Active metasurfaces with dynamically switchable functionalities are highly in demands in various practical applications. In this paper, we experimentally present an active metasurface based on PIN diodes which can realize nearly perfect reflection, transmission and absorption in a single design. Such switchable functionalities are accomplished by controlling the PIN diodes integrated in both layers of the metasurface. A transmission line model is employed to further investigate the underlying mechanism of the metasurface. This proposal is confirmed by numerical simulations and experiments. As a novel metasurface with multiple switchable functionalities, our design may find some practical applications such as smart radomes.

Switchable metasurface for nearly perfect reflection, transmission, and absorption using PIN diodes

Looking back on the development of metamaterials in the past 20 years, metamaterials have gradually developed from three-dimensional complex electromagnetic structures to a two-dimensional metasurface with a low profile, during which a series of subversive achievements have been produced. The form of electromagnetic manipulation of the metasurface has evolved from passive to active tunable, programmable, and other dynamic and real-time controllable forms. In particular, the proposal of coding and programmable metasurfaces endows metasurfaces with new vitality. By describing metamaterials through binary code, the digital world and the physical world are connected, and the research of metasurfaces also steps into a new era of digitalization. However, the function switch of traditional programmable metamaterials cannot be achieved without human instruction and control. In order to achieve richer and more flexible function regulation and even higher level metasurface design, the intelligence of metamaterials is an important direction in its future development. In this paper, we review the development of tunable, programmable, and intelligent metasurfaces over the past 5 years, focusing on basic concepts, working principles, design methods, manufacturing, and experimental validation. Firstly, several manipulation modes of tunable metasurfaces are discussed; in particular, the metasurfaces based on temperature control, mechanical control, and electrical control are described in detail. It is demonstrated that the amplitude and phase responses can be flexibly manipulated by the tunable metasurfaces. Then, the concept, working principle, and design method of digital coding metasurfaces are briefly introduced. At the same time, we introduce the active programmable metasurfaces from the following aspects, such as structure, coding method, and three-dimensional far-field results, to show the excellent electromagnetic manipulation ability of programmable metasurfaces. Finally, the basic concepts and research status of intelligent metasurfaces are discussed in detail. Different from the previous programmable metamaterials, which must be controlled by human intervention, the new intelligent metamaterials control system will realize autonomous perception, autonomous decision-making, and even adaptive functional manipulation to a certain extent.

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Evolution of the Electromagnetic Manipulation: From Tunable to Programmable and Intelligent Metasurfaces.

The reconfigurable metasurface (MS) with tunable electromagnetic scattering properties has had an impact on innovative communication applications. However, the widely studied MS controlled by direct-current (DC) bias requires complex physical control circuits in array wavefront encoding, while the light-controlled MS does not. In this paper, we report a new approach for a light-controlled MS with a controllable modulation range of reflection phase, and its unit is composed of a reflection phase element based on the varactor and an optical interrogation network (OIN) based on the photoresistor so that the phase distribution of each MS unit could be independently programmed optically. To illustrate our approach, a 2-bit reflection phase MS with 10 × 10 units was fabricated and tested to achieve pencil beam scanning and orbital angular momentum beam at microwave frequencies. The simulated and measured results verify the feasibility of the proposed design. Furthermore, all the OINs on the MS are connected in parallel and powered by the same DC voltage source, which simplifies the difficulty of array expansion in MS design with a large-scale array.

Light-controlled metasurface with a controllable range of reflection phase modulation

Varifocal lenses, conventionally implemented by changing the axial distance between multiple optical elements, have a wide range of applications in imaging and optical beam scanning. The use of conventional bulky refractive elements makes these varifocal lenses large, slow, and limits their tunability. Metasurfaces, a new category of lithographically defined diffractive devices, enable thin and lightweight optical elements with precisely engineered phase profiles. Here, we demonstrate tunable metasurface doublets, based on microelectromechanical systems (MEMS), with more than 60 diopters (about 4%) change in the optical power upon a 1-um movement of one metasurface, and a scanning frequency that can potentially reach a few kHz. They can also be integrated with a third metasurface to make compact microscopes (~1 mm thick) with a large corrected field of view (~500 um or 40 degrees) and fast axial scanning for 3D imaging. This paves the way towards MEMS-integrated metasurfaces as a platform for tunable and reconfigurable optics.

/pdf/mems-tunable-dielectric-metasurface-lens-1cepj63gy1.pdf

MEMS-tunable dielectric metasurface lens

Metamaterials and metasurfaces have inspired worldwide interest in the recent two decades due to their extraordinary performance in controlling material parameters and electromagnetic properties. However, most studies on metamaterials and metasurfaces are focused on manipulations of electromagnetic fields and waves, because of their analog natures. The concepts of digital coding and programmable metasurfaces proposed in 2014 have opened a new perspective to characterize and design metasurfaces in a digital way, and made it possible to control electromagnetic fields/waves and process digital information simultaneously, yielding the birth of a new direction of information metasurfaces. On the other hand, artificial intelligence (AI) has become more important in automatic designs of metasurfaces. In this review paper, we first show the intrinsic natures and advantages of information metasurfaces, including information operations, programmable and real-time control capabilities, and space–time-coding strategies. Then we introduce the recent advances in designing metasurfaces using AI technologies, and particularly discuss the close combinations of information metasurfaces and AI to generate intelligent metasurfaces. We present self-adaptively smart metasurfaces, AI-based intelligent imagers, microwave cameras, and programmable AI machines based on optical neural networks. Finally, we indicate the challenges, applications, and future directions of information and intelligent metasurfaces.

Information metasurfaces and intelligent metasurfaces

Applying multiple physical fields to artificial manipulate electromagnetic waves is a highly stirring research. In this paper, we creatively combine light control with microwave scattering, realizing an optically coding metasurface for beam deflection based on anomalous reflection. A photoresistor and a voltage-driven module are connected to control each row of PIN-diode-loaded unit cells, endowing the reflection phase of the elements with a strong dependence on light. Owing to the high sensitivity of photoresistor, the digital element state “0” or “1” can be switched effectively via light variation sensed by the photoresistor. By modulating the light signal, the arrangement of digital elements can be reprogrammed, generating the specific scattering field. Therefore, the electromagnetic field can be determined by the spatial distribution of light, which induces the connect with the optical information and microwave field. The simulated and experimental results demonstrate the feasibility of our design. This light-steering approach provides a dimension for electromagnetic wave modulation.

Light-controllable metasurface for microwave wavefront manipulation.

Previous programmable metasurfaces integrated with diodes or varactors require external instructions for field programmable gate arrays (FPGAs), which usually rely on computer-inputs or pre-loaded algorithms. But the complicated external devices make the coding regulation process of the programmable metasurfaces cumbersome and difficult to use. To simplify the process and provide a new interaction manner, a touch-programmable metasurface (TPM) based on touch sensing modules is proposed to realize various electromagnetic (EM) manipulations and encryptions. By simply touching the meta-units of the TPM, the state of the diodes can be changed. Through the touch controls, the TPM can achieve independent and direct manipulations of meta-units and efficient inputs of coding patterns without using a FPGA or other control modules. Various coding patterns are demonstrated to achieve diverse scattering-field control and flexible near-field EM information encryptions, which verifies the feasibility of the TPM design. The presented TPM will have wide application prospects in imaging displays, wireless communications, and EM information encryptions.

Touch-Programmable Metasurface for Various Electromagnetic Manipulations and Encryptions.

In this paper, we present a flexible, breathable and optically transparent metasurface with ultra-wideband absorption. The designed double layer of indium tin oxide (ITO) films with specific carved structure realizes absorption and electromagnetic (EM) isolation in dual-polarization, as well as good air permeability. Under the illumination of x- and y-polarization incidence, the metasurface has low reflectivity and transmission from about 2 to 18 GHz. By employing ITO film based on polyethylene terephthalate (PET), the presented metasurface also processes the excellent flexibility and optically transparency, which can be utilized for wearable device application. In addition, the dual-layer design enables mechanically-reconfigurable property of the metasurface. The transmission and reflection coefficients in two polarizations show distinct difference when arranging the different relevant positions of two layers of the metasurface. A sample with 14*14 elements is designed, fabricated and measured, showing good agreement with the simulation results. We envision this work has various potentials in the wearable costume which demands both EM absorption and isolation.

https://www.mdpi.com/2072-666X/11/12/1032/pdf

Optically Transparent Metasurface Absorber Based on Reconfigurable and Flexible Indium Tin Oxide Film.

Combining digital information science with metasurface technology is critical for achieving arbitrary electromagnetic wave manipulation. However, there is a scarcity of contemporary scholarly studies on this subject. In this paper, we propose an Ultraviolet (UV) sensing metasurface for programmable electromagnetic scattering field manipulation by combining light control with a microwave field. The active sensing of UV light and the real-time reaction of the scattering are achieved by integrating four UV sensors on the metasurface. On the metasurface, a UV sensor ML8511 and a voltage driver module are coupled to control each row of the Positive-Intrinsic-Negative (PIN) diodes. Due to the light sensing capability of the UV sensor, the on or off state of the PIN diode integrated into the programmable metasurface can be switched efficiently through the change of light. When the incident wave changes, various discrete data are transmitted to the FPGA. Then the FPGA performs the corresponding voltage distribution to control the state of the PIN diode. Finally, different metasurface coding sequences are generated to realize different electromagnetic functions. As a result, the spatial distribution of sensing light by sensors can be used to determine the electromagnetic field and connect sensing optical information with the microwave field. The simulation and measured results show that this design is feasible. This work provides a dimension for electromagnetic waves modulation.

Ultraviolet-sensing metasurface for programmable electromagnetic scattering field manipulation by combining light control with a microwave field.

In recent years, plentiful works have focused on anomalous reflections, but only few works achieved dual-polarized reflection with high efficiency. In this paper, we present a reflection-type metasurface based on a dual-layer metallic structure at microwave frequency. By designing various periods of metallic structures, the metasurface can achieve high-efficiency anomalous reflections with a wide deflected angle range for orthogonal linearly polarized plane waves (x and y). By arranging distinct periods of units, various scattering angles of the reflected beam are achieved in two orthogonal directions. Besides, there is little interference between x-polarization and y-polarization reflected waves. To realize the aforementioned functionality, a unit cell with a stacked strip structure is employed. Based on the specific elements, we propose and simulate four metasurface schemes. Two of them are fabricated and measured, the measured results of which show good agreement with simulations validating our design. The high performance potentially makes this work diverse and intriguing with applications such as focusing reflectors and holograms.

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Si Si Luo

Papers

Light-controllable metasurface for microwave wavefront manipulation.

Touch-Programmable Metasurface for Various Electromagnetic Manipulations and Encryptions.

Optically Transparent Metasurface Absorber Based on Reconfigurable and Flexible Indium Tin Oxide Film.

Ultraviolet-sensing metasurface for programmable electromagnetic scattering field manipulation by combining light control with a microwave field.

Dual-polarization wave-front manipulation with high-efficiency metasurface