Reconfigurable all-dielectric Fano metasurfaces for strong full-space intensity modulation of visible light

Recent Progress in Reconfigurable and Intelligent Metasurfaces: A Comprehensive Review of Tuning Mechanisms, Hardware Designs, and Applications

Intelligent metasurfaces have gained significant importance in recent years due to their ability to dynamically manipulate electromagnetic (EM) waves. Their multifunctional characteristics, realized by incorporating active elements into the metasurface designs, have huge potential in numerous novel devices and exciting applications. In this article, recent progress in the field of intelligent metasurfaces are reviewed, focusing particularly on tuning mechanisms, hardware designs, and applications. Reconfigurable and programmable metasurfaces, classified as space gradient, time modulated, and space-time modulated metasurfaces, are discussed. Then, reconfigurable intelligent surfaces (RISs) that can alter their wireless environments, and are considered as a promising technology for sixth-generation communication networks, are explored. Next, the recent progress made in simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) that can achieve full-space EM wave control are summarized. Finally, the perspective on the challenges and future directions of intelligent metasurfaces are presented.

Active Fano resonance switch using dual-layer graphene in an embedded dielectric metasurface

We propose an active optical Fano switch (OFS) based on an embedded dielectric metasurface (EDM) including dual-layer graphene (DLG). An EDM is a dielectric grating overlapped by two cladding layers, and it excites a Fano resonance. DLG is positioned inside the upper cladding layer to maximize light-graphene interaction. Thus, with a small change of the chemical potential (µ_c) of graphene, a resonance wavelength is tuned to switch the OFS on and off. First, a red-parity asymmetric Fano resonance is realized, and a sharp asymmetric lineshape is achieved by controlling the structural parameters of the EDM and the interaction between the Fano resonance and additional weak Fabry-Perot interference for efficient switching. The distance of a peak-to-dip wavelength (Δλ_p-d) and the change of chemical potential (Δµ_c) for switching is analyzed by varying the duty cycle (DC) and grating thickness (t_g) of the EDM. Furthermore, switching contrast as a figure of merit (FoM) is analyzed. With DC of 0.5 and t_g of 70 nm, the OFS requires Δλ_p-d of 7.3 nm and Δµ_c of 0.25 eV. The FoM of 0.97 is achieved. By adjusting the two parameters, the switching condition is tuned. In the case of a blue parity, the effect of the two parameters exhibits a similar trend to that of the red parity. The FoM, however, is lower due to the reversed parity.

Design of highly perceptible dual-resonance all-dielectric metasurface colorimetric sensor via deep neural networks

Colorimetric sensing, which provides effective detection of bio-molecular signals with one's naked eye, is an exceptionally promising sensing technique in that it enables convenient detection and simplification of entire sensing system. Though colorimetric sensors based on all-dielectric nanostructures have potential to exhibit distinct color variations enabling manageable detection due to their trivial intrinsic loss, there is crucial limitation that the sensitivity to environmental changes lags behind their plasmonic counterparts because of relatively small region of near field-analyte interaction of the dielectric Mie-type resonator. To overcome this challenge, we proposed all-dielectric metasurface colorimetric sensor which exhibits dual-resonance in the visible region. Thereafter, we confirmed with simulation that, in the elaborately designed dual-Lorentzian-type spectra, highly perceptible variations of structural color were manifested even in minute change of peripheral refractive index. In addition to verifying physical effectiveness of the superior colorimetric sensing performance appearing in the dual-resonance type sensor, by combining advanced optimization technique utilizing deep neural networks, we attempted to maximize sensing performance while obtaining dramatic improvement of design efficiency. Through well-trained deep neural network that accurately simulates the input target spectrum, we numerically verified that designed colorimetric sensor shows a remarkable sensing resolution distinguishable up to change of refractive index of 0.0086.

Metasurface-Driven Optically Variable Devices

Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurface-driven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahigh-resolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

Stimuli-Responsive Phase Change Materials: Optical and Optoelectronic Applications

Stimuli-responsive materials offer a large variety of possibilities in fabrication of solid- state devices. Phase change materials (PCMs) undergo rapid and drastic changes of their optical properties upon switching from one crystallographic phase to another one. This peculiarity makes PCMs ideal candidates for a number of applications including sensors, active displays, photonic volatile and non-volatile memories for information storage and computer science and optoelectronic devices. This review analyzes different examples of PCMs, in particular germanium–antimonium tellurides and vanadium dioxide (VO₂) and their applications in the above-mentioned fields, with a detailed discussion on potential, limitations and challenges.

Nanophotonics for light detection and ranging technology

Light detection and ranging (LiDAR) technology, a laser-based imaging technique for accurate distance measurement, is considered one of the most crucial sensor technologies for autonomous vehicles, artificially intelligent robots and unmanned aerial vehicle reconnaissance. Until recently, LiDAR has relied on light sources and detectors mounted on multiple mechanically rotating optical transmitters and receivers to cover an entire scene. Such an architecture gives rise to limitations in terms of the imaging frame rate and resolution. In this Review, we examine how novel nanophotonic platforms could overcome the hardware restrictions of existing LiDAR technologies. After briefly introducing the basic principles of LiDAR, we present the device specifications required by the industrial sector. We then review a variety of LiDAR-relevant nanophotonic approaches such as integrated photonic circuits, optical phased antenna arrays and flat optical devices based on metasurfaces. The latter have already demonstrated exceptional functional beam manipulation properties, such as active beam deflection, point-cloud generation and device integration using scalable manufacturing methods, and are expected to disrupt modern optical technologies. In the outlook, we address the upcoming physics and engineering challenges that must be overcome from the viewpoint of incorporating nanophotonic technologies into commercially viable, fast, ultrathin and lightweight LiDAR systems.

Holographic metasurface gas sensors for instantaneous visual alarms

The rapid detection of biological and chemical substances in real time is particularly important for public health and environmental monitoring and in the military sector. If the process of substance detection to visual reporting can be implemented into a single miniaturized sensor, there could be a profound impact on practical applications. Here, we propose a compact sensor platform that integrates liquid crystals (LCs) and holographic metasurfaces to autonomously sense the existence of a volatile gas and provide an immediate visual holographic alarm. By combining the advantage of the rapid responses to gases realized by LCs with the compactness of holographic metasurfaces, we develop ultracompact gas sensors without additional complex instruments or machinery to report the visual information of gas detection. To prove the applicability of the compact sensors, we demonstrate a metasurface-integrated gas sensor on safety goggles via a one-step nanocasting process that is attachable to flat, curved, and flexible surfaces.

Multistate Switching of Photonic Angular Momentum Coupling in Phase-Change Metadevices

The coupling between photonic spin and orbital angular momenta is significantly enhanced at the subwavelength scale and has found a plethora of applications in nanophotonics. However, it is still a great challenge to make such kind of coupling tunable with multiple sates. Here, a versatile metasurface platform based on polyatomic phase-change resonators is provided to realize multiple-state switching of photonic angular momentum coupling. As a proof of concept, three coupling modes, namely, symmetric coupling, asymmetric coupling, and no coupling, are experimentally demonstrated at three different crystallization levels of structured Ge₂ Sb₂ Te₅ alloy. In practical applications, coded information can be encrypted in asymmetric mode using the spin degree of freedom, while revealing misleading one without proper phase change or after excessive crystallinity. With these findings, this study may open an exciting direction for subwavelength electromagnetics with unprecedented compactness, allowing to envision applications in active nanophotonics and information security engineering.