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Origami is the art of paper folding that allows a single flat piece of paper to assume different 3D shapes depending on the fold patterns and the sequence of folding. Using the principles of origami along with computation imaging technique the authors demonstrate a versatile shape‐morphing microwave imaging array with reconfigurable field‐of‐view and scene‐adaptive imaging capability. Microwave/millimeter‐wave based array imaging systems are expected to be the workhorse for sensory perception of future autonomous intelligent systems. The imaging capability of a planar array‐based systems operating in complex scattering conditions have limited field‐of‐view and lack the ability to adaptively reconfigure resolution. To overcome this, here, deviations from planarity and isometry are allowed, and a shape‐morphing computational imaging system is demonstrated. Implemented on a reconfigurable Waterbomb origami surface with 22 active metasurface panels that radiate near‐orthogonal modes across 17–27 GHz, capability to image complex 3D objects in full details minimizing the effects of specular reflections in diffraction‐limited sparse imaging with scene adaptability, reconfigurable cross‐range resolution, and field‐of‐view is demonstrated. Such electromagnetic origami surfaces, through simultaneous surface shape‐morphing ability (potentially with shape‐shifting electronic materials) and electromagnetic field programmability, opens up new avenues for intelligent and robust sensing and imaging systems for a wide range of applications. [ABSTRACT FROM AUTHOR]

In this talk, we will present a new platform for a virtual/augmented reality system based on a metalens and a fiber scanning display. The metalens was designed for achromatic focusing of blue, green, and red light. The fiber scanning display features combined desired merits of high resolution, high brightness, high dynamic range and wide color gamut. [ABSTRACT FROM AUTHOR]

Reconfigurable metasurfaces exhibit the capability of flexibly controlling electromagnetic (EM) waves, and the real‐time manipulation of tailored EM wavefront in full‐space is one of the steadily increasing interests. This paper proposes a reconfigurable transmission‐reflection‐integrated (TRI) coding metasurface, combining the functionalities of reconfigurable transmission and reflection metasurfaces. The meta‐particle is composed of a multilayer structure integrated with three positive‐intrinsic‐negative (PIN) diodes. By dynamically controlling the states of these diodes with the direct‐current (DC) bias voltage from a field‐programmable‐gate‐array (FPGA), meta‐particles achieve switching between transmission mode and reflection mode and independently modulate phase response. Metasurfaces can manipulate the reflected and transmitted wavefront by array encoding, allowing for full‐space control of the EM wavefront. Additionally, this paper demonstrates various functions implemented on this metasurface, including mode switching, beam scanning, shaping, focusing, and spatial scattering. As a proof of concept, a metasurface array prototype is fabricated and experimented. The simulation and measurement results agree well, confirming the effective ability of reconfigurable TRI metasurfaces to manipulate EM waves throughout full‐space. The proposed reconfigurable TRI coding metasurface opens the door for flexible wavefront control in the full‐space domain, with broad application prospects in areas such as microwave imaging, EM stealth, and smart radomes. [ABSTRACT FROM AUTHOR]

Diffractive Optical Elements (DOEs) are indispensable tools across numerous technological domains due to their capacity to manipulate light with sophistication and versatility. Their compact dimensions, lightweight nature, and compatibility with diverse materials render DOEs as prime candidates for integration into various optical systems, spanning from consumer electronics to state‐of‐the‐art scientific devices. Thus, DOEs stand as essential catalysts in technological advancement, facilitating innovation and unlocking new applications across a broad spectrum of disciplines. In this comprehensive review paper, Numerous types of DOEs widely acknowledge for their efficacy in both free space optics and imaging applications are delved. Beyond mere enumeration, their practical applications, elucidating their transformative impact on these fields are carefully examined. Furthermore, the challenges encountered in their implementation are dissected, paving the way for insightful discussions on future trajectories and advancements. [ABSTRACT FROM AUTHOR]

The holographic display is based on wavefront‐based light field methods, enabling the implementation of photorealistic three‐dimensional (3D) images. Thus, it is probably the final 3D visualization technology. The creation of a high‐quality holographic display involves three fundamental components: the generation of hologram, the utilization of hologram carrier, and the implementation of optical reconstruction system. In particular, the hologram carrier directly determines the resolution and field of view (FOV) of holographic display. The spatial light modulators (SLMs), serving as a device capable of manipulating the amplitude, polarization, or phase of incident light field, constitutes the primary hologram carrier in 3D holography. However, SLMs also present challenges such as limited FOV, complex integration, and multi‐order diffraction, which impede the enhancement of holographic display performance. The utilization of metasurface comprising subwavelength nanostructures enables precise manipulation of the light field, serving as an exceptional hologram carrier with the potential to replace conventional SLMs. Here, this paper will review the current research progress of metasurface‐based holographic systems to provide a comprehensive overview of the existing status. This work focuses on two aspects (i.e., static and dynamic metasurface‐based holographic systems), highlighting the principles, techniques, and advancements. [ABSTRACT FROM AUTHOR]

Dynamically tunable flexible metasurfaces with optical transparency are essentially useful in stealth, electromagnetic sensing, and camouflage while keeping continuous optical observation and communication. In this paper, a dual‐tunable graphene‐based metasurface is designed based on patterned graphene sandwich structures, showing independent dynamic control of reflection amplitude and resonance frequency, excellent optical transparency, and outstanding flexibility. Both the simulated and measured results demonstrate that the transition from dual band absorption at 12 and 24.8 GHz to broad band absorption from 10 to 25.7 GHz, and the absorption amplitude can be tuned between −5 and −15 dB on average in each case dynamically and independently by applying bias voltage to graphene layers. Lastly, the distributions of the induced electric field and surface currents, as well as an equivalent circuit model, are provided to illustrate the tunable electromagnetic absorption mechanism. The designed metasurface has potential applications that are speculated in the fields of electromagnetic tune and electromagnetic stealth. [ABSTRACT FROM AUTHOR]

This paper designs and validates a composite functional metasurface that combines microwave absorption and vibrational energy harvesting. The metasurface is composed of an indium tin oxide (ITO) electrically conductive glass pattern layer, arrays of 1‐ethyl‐3‐methylimidazolium dicyanamide [EMIM][N(CN)2] droplets, and an underlying indium tin oxide (ITO) electrically conductive glass layer. The composite functional metasurface possesses broadband absorption performance when the microwave frequency bands are between 7.34 and 22.63 GHz, which can transform vibration energy into electrical energy simultaneously. The relationship between output power, the number of droplets, internal resistance, and the matching load of the composite functional metasurface is deeply studied. It is found that the internal resistance decreases with the increase in the number of droplet arrays. The effective output power reaches up to 125nW when there are 48 droplet arrays. This technology has promising applications in the field of intelligent and portable electromagnetic devices. [ABSTRACT FROM AUTHOR]

Reconfigurable metasurfaces have shown unprecedented capabilities in controlling electromagnetic (EM) waves in recent years and play an essential role in information manipulation. However, most research on reconfigurable metasurfaces is limited to planar rigid structures, with only one control function and suitable for specific single polarization. Achieving simultaneous control of phase, polarization, and frequency in the conformal structure has significant practical implications in many applications, but remains obviously challenging. In this paper, a flexible multifunctional time‐varying phase‐modulated metasurface (MTPM) with orthogonal polarization control is proposed for conformal applications to manipulate the spectrum distribution. It enables time encoding capability in controlling the phase characteristics of x‐ and y‐polarized EM waves individually by switching the bias voltage of the PIN diode in two directions. Discrete harmonic spectrum and continuous Doppler frequency shifts are implemented separately on two orthogonal polarizations by applying different time coding sequences. As a practical application, a conformal MTPM for radar feature transformation is explored. Multiple discrete false targets and defocusing phenomena can be realized respectively for x‐ and y‐polarized waves. Both simulated and measured results verify the validity of the proposed method and prototype. [ABSTRACT FROM AUTHOR]

Light reflection plays a crucial role in a number of modern technologies. In this paper, analytical expressions for maximal reflected power in any direction and for any polarization are given for generic planar structures made of a single material represented by a complex scalar susceptibility. The problem of optimal light‐matter interactions to maximize reflection is formulated as the solution of an optimization problem in terms of the induced currents, subject to energy conservation and passivity, which admits a global upper bound by using Lagrangian duality. The derived upper bounds apply to a broad range of planar structures, including metasurfaces, gratings, homogenized films, photonic crystal slabs, and more generally, any inhomogeneous planar structure irrespective of its geometrical details. These bounds also set the limit on the minimum possible thickness, for a given lossy material, to achieve a desired reflectance. Moreover, the results allow the discovery of parameter regions where large improvements in the efficiency of a reflective structure are possible compared to existing designs. Examples are given of the implications of these findings for the design of superior and compact reflective components made of real, imperfect (i.e., lossy) materials, such as ultra‐thin and efficient gratings, polarization converters, and light‐weight mirrors for solar/laser sails. [ABSTRACT FROM AUTHOR]

Programmable and information metasurfaces have shown great potentials in wireless communications, but there are few reports on encrypted communications. In this paper, a programmable polarization‐modulated (PoM) information metasurface is proposed, which can not only customize arbitrarily linearly polarized reflected waves, but also modulate their amplitudes in real time. Based on this feature, a physical‐level wireless communication encryption scheme is presented and experimentally demonstrated by introducing a meta‐key, which can be encrypted and sent by the programmable PoM information metasurface. To be specific, the key is encoded and concealed into different linear polarization channels, and then modulated and transmitted by the information metasurface at the transmitting end. At the receiving end, the modulated signal can be received and decoded by using a pair of polarization discrimination antennas. A wireless transceiver system is established to verify the feasibility of the scheme. It is shown that, once the meta‐key is obtained, the corresponding encrypted target information that has been sent to the user in advance can be recovered. [ABSTRACT FROM AUTHOR]

Programmable metasurfaces enable fine‐grained real‐time modulation over electromagnetic (EM) waves in a digital manner, which have gained remarkable attention in antenna engineering and wireless communications. However, either a reflective or transmissive programmable metasurface requires an external feed source with significant distance to the metasurface, making the whole programmable device a relatively high profile as well as low efficiency as a result of the feed spillover losses. In this paper, an innovative design of radiation‐type programmable metasurface for dynamic modulation of radiant EM wave is presented, which perfectly combines the functions of 1‐bit phase shifter and efficient power radiator in the single unit framework. By employing the integrated series‐parallel hybrid microstrip network to stimulate the topside metasurface units, versatile capacities, including dynamic beam scanning and multimode vortex beam generation, are verified both numerically and experimentally. Due to the direct‐radiation characteristic of the metasurface design, the programmable metasurface possesses the aperture efficiency over 30% and a low profile reduced by almost two orders of magnitude, when comparing with the conventional reflective or transmissive programmable metasurfaces illuminated by the external feeds. The proposed design can offer a novel approach of programmable metasurface framework in dynamic radiant EM modulation for low‐profile conformal applications. [ABSTRACT FROM AUTHOR]

Metamaterials are structures engineered at a small scale with respect to the wavelength of the excitations they interact with. These structures behave as artificial materials whose properties can be chosen by design, mocking and even outperforming natural materials, and making them the quintessential tool for manipulation of wave systems. In this paper it is shown how the acoustic properties of a silicon nitride membrane can be affected by nanopatterning. The degree of asymmetry in the pattern geometry induces an artificial anisotropic elasticity, resulting in the splitting of otherwise degenerate mechanical modes. The artificial material introduced has a maximum Ledbetter–Migliori anisotropy of 1.568, favorably comparing to most bulk natural crystals. With an additional freedom in defining arbitrary asymmetry axes by pattern rotation, the described approach can be useful for fundamental investigation of material properties as well as for devising improved sensors of light, mass, or acceleration based on micromechanical resonators. [ABSTRACT FROM AUTHOR]

In this paper, a 2‐bit water‐based programmable digital metasurface for beam steering and backscatter field reduction applications is presented, a reflective and transmissive type water based programmable metasurface (WPMS) for RCS reduction and the antenna's beam steering is suggested, respectively. It has been demonstrated that a 2‐bit water‐based programmable unit cell can reduce RCS by changing its state (0/1). If the unit cell channel is empty, the state is "0," if the channel is filled, the state is "1." So at different resonance frequencies, the four phase and reflection coefficient responses are achieved as "00"01"10"11". This functionality enables beam steering in both the elevation and azimuth axis and also backscattering reduction of the antenna. Furthermore, the impacts of the number of unit cells and reflection phase states on the far‐field pattern are examined. Beam steering of ±45° in the elevation is attained with 10‐dB impedance bandwidth of 4.25–4.40 GHz, radiation gain of more than 7.1 dBi is maintained. With the present antenna, it is possible to achieve volumetric beamsteering performance directly. The patterns of far‐field radiation that are predicted theoretically coincide well with full waves simulations. As such, the proposed prototype can be a good option for applications that require a low RCS platform including beam steering in radars, 5G/6G, etc. Key Points: A 2‐bit water‐based programmable digital metasurface is designed for real‐time beam steering and backscatter field reduction applicationsIt has been demonstrated that a 2‐bit water‐based programmable unit cell can reduce RCS by changing its state (0/1)This functionality enables beam steering in both the elevation and azimuth axis and also backscattering reduction of the antenna [ABSTRACT FROM AUTHOR]

The strong light−matter coupling, occurring when the light−matter interaction prevails on the damping, has found applications beyond the domain of optics in chemistry or transport. These advances make the development of various structures in strong coupling crucial. In this paper, a new way to hybridize two materials and transfer energy through a surface plasmon over micrometric distances is proposed. For this purpose, two patterned interlocked dye arrays, one donor and one acceptor, are deposited on a silver surface by successive micro‐contact printing, leading to a pattern of 5 microns period. The dispersion relation of the structure is measured with reflectometry experiments, showing the hybridization with the plasmon, and the formation of states that mix both excitons and the plasmon with similar weights. The mixing in these polaritonic metasurfaces enables an energy transfer mechanism in the strong coupling, which is observed with luminescence experiments. As the donor and acceptor are spatially separated by a distance larger than the diffraction limit the excitation transfer is directly measured and evaluated by comparison with dye arrays without silver. [ABSTRACT FROM AUTHOR]

Accelerated beams have many applications because of their remarkable ability to propagate along curved trajectories. An accelerated beam with a non‐monotonic trajectory provides more beam deformability and trajectory concealment. This paper proposes direction‐reversible accelerated beam generation on nanometer scale based on metasurfaces. The non‐monotonic trajectory of the beam can be obtained by designing the geometric metasurface, while the curvature of the light trajectory can be adjusted by changing the angles of rectangular nanoholes. A theoretical expression for the modified wavefront of the proposed beam is deduced, and simulation and experiment results confirm the performance of the metasurface for generating a direction‐reversible accelerated beam. Discussions on the self‐healing of the accelerated beam and the influence of incident conditions on the curve trajectory show the trajectory controllability of the accelerated beam during the propagation. This work paves one way for tailoring the accelerated fields with complex trajectories, and it is helpful to expand the applications of accelerated beams. [ABSTRACT FROM AUTHOR]

Zoom lenses with variable focal lengths and magnification ratios are essential for many optical imaging applications. Conventional zoom lenses are composed of multiple refractive optics, and optical zoom is attained via translational motion of one or more lens elements, which adds to module size, complexity, and cost. In this paper, a zoom lens design based on multi‐functional optical metasurfaces is presented, which achieves large step zoom ratios, minimal distortion, and diffraction‐limited optical quality without requiring mechanical moving parts. Two embodiments of the concept are experimentally demonstrated based on polarization‐multiplexing in the visible and phase change materials in the mid‐infrared, both yielding 10× parfocal zoom in accordance with the design. [ABSTRACT FROM AUTHOR]

The emergence of new materials and fabrication techniques provides progress in the development of advanced photonic and communication devices. Transition metal dichalcogenides (e.g., molybdenum disulfide, MoS2) are novel materials possessing unique physical and chemical properties promising for optical applications. In this paper, a metasurface composed of particles made of bulk MoS2 is proposed and numerically studied considering its operation in the near‐infrared range. In the bulk configuration, MoS2 has a layered structure being a uniaxial anisotropic crystal demonstrating an optical birefringence property. It is supposed that the large‐scale and uniform MoS2 layers are synthesized in a vertical‐standing morphology, and then they are patterned into a regular 2D array of disks to form a metasurface. The natural anisotropy of MoS2 is utilized to realize the splitting of electric and magnetic dipole modes of the disks while optimizing their geometric parameters to bring the desired modes into overlap. At the corresponding resonant frequencies, the metasurface behaves as either an electric or a magnetic mirror, depending on the polarization of incident light. Based on the extraordinary reflection characteristics of the proposed metasurface, it can be considered an alternative to traditional mirrors and optical splitters when designing compact and highly efficient metadevices, which provide polarization and phase manipulation of electromagnetic waves on a subwavelength scale. [ABSTRACT FROM AUTHOR]

Camouflage skills are important for understanding the relationship between predators and prey in ecological systems. Mimicry is a common strategy used by creatures to alter their appearance in order to blend in with their surroundings and avoid detection. This paper unveils a previously unknown camouflage strategy that creates false object information and positions of an object in the visual range of radar sensor vision. A dispersion‐modulated illusionary metasurface to realize focal spot coding in a 3‐dimensional space to fit the radar imaging process is proposed. As the radar image indicates, an aircraft image is created by a flat metasurface plane with good polarization adaptation ability. Design, simulation, and experimental results validate the study. This camouflage strategy indicates the complexity of the food chain and can be used by both predators and prey, providing accessible insights for maintaining biological diversity. The metasurface‐based system can be potentially utilized in future radar markers for marine traffic control systems. [ABSTRACT FROM AUTHOR]

Graphene, a 2D material with tunable optical properties, has recently attracted intense interest for reconfigurable metasurfaces. So far, the working wavelength of graphene‐based or hybrid graphene metasurfaces has been limited in the mid‐infrared and terahertz spectra. In this paper, by combining graphene with Au nanostructures, the authors demonstrate a near‐infrared tunable metasurface with decent modulation efficiency, weak dependence on graphene's carrier mobility, and small gate voltages, attributing to the unique interband transition of graphene. The experimental results agree well with numerical simulations. It is also shown that by properly designing the structural parameters of Au nanostructures, the hybrid graphene metasurface can be tunable in both near‐infrared and mid‐infrared regions. [ABSTRACT FROM AUTHOR]

Metasurfaces have provided a novel and promising platform for realizing compact and high‐performance optical devices. The conventional metasurface design approach assumes periodic boundary conditions for each element, which is inaccurate in most cases since near‐field coupling effects between elements will change when the element is surrounded by nonidentical structures. In this paper, a deep learning approach is proposed to predict the actual electromagnetic (EM) responses of each target meta‐atom placed in a large array with near‐field coupling effects taken into account. The predicting neural network takes the physical specifications of the target meta‐atom and its neighbors as input, and calculates its actual phase and amplitude in milliseconds. This approach can be used to optimize metasurfaces' efficiencies when combined with optimization algorithms. To demonstrate the efficacy of this methodology, large improvements in efficiency for a beam deflector and a metalens over the conventional design approach are obtained. Moreover, it is shown that the correlations between a metasurface's performance and its design errors caused by mutual coupling are not bound to certain specifications (materials, shapes, etc.). As such, it is envisioned that this approach can be readily applied to explore the mutual coupling effects and improve the performance of various metasurface designs. [ABSTRACT FROM AUTHOR]

Achromatic devices have wide application prospects in radar and imaging fields. However, chromatic aberration and limited bandwidth restrict their development. Moreover, broadband and highly efficient achromatic devices working in transmission mode are still difficult to realize. In this paper, broadband highly efficient achromatic transmission in the microwave region by a metasurface is achieved. First, the ideal dispersion conditions of achromatic meta‐atoms are given. Then, a polarization selective grating metasurface and a split ring slot metasurface are designed using the transfer matrix method and equivalent circuit theory, respectively. The former is used to control phase characteristics while the latter enables controlling dispersion. Phase and dispersion can be controlled independently by cascading them and any phase curve can be designed as is desired. In order to verify the strategy, an achromatic deflector and an achromatic lens are designed and samples are fabricated. The experimental results show that the deflector can realize achromatic refraction from 9.3 to 12.3 GHz with average efficiency 77.5% and the lens can realize achromatic focusing from 9.8 to 12.2 GHz with average efficiency 78.9%, respectively. The experimental results are in good agreement with theory. The findings provide valuable strategy for achromatic devices design, which can be widely applied. [ABSTRACT FROM AUTHOR]

A metasurface (MS) antenna for the 5 GHz wireless local area network (WLAN) band is presented in this paper. The antenna is feed by a modified aperture‐coupled structure with an optimized rectangular nonuniform MS to widen the bandwidth and improve the gain. The proposed antenna has a thickness of 4 mm and occupies an area of 50 mm × 50 mm. The operating frequency range of the antenna is 4.72–6.19 GHz, covering the 5.15–5.825 GHz band used by the 5 GHz WLAN. The measured average gain is 7.23 dBi. The simulated specific absorption rate values under 1 g and 10 g standard are lower than the limitation of U.S. and European. Due to its simple structure, wide bandwidth and small size, the proposed MS antenna has great potential in the wearable applications. [ABSTRACT FROM AUTHOR]

Control of optical fields at the nanoscale holds the promise of fast, efficient imaging methods, but is elusive due to the diffraction limit. This paper investigates how a single metasurface patch in the near field of a sample plane may be used to create a wide variety of intensity patterns by applying different illumination profiles from the far field. Numerical analysis shows that one metasurface patch may be used to generate complete bases of illumination patterns on a grid as fine as λ/16. The limits of control are explored in terms of degrees of freedom on the illumination side and spatial resolution on the sample side. These illumination patterns are expected to enable sub‐wavelength structured illumination microscopies, compressive imaging and sensing. Quantitative analysis of how the engineered fields may be used for detection of small scattering particles demonstrates the potential the approach holds for nanoscale optical sensing. [ABSTRACT FROM AUTHOR]

Polarization plays a critical role in optical systems that range from optical communications to imaging, lithography, metrology, and data storage. Thus, in systems that need to generate a certain polarization state, a light source (e.g., laser) is combined with polarization control elements such as polarizers, polarizing beam splitters, and waveplates. Similarly, in systems requiring polarization state detection, such elements are combined with photodetectors. There is currently a trend toward miniaturized optical systems. This motivates the question of how to achieve what may be argued as an ultimate level of miniaturization: a single chip that can both generate light with a prescribed polarization state and detect the polarization state of light impinging upon it. This paper demonstrates this via vertical cavity surface emitting lasers (VCSELs) with on‐facet metasurfaces. Two classes of devices are demonstrated. The first class uses high‐index dielectric metasurfaces (amorphous silicon nanofins), whereas the second class uses plasmonic metasurfaces (aluminum bilayer gratings). Each can operate as a laser (to generate) and as a photodetector (to detect) circularly or linearly polarized light. [ABSTRACT FROM AUTHOR]

This paper explores time‐modulated metasurface‐assisted waveguide setups for the unique and accurate permittivity and permeability extraction of lossy dispersive samples with phase‐less measurements. It is theoretically demonstrated that when the position of the short‐circuit termination is dynamically modulated in a predefined way, the phase information of the reflection S‐parameter manifests itself into the amplitude level of the emerging harmonics. Being insensitive to the calibration plane shifts and phase uncertainties in reflection measurements while bypassing a priori knowledge about the material under test (MUT) and also the transmission information, can be regarded as the main advantages of the proposed time‐modulated material characterization approach. Using a simple post‐processing step, several illustrative examples are presented to numerically verify the validity of the proposed approach for some applicable and practical types of homogeneous materials. Two possible realization schemes based on mechanical actuation and electrical phase control are comprehensively discussed. An uncertainty analysis to examine how realization tolerances can affect the accuracy of results is also performed. The effectiveness of the proposed method is further experimentally demonstrated using a programmable phase‐agile metasurface. By involving the temporal dimension, the proposed strategy takes a great step forward in the phase‐less reconstruction of electromagnetic parameters. [ABSTRACT FROM AUTHOR]

Metasurfaces have enabled precise electromagnetic (EM) wave manipulation with strong potential to obtain unprecedented functionalities and multifunctional behavior in flat optical devices. These advantages in precision and functionality come at the cost of tremendous difficulty in finding individual meta‐atom structures based on specific requirements (commonly formulated in terms of EM responses), which makes the design of multifunctional metasurfaces a key challenge in this field. In this paper, a generative adversarial network that can tackle this problem and generate meta‐atom/metasurface designs to meet multifunctional design goals is presented. Unlike conventional trial‐and‐error or iterative optimization design methods, this new methodology produces on‐demand free‐form structures involving only a single design iteration. More importantly, the network structure and the robust training process are independent of the complexity of design objectives, making this approach ideal for multifunctional device design. Additionally, the ability of the network to generate distinct classes of structures with similar EM responses but different physical features can provide added latitude to accommodate other considerations such as fabrication constraints and tolerances. The network's ability to produce a variety of multifunctional metasurface designs is demonstrated by presenting a bifocal metalens, a polarization‐multiplexed beam deflector, a polarization‐multiplexed metalens, and a polarization‐independent metalens. [ABSTRACT FROM AUTHOR]

In this paper, a novel bi‐layered chiral metamaterial (CMM) based design is proposed, which exhibits broadband operation of asymmetric transmission (AT) of linearly polarized waves. Simulation results of the proposed design shows wide‐band AT response (transmission efficiency greater than 90%) in the microwave region from 8.48 to 10.16 GHz (1.68 GHz bandwidth) with a simultaneous cross‐polarization conversion (CPC) response (conversion efficiency greater than 92%). The transmission and reflection response clearly indicate the AT and CPC results of our design. Both behaviors are explained in terms of cross‐coupling between induced electric and magnetic dipoles due to surface currents. Finally, it is shown that AT response of the proposed design is stable with respect to variations in incident angle of the incoming wave up to 45°. Due to its high efficiency, low‐profile, broadband response, and angular stability this design has numerous applications in modern telecommunication systems. [ABSTRACT FROM AUTHOR]

Increasing information capacity is crucial for high‐capacity and high‐speed communication, especially for sub‐terahertz communication. Over the last decade, spatial multiplexing based on the orbital angular momentum (OAM) by adopting a multi‐mode OAM metasurface has attracted a lot of attention. However, current metasurface‐based OAM multiplexing methods suffer from complex and limited multiplexing channels. In this paper, a novel method to realize multi‐dimensional multiplexing combining OAM and frequency based on an angle‐multiplexed metasurface over a broadband terahertz region is proposed and investigated. A frequency‐independent phase profile formula of the angle‐multiplexed metasurface is derived. A reflective metasurface operating from 0.25 to 0.35 terahertz (THz) is designed based on this formula. For proof of concept, nine‐channel multiplexing is illustrated based on this novel method. The simulation results verify that nine‐channel off‐axis left‐hand circularly polarized beams are converted to nine orthogonal coaxial beams. Besides, according to the conventional method for OAM multiplexing, an angle‐multiplexed reflective metasurface working at 0.3 THz is designed for comparison. The simulation results show that only three‐channel multiplexing can be obtained by this model with nine‐channel incident waves. The proposed method has a great potential to enhance the transmission capacity of the communication system. [ABSTRACT FROM AUTHOR]

Metasurfaces have been extensively studied for generating electromagnetic waves carrying orbital angular momentum (OAM). In particular, programmable metasurfaces enable real‐time switching between multiple OAM modes in a digital manner. However, the current programmable metasurfaces are mostly based on reflective mode, which suffer from low efficiency as well as serious feed blockage. In this paper, a transmissive programmable metasurface is presented for the highly efficient generation of multimode convergent OAM beams. The proposed transmissive metasurface is composed of electronically reconfigurable units with 1‐bit phase resolution (0/π), which are obtained by integrating two PIN diodes in the radiating layer for current direction modulation. Through the antisymmetry configuration of the two PIN diodes, nearly uniform transmission magnitudes but inversed phase states in a wide band can be obtained. The simulation results show that the proposed reconfigurable unit can achieve good 1‐bit phase tuning, with minimum insertion loss of 0.2 dB and 2 dB transmission bandwidth of more than 10%. Through the dynamic modulation of the quantized code distributions on the metasurface, programmable multimode OAM beams can thus be constructed. Both simulated and measured results verify the effectiveness of the proposed design. [ABSTRACT FROM AUTHOR]

Metasurfaces can provide novel functionality for absorbing electromagnetic waves through a periodic array of inductive/capacitive resonators by controlling the material's permittivity and permeability. This attractive characteristic makes them widely applicable to various electromagnetic applications. However, conventional metasurfaces are dispersive and suffer from a narrow bandwidth with uncontrollable and unchangeable functions because the effective permittivity and permeability are tailored to a certain resonance frequency. Therefore, many studies have focused on how to increase the absorption frequency bandwidth and control the absorption frequency by using reconfigurable metasurfaces. This paper presents a report on the most recent progress in the realization of reconfigurable metasurfaces based on advanced materials for frequency‐selective absorption in the microwave, millimeter‐wave, sub‐terahertz, terahertz, and visible ranges. Furthermore, their tuning ratios for different advanced materials and spectra are summarized and compared. This progress report provides guidelines for the material selection for and the design of reconfigurable metasurfaces. [ABSTRACT FROM AUTHOR]

Existing metasurfaces with ultrathin volume for asymmetric transmission are often constructed from metal with low efficiency in optical frequencies, and cannot realize optical asymmetric transmission and focusing simultaneously. Although acoustic asymmetric focusing in asymmetric focusing lenses (AFL) has been realized, the obtained focal point is accompanied by a large sidelobe. To solve these problems, an AFL with dual‐layer metasurfaces designed by using impedance matching materials can realize optical asymmetric transmission and focusing simultaneously in this paper. Furthermore, through optimizing the design theory of AFL, near‐zero‐sidelobe (NZS) focusing can be realized on the subwavelength scale. NZS asymmetric focusing of the optimized AFL is effective in broadband, which is expected to show tremendous potential in optical micro‐nanoprocessing, quantum communication, optical micromanipulation, etc. [ABSTRACT FROM AUTHOR]

Planar optics based on patterned cholesteric liquid crystals (CLCs) has attracted increasing attention owing to the self‐organized helical structure and the ability to create arbitrary reflected wavefront through spatial orientation control. However, because of the subwavelength‐orientation requirement, it is challenging for liquid crystal lens to achieve a low f‐number (f/#) and large deflection angle simultaneously. Furthermore, with the increasing demand for compact size in novel optical systems, reflective lens has advantage over the transmissive one because it can fold the optical path. Here, a new off‐axis reflective polarization volume lens (PVL) with f/# = 0.825, large aperture size, simple fabrication process, thin profile, circular polarization selectivity, and large diffraction angle is proposed. In contrast to traditional vertical spiral structure, PVL is based on patterned CLCs with a slanted helical axis. In this paper, the PVL is theoretically evaluated and then three reflective PVLs at red, green, and blue wavelengths (R = 605 nm, G = 532 nm, and B = 450 nm) are fabricated. Meanwhile, a simple approach is utilized to achieve 20 mm diameter and 16.5 mm focal length. The low scattering and good image quality of reflective PVL enrich these functional devices and provide promising applications to novel foldable optical systems and waveguide‐based wearable near‐eye displays. [ABSTRACT FROM AUTHOR]

In this paper, a dielectric metasurface is demonstrated to manipulate the photoluminescence of the G‐centers introduced by nanopatterning of crystalline silicon. The metasurface consists of asymmetric holes arranged in a square array, which can transform the bound states in the continuum (BICs) with infinite quality factor to a quasi‐BICs, while maintaining high quality factor of the optical resonance. Compared with the photoluminescence of G‐centers with nonresonance enhancement, ≈40 times photoluminescence enhancement is achieved, accompanied by a near‐zero threshold at cryogenic temperature. In addition, the polarization of the photoluminescence is controlled by the structural asymmetry. These findings provide a novel approach to enhance and manipulate the photoluminescence of G‐centers and may anticipate for realizing light source in silicon. [ABSTRACT FROM AUTHOR]

Subwavelength resolution imaging entails a sharpened superfocusing to overcome the diffraction limit. However, conventional high‐numerical‐aperture superfocusing lenses are bulky and expensive. Relying on the abrupt amplitude and phase discontinuities, metasurfaces have been recently explored as an ultrathin platform to generate arbitrary wavefronts over subwavelength thicknesses. In this paper, a dual‐field superfocusing is demonstrated for terahertz wavelengths using a near‐field metasurface by simultaneously tailoring the amplitude and phase of the evanescent field, which satisfies both electrostatic and magnetostatic limits. A combined analytic theory is introduced to describe the physical mechanism and the entire design process of the near‐field metasurface. With three‐layer minuscule elements characterized by extremely small electric sizes, the metasurface can force both incident electric and magnetic fields to converge to a sharpened focus spot (0.066 and 0.045 wavelength focus spots at a distance of 0.073 wavelength) through radiationless electromagnetic interference. Practical implementations of these metasurfaces hold promise for near‐field terahertz beam steering, laser‐based microscopy, noncontact sensing, imaging, and lithography applications. [ABSTRACT FROM AUTHOR]

The diversity and practicability of terahertz metamaterials have experienced rapid development in the past decade due to the increasing demand for various devices. This topic has attracted significant interest from researchers. Among the key functional devices in terahertz metamaterial systems, the active control ability of terahertz metamaterials is highly valuable and captivating. This implies that the electromagnetic properties of metamaterials can be modulated over a wide dynamic range by external stimuli. This review categorizes the different types of tunable terahertz metamaterials based on the external stimuli to which they respond, namely, mechanical modulation, electrical modulation, magnetic modulation, and optical modulation. Mechanically modulated devices offer simple yet efficient modulation, while electrical and magnetic modulation provide effective active modulation through electrical mechanisms. Optical modulation, in contrast, focuses on incorporating various materials to achieve active modulation. [ABSTRACT FROM AUTHOR]

Metamaterials research has been ongoing for more than 20 years and it has gained much public and scientific interest. There have been many experts forecasting on the road ahead for metamaterials, notably, in lieu of "knowledge tree" in 2010. Ten years on, it is proposed to re‐examine these claims by using automated computer tools, such as natural language processing (NLP), to extract research information for processing and analyzing from unstructured texts in publications. In this study, a fully auto‐generated database of 43 678 abstracts related to metamaterials published between 2000 and 2021 using Scopus Search API (Application Programming Interface) is built. Applying word embedding, each keyword is studied in a hyperdimensional vector space and clusters so that their relationships can be visualized for assessing the popularity and trends of research themes. A neural network model developed based on the encoder–decoder long short‐term memory (LSTM) architecture is finally trained to predict future directions and theme evolutions in the next four years for selected topics. This study not only provides vital information in terms of impact of metamaterials research but also lays down a solid foundation for the development of future metamaterial research roadmap in the form of Gartner hype cycle. [ABSTRACT FROM AUTHOR]

Metasurfaces is a rapidly developing area with quite many prospective applications in light manipulation, aberration-free optical imaging, and security. One potential area where metasurface approaches may provide a valuable contribution is medical imaging. In this paper, a novel subwavelength metasurface-inspired resonator design is presented, which is employed to enhance the sensitivity of magnetic resonance imaging (MRI). To this end, the resonator is formed by an array of capacitively loaded telescopic wires is placed inside the scanner in a close proximity of the studied object. The telescopic design of the structure permits to mechanically adjust its eigenmode resonance frequency to the operational frequency of a 1.5T MRI machine, thus making it possible to enhance and redistribute the radiofrequency magnetic field of an available radiofrequency coil in the region of interest and therefore significantly increase the sensitivity of the receiver. [ABSTRACT FROM AUTHOR]

A reconfigurable transmission metasurfaces (RTMs) excited by a multiple‐input multiple‐output (MIMO)(RTMs‐MIMO) antenna is designed to implement multi‐domain metamaterial computational imaging (MCI). Based on the receiver‐transmitter metasurfaces structure, four PIN diodes are applied to control the operating state of the RTMs. It enables arbitrary switching among four polarization transmission waves. A reverse‐induced current can be generated by adjusting the PIN diodes in the symmetrical position, resulting in a perfect 1‐bit phase modulation with a 3dB‐loss bandwidth of 15.38~19.89 GHz. A microstrip antenna with mushroom‐shaped parasitic patches is employed to implement the MIMO feeds. The ‐10dB bandwidth of MIMO antenna is compatible with the RTMs, and the maximum envelope correlation coefficient of 0.16 demonstrates good isolation between the MIMO feeds. In the MCI system constructed by the RTMs‐MIMO antennas, a transmission matrix H can be achieved by MIMO space beams, metasurfaces phase, metasurfaces polarization, and frequency modes. The correlation coefficient µg = 0.018 is achieved by the multi‐domain integration, allowing H to be highly non‐correlated. Based on the detection results of H, an imaging result with a relative reconstruction error of 0.372 is achieved. The designed MCI system therefore has unparalleled application prospects in the broad field of medical and military detection. [ABSTRACT FROM AUTHOR]

Optical properties of a metasurface which can be considered a monolayer of uniaxial metamaterials - parallel-plate and nanorod arrays - are investigated. It is shown that such metasurface acts as an ultimately thin sub-100 nm wave plate. This is achieved via an interplay of epsilon-near-zero and epsilon-near-pole behavior along different axes in the plane of the metasurface allowing for extremely rapid phase difference accumulation in very thin metasurface layers. These effects are shown to not be disrupted by non-locality and can be applied to the design of ultrathin wave plates, Pancharatnam-Berry phase optical elements and plasmon-carrying optical torque wrench devices. [ABSTRACT FROM AUTHOR]

Radiative cooling, a technology that lowers the temperature of terrestrial objects by dissipating heat into outer space, presents a promising ecologically‐benign solution for sustainable cooling. Recent years witness substantial progress in radiative cooling technologies, bringing them closer to commercialization. This comprehensive review provides a structured overview of radiative cooling technologies, encompassing essential principles, fabrication techniques, and practical applications, with the goal of guiding researchers toward successful commercialization. The review begins by introducing the fundamentals of radiative cooling and the associated design strategies to achieve it. Then, various fabrication methods utilized for the realization of radiative cooling devices are thoroughly discussed. This discussion includes detailed assessments of scalability, fabrication costs, and performance considerations, encompassing both structural designs and fabrication techniques. Building upon these insights, potential fabrication approaches suitable for practical applications and commercialization are proposed. Further, the recent efforts made toward the practical applications of radiative cooling technology, including its visual appearance, switching capability, and compatibility are examined. By encompassing a broad range of topics, from fundamental principles to fabrication and applications, this review aims to bridge the gap between theoretical research and real‐world implementation, fostering the advancement and widespread adoption of radiative cooling technology. [ABSTRACT FROM AUTHOR]

Since the surface plasmon polariton (SPP) has received a great deal of attention because of its capability of guiding light within the subwavelength scale, finding methods for arbitrary SPP field generation has been a significant issue in the area of integrated optics. To achieve such a goal, it will be necessary to generate a plasmonic complex field. In this paper, we propose a novel method for generating a plasmonic complex field propagating with arbitrary curvatures by using double-lined distributed nanoslits. As a unit cell, two facing nanoslits are used for tuning both the amplitude and the phase of excited SPPs as a function of their tilted angles. For verification of the proposed design rule, we experimentally demonstrate some plasmonic caustic curves and Airy plasmons. [ABSTRACT FROM AUTHOR]

Metasurface holography has been well studied due to its features that are superior to the traditional holographic technologies. However, high profiles of the reflection‐ and transmission‐type configurations in the metasurface holography restrict its applications. On the other hand, digital coding metasurfaces have received great attention recently because they build a link between the information theory and surface electromagnetic fields, which also bring new viewpoint and convenience for digital computational holography. Here, a real low‐profile holographic method is proposed using a leaky‐type 2‐bit coding Fabry–Perot metasurface with in‐plane feeding in the microwave frequency. For constructing the metasurface hologram, the weighted Gerchberg–Saxton algorithm is presented to modulate the energy proportion of focal points by adjusting the phase distribution. As proof of concept, some typical metasurface holograms are demonstrated in microwave experiments at the height of 35 mm (≈3 wavelengths), which show good agreement with theoretical analyses and full‐wave simulations. The design method can be easily extended to the millimeter wave and terahertz bands. [ABSTRACT FROM AUTHOR]

Abstract: Metasurfaces, planar metamaterials consisting of a single layer or several layers of artificial structures, not only form the basis for fundamental physics research but also have considerable technological significance. Metasurfaces can locally modify the optical property within a subwavelength range, which can facilitate device miniaturization and system integration. Metasurfaces have shown extraordinary capabilities in the local manipulation of the light's amplitude, phase, and polarization, leading to a plethora of novel applications such as generalized Snell's law of refraction and photonic spin Hall effect. This progress report is focused on the recent advancements in the fundamental research of geometric metasurfaces and their applications in ultrathin optical devices, including planar metalenses, helicity multiplexed holograms, functionality switchable devices, polarization beam splitters, vector beam generation, arbitrary polarization control, and so on. The compactness, ease of fabrication, and unusual functionalities of these devices render geometric optical metasurface devices very attractive for new applications such as encryption, imaging, anti‐counterfeiting, optical communications, quantum science, and fundamental physics. This paper aims to bring readers some new insights, and to broaden the applications of geometric metasurfaces in more research fields of science and technology. [ABSTRACT FROM AUTHOR]

Bioinspired nanoplasmonic 3D crossed surface relief gratings and metasurfaces are fabricated on azobenzene molecular glass thin films to create effective antibacterial surfaces. A synergetic mechanical and photothermal interaction at the interface between the nanostructures and the Escherichia coli (E. coli) bacteria results in a significant decrease in the viable bacterial population. In particular, combined exposure to the interfacial nanospikes as well as the evanescent blue and red electromagnetic fields induced by the nanoplasmonic metasurface, results in a 97% reduction of the viable E. coli in only 25 min, when illuminated with a low‐power white light. [ABSTRACT FROM AUTHOR]

The capability to dynamically modulate electromagnetic wavefronts can revolutionize and be crucial for future wireless technology. Electromagnetic waves can be fundamentally described in terms of amplitude, phase, polarization, and angular frequency. However, reported reconfigurable metasurfaces can only control one or two fundamental parameters and require different tuning/switching elements or materials that remain challenging to control those continuously. In this work, an approach is presented for designing electrically tunable reflective metasurfaces that enable independent and simultaneous control of amplitude and phase, while also providing frequency tuning capability. This is achieved by arranging two varactor diodes on the top layer and a lumped resistor on the bottom layer. The proposed metasurface is fabricated and several electromagnetic functionalities experimentally demonstrated as proof‐of‐concept applications, including reflector, radar absorbing, dual and single‐beam steering, and amplitude control. The proposed metasurface will open avenues for realizing advanced multifunctional devices to fully control electromagnetic parameters, offering numerous applications in future communication technologies, radar systems, and information processing. [ABSTRACT FROM AUTHOR]

Beam diffraction management with on‐demand efficiency over compact devices is important in various applications, such as communications, spectroscopy, wireless power transfer, and others. Recently, the in‐depth study of metasurfaces, such as phase gradient metasurfaces (PGMs) or metagratings (MGs) made of discrete elements, has promoted an ultrathin platform to manipulate diffractions. However, most studies only focus on symmetrical diffraction orders or different propagating diffraction orders with equally distributed energy. It is difficult to efficiently excite beams with arbitrary energy distribution by phase‐only metasurfaces due to the complex optimization procedure. Here, to address these challenges, Fourier harmonic component engineering is proposed to allocate the energy between multiple diffraction beams. By introducing phase‐corrected gradient (PCG) on the metasurface platform, lossless transformation from the incidence to far‐field patterns can be obtained. A variety of diffraction situations are considered (symmetric and asymmetric, with equal or arbitrary energy ratio), where the simulated and measured far‐field patterns are in excellent agreement with the theoretical predictions and the achieved diffraction efficiency is up to 98.3%. The proposed method paves the way for multichannel wireless communication applications and can be readily extended to other frequency regions. [ABSTRACT FROM AUTHOR]

Full wavefront control by photonic components requires that the spatial phase modulation on an incoming optical beam ranges from 0 to 2π. Because of their radiative coupling to the environment, all optical components are intrinsically non‐Hermitian systems, often described by reflection and transmission matrices with complex eigenfrequencies. Here, it is shown that parity or time symmetry breaking—either explicit or spontaneous—moves the position of zero singularities of the reflection or transmission matrices from the real axis to the upper part of the complex frequency plane. A universal 0 to 2π‐phase gradient of an output channel as a function of the real frequency excitation is thus realized whenever the discontinuity branch bridging a zero and a pole, that is, a pair of singularities, is crossing the real axis. This basic understanding is applied to engineer electromagnetic fields at interfaces, including, but not limited to, metasurfaces. Non‐Hermitian topological features associated with exceptional degeneracies or branch cut crossing are shown to play a surprisingly pivotal role in the design of resonant photonic systems. [ABSTRACT FROM AUTHOR]

Polarization and phase are two essential properties of electromagnetic (EM) waves. A transmissive metasurface is proposed that can simultaneously manipulate the polarization and phase of the EM waves. The design has a powerful capability of rotating the polarization to any desired azimuth direction for a linearly polarized (LP) wave and achieving the cross‐polarization conversion for a circularly polarized (CP) wave. Meanwhile, the transmission phase of the cross‐polarization can be dynamically controlled for the CP wave. The proposed metasurface is composed of two types of substrate‐integrated waveguides (SIW) in a cross arrangement, which has an insertion loss of less than 0.3 dB from 9.7 to 10.3 GHz. Some samples are fabricated and measurements are performed, which are in satisfactory agreement with the theoretical and simulated results, indicating the feasibility of the proposed method. It is believed that the proposed design has many potential applications in the fields of imaging, communication, and radar systems. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an optically active reflective metasurface with tunable phase performance is designed which operates based on the excitation of both electric and magnetic resonances in a silicon‐made nanoantenna element and the use of layered indium tin oxide (ITO)–alumina under the influence of external multigate biasing. The capability of achieving robust control over the reflection characteristics of two closely spaced electric and magnetic resonances is investigated. Adopting the multigate biasing enables relatively high reflection amplitude of 0.4 over the entire phase‐change coverage (≈180°) in the near‐infrared regime by exploiting the constructive interference between the geometrical resonances. The physical mechanism behind this ITO‐assisted metal–insulator–semiconductor building block can be described as the strong confinement of electromagnetic fields at the vicinity of ITO layers and the possibility of modulating the carrier concentrations of the integrated active materials by electrically varying the bias voltage. The uniformity of reflection magnitude for all reflection phase coverage, relatively high reflectivity, and electro‐optical tunability is leveraged to design three multifunctional devices with step‐by‐step increase in complexity of biasing network including ultrathin reconfigurable linear/circular polarizer (simple identical biasing), dynamical beam steering platform (two‐state biasing), and tunable metalens with controllable on‐ and off‐axes focusing pattern in real‐time (advanced element‐by‐element biasing). [ABSTRACT FROM AUTHOR]

Metamaterial absorbers have been widely studied and continuously concerned owing to their excellent resonance features of ultra‐thin thickness, light‐weight, and high absorbance. Their applications, however, are typically restricted by the intrinsic dispersion of materials and strong resonant features of patterned arrays (mainly referring to narrow absorption bandwidth). It is, therefore essential to reassert the principles of building broadband metamaterial absorbers (BMAs). Herein, the research progress of BMAs from principles, design strategies, tunable properties to functional applications are comprehensively and deeply summarized. Physical principles behind broadband absorption are briefly discussed, typical design strategies in realizing broadband absorption are further emphasized, such as top‐down lithography, bottom‐up self‐assembly, and emerging 3D printing technology. Diversified active components choices, including optical response, temperature response, electrical response, magnetic response, mechanical response, and multi‐parameter responses, are reviewed in achieving dynamically tuned broadband absorption. Following this, the achievements of various interdisciplinary applications for BMAs in energy‐harvesting, photodetectors, radar‐IR dual stealth, bolometers, noise absorbing, imaging, and fabric wearable are summarized. Finally, the challenges and perspectives for future development of BMAs are discussed. [ABSTRACT FROM AUTHOR]