. This design could provide some prospects in designing switchable terahertz devices, such as the modulator, filter, and optoelectronic components.

We propose an asymmetric to quasisymmetric behavioral tunability of a structured filter operating in the mid-infrared (IR) spectral region, where the atmospheric transparency is high. The structure is designed through electromagnetic wave analysis using finite-difference time-domain computations. The behavioral tunability provides dynamic control of the optical behavior of the IR filter without geometric and structural changes and opens up a field of research area in the tunable IR devices. It is shown that optical characteristic can be switched from an optical diode (unidirectional isolator or asymmetric light transmission) to the bidirectional isolator (quasisymmetric behavior) based on the phase transition of vanadium dioxide in the entire mid-IR spectrum. The proposed structure can be fabricated with the current nano and microfabrication techniques and can be utilized in smart front IR windows for protecting delicate sensors in the IR imaging systems and IR missile seekers under strong IR laser radiation.

Wideband multimode smart modulator based on phase change materials