Active Metasurface Research Articles

Thermoplasmonic Controlled Optical Absorber Based on a Liquid Crystal Metasurface.

Metasurfaces can be realized by organizing subwavelength elements (e.g., plasmonic nanoparticles) on a reflective surface covered with a dielectric layer. Such an array of resonators, acting collectively, can completely absorb the resulting resonant wavelength. Unfortunately, despite the excellent optical properties of metasurfaces, they lack the tunability to perform as adaptive optical components. To boost the utilization of metasurfaces and realize a new generation of dynamically controlled optical components, we report our recent finding based on the powerful combination of an innovative metasurface-optical absorber and nematic liquid crystals (NLCs). The metasurface consists of self-assembled silver nanocubes (AgNCs) immobilized on a 50 nm thick gold layer by using a polyelectrolyte multilayer as a dielectric spacer. The resulting optical absorbers show a well-defined reflection band centered in the near-infrared of the electromagnetic spectrum (750-770 nm), a very high absorption efficiency (∼60%) at the resonant wavelength, and an elevated photothermal efficiency estimated from the time constant value (34 s). Such a metasurface-based optical absorber, combined with an NLC layer, planarly aligned via a photoaligned top cover glass substrate, shows homogeneous NLC alignment and an absorption band photothermally tunable over approximately 46 nm. Detailed thermographic studies and spectroscopic investigations highlight the extraordinary capability of the active metasurface to be used as a light-controllable optical absorber.

Design of continuously tunable varifocal metalenses

Active metasurfaces with tunable optical properties enable reconfigurable or programmable photonic systems that can swiftly adapt to different functional needs. Realizing continuous tuning of phase-gradient metasurfaces, however, remains elusive. This is because active modulation of individual or few meta-atoms is traditionally deemed essential to continuous tuning, which nonetheless introduces significant technical difficulties associated with addressing and control of large meta-atom arrays. In this paper, we propose a novel active metasurface architecture to circumvent this challenge. The key notion is to modulate the gradients of metasurface phase profiles over large regions instead of setting the phase at individual locations, thereby enabling continuous tuning with only a minimal number of independently controlled elements. The design principle can be generically applied to different active tuning mechanisms. As an example, we modeled a varifocal metalens with focal length continuously tunable between 4 and 10 mm based on electrically switched optical phase change materials.

Wideband low-scattering metasurface with an in-band reconfigurable transparent window

Active metasurfaces with dynamically reconfigurable functionalities are highly demanded in various practical applications. Here, we propose a wideband low-scattering metasurface that can realize an in-band reconfigurable transparent window by altering the operation states of the PIN diodes loaded on the structures. The metasurface is composed of a band-pass frequency selective surface (FSS) sandwiched between two polarization conversion metasurfaces (PCMs). PIN diodes are integrated into the FSS to switch the transparent window, while a checkerboard configuration is applied in PCMs for the diffusive-reflective function. A sample with 20×20 elements is designed, fabricated, and experimentally verified. Both simulated and measured results show that the in-band functions can be dynamically switched between beam-splitting scattering and high transmission by controlling the biasing states of the diodes, while low backscattering can be attained outside the passband. Furthermore, the resonant structures of FSS also play the role of feeding lines, thus significantly eliminating extra interference compared with conventional feeding networks. We envision that the proposed metasurface may provide new possibilities for the development of an intelligent stealth platform and its antenna applications.

Precisely constructing hybrid nanogap arrays via wet-transfer of dielectric metasurfaces onto a plasmonic mirror.

We propose a new method for fabricating hybrid metasurfaces by combining Mie and plasmonic resonances. Our approach involves obtaining an ultrasmooth gold film and separately structuring monocrystalline silicon (c-Si) nanoantenna arrays, which are then wet-transferred and finally immobilized onto the gold film. The experimental and simulation analysis reveals the importance of the native oxide layer of Si and demonstrates fascinating dispersion curves with nanogap resonances and bound states in the continuum. The localized field enhancements in the nanogap cavities result from the coupling between multipolar Mie resonances and their mirror images in the gold film. This effective method improves our understanding of hybrid modes and offers opportunities for developing active metasurfaces, such as depositing c-Si nanoantenna arrays onto stretchable polydimethylsiloxane substrates or electro-optic and piezoelectric sensitive lithium niobate films for potential applications in MEMS, LiDAR, and beyond.

Multifunctional active terahertz metasurface with electromagnetically induced transparency, perfect absorption, and circular dichroism

Navigated by the continued demand for design integration and miniaturization, incorporating multiple diversified operations into a single metasurface hybridized with the tunable metasurface is highly demanding at the terahertz (THz) range. However, up to now, because of the limitation of the reconfigurable metasurface at the terahertz range, most metasurfaces only feature a single function or process similar functionalities at a single frequency. A multi-functional terahertz metasurface based on vanadium dioxide (VO2) is proposed in this paper, which consists of a gold layer, a VO2 bridge, a SiO2 spacer, and a VO2 layer on the back of the spacer. Under different conductivity of different VO2 layers, the proposed metasurface achieves various terahertz device functions, including single-band perfect terahertz absorption, circular dichroism (CD) for the reflected and transmitted fields, and electromagnetically induced transparency (EIT) effect. Furthermore, the proposed metasurface exhibits robust response against oblique incidences. The mechanisms of perfect absorption, chiral-selective absorption and EIT are illustrated by impedance matching theory, surface current and electric field distributions, respectively. The function-switching ability of proposed metasurface provides more options for the design of terahertz devices in the future.

A Broadband Active Solar Cell Meta-Surface Antenna

In this letter, a broadband active solar cell meta-surface (SCMS) antenna powered by solar energy is proposed. This active antenna is composed of a programmable frequency source circuit and a SCMS antenna. The SCMS antenna is constructed by a spade shaped monopole and two SCMSs. The two SCMSs are located above and below the spade shaped monopole. We analyzed the working mechanism of SCMS using characteristic mode analysis (CMA) method. It shows the SCMS improves the voltage standing wave ratio (VSWR) and gain of the original antenna, and raise the coverage ratio of the solar cell. The VSWR of the SCMS antenna is less than 1.5 at 2.14-3.68 GHz, less than 2 at 2.08-4.1 GHz, and the gain of the antenna increases 1.74 dB on average in the frequency band of 2.6-4.1 GHz. The solar cell coverage ratio is 75.5%, with 100% insolation. This design provides an idea to achieve an active antenna powered by solar energy, which can be used for green wireless communication.

Continuously Tunable Optical Modulation Using Vanadium Dioxide Huygens Metasurfaces.

Efficient and dynamic light manipulation at small scale is highly desirable for many photonics applications. Active optical metasurfaces represent a useful way of achieving this due to their creative design potential, compact footprint, and low power consumption, paving the way toward the realization of chip-scale photonic devices with tunable optical functionality on demand. Here, we demonstrate a dynamically tunable, dual-function metasurface based on dielectric resonances in vanadium dioxide that is capable of independent active amplitude and phase control without the use of mechanical parts. Significant developments in the nanofabrication of vanadium dioxide have been shown to enable this metasurface. Gradual thermal control of the metasurface yields a computationally predicted continuously tuned amplitude modulation of 19 dB with negligible phase modulation and a continuously tuned phase modulation of 228° with negligible amplitude modulation, both at near-infrared wavelengths. Experimentally, a maximum continuously tuned amplitude modulation of 9.6 dB and phase modulation of 120° are shown, along with demonstration of stable intermediate states and repeated modulation without degradation. Reprogrammable optical functionality can thus be achieved in precisely engineered nanoantenna arrays for adaptive modulation of amplitude and phase of light for applications such as tunable holograms, lenses, and beam deflectors.

Active polarization-converting metasurface with electrically controlled magnitude amplification.

Recently, reconfigurable polarization-manipulation metasurfaces controlled with active components have gained widespread interest due to their adaptability, compact configuration, and low cost. However, due to the inherent non-negligible ohmic loss, the output energy of these tunable metasurfaces is typically diminished, particularly in the microwave region. To surmount the loss problem, herein, we propose an active polarization-converting metasurface with non-reciprocal polarization responses that is integrated with amplifying transistors. In addition, we provide a design strategy for a polarizer that is insensitive to polarization and has energy amplification capabilities. Experiments are conducted in the microwave region, and amplification of the polarization-converting behaviors is observed around 3.95 GHz. The proposed metasurface is prospective for applications in future wireless communication systems, such as spatial isolation, signal enhancement, and electromagnetic environment shaping.

Accurate 2-D DoA Estimation Based on Active Metasurface With Nonuniformly Periodic Time Modulation

A novel direction of arrival (DoA) estimation method based on amplifier-integrated active metasurface is proposed to find multisource directions by a single-channel system. Nonuniformly periodic time modulation (NPTM) on the metasurface reduces the hardware complexity by taking full advantage of the spectral resources. As a result, only partial metasurface elements are required to achieve 2-D DoA estimation, while the remaining elements can be redesigned and encoded to control electromagnetic waves. Thus, adaptive beamsteering can be expected to realize in a full-duplex way. Moreover, the amplifiers in the metasurface can function as modulation units to generate the required harmonics and enhance signal quality. The nonideality of the modulated square wave due to the active devices is allowed without affecting the estimation accuracy. An active metasurface prototype composed of 5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 5 elements is fabricated and measured in the microwave anechoic chamber for multisource direction finding. The measured maximum error is 0.31 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , which is high accuracy compared with the state-of-the-art works. Thus, the proposed method has application values in the Internet of Things (IoT), wireless communications, and radar.

The Design of Wideband Nonreciprocal Transmission by Active Metasurface

The Design of Wideband Nonreciprocal Transmission by Active Metasurface

Spin-controlled active geometric metasurfaces

Spin-controlled active geometric metasurfaces

High‐Speed Modulations of Guided Terahertz Waves via 2DEG Tiny Metasurfaces

AbstractMetasurfaces have emerged as fascinating planar optical components and devices, and, in particular, active metasurfaces confer more multifunctional expansions. However, the metasurface research has mainly focused on operating the light in free space, which hardly satisfies the system‐level compactness and miniaturization. The potential of the metasurface for guided wave modulation remains to be further developed. Here, an integrated electronic device is reported by embedding a tiny metasurface, which consists of several patterned 2D electron gas (2DEG) microstructures in a waveguide. This device suppresses excessive parasitism to improve the modulation speed and enhances the electromagnetic field interaction to improve the modulation efficiency. It is experimentally demonstrated that high‐speed modulations are electrically excited by the guided terahertz waves on a time scale of 40 ps via the tiny metasurface. This scheme represents a new platform for compact and high‐speed integrated electronics.

Active metasurface in reflection manner for generation of reconfigurable OAM vortex beams

In this paper, we proposed an active metasurface in reflection manner that can generate reconfigurable OAM vortex beams with high purity in the X-band. The metasurface has a high reflectance of 0.94 and achieves a phase coverage of 320° between 9.8 ​GHz and 11 ​GHz. Then, by encoding the phase distribution of the meta-atoms, various OAM vortex beams including ±1, ±2, ±3, and ±4 orders are generated, where the purity of all modes can be above 70%. Moreover, the metasurface can also deflect the OAM beam with a certain angle while maintaining high purity, which can be applied to reduce the influence of the alignment deviation between transmitting and receiving antennas during the communication processes. As a validation, the metasurface composed of 30 ​× ​30 meta-atoms is fabricated and measured. Both simulation and measurement results demonstrate the capability of the proposed metasurface to generate reconfigurable OAM beams with high purity, indicating the application potentials of proposed meta-devices in future OAM communications.

Reversibly reconfigurable GSST metasurface for broadband beam steering and achromatic focusing in the long-wave infrared.

Active optical metasurfaces promise compact, lightweight, and energy-efficient optical systems with unprecedented performance. Chalcogenide phase-change material Ge2Sb2Se4Te1 (GSST) has shown tremendous advantages in the design of mid-infrared active metasurfaces. However, most of the GSST-based active metasurfaces can only work efficiently within a narrow frequency range. Furthermore, their design flexibility and reversible switching capability are severely restricted by the melting of GSST during re-amorphization. Here, we propose broadband, reversibly tunable, GSST-based transmissive metasurfaces operating in the long-wave infrared spectrum, where the GSST micro-rods are cladded by refractory materials. To accurately evaluate the performance of the proposed metasurfaces, two figures of merits are defined: FOMΦ for the evaluation of wavefront matching, and FOMop for the assessment of the overall performance incorporating both wavefront modulation efficiency and switching contrast ratio. For the proof of concept, two meta-devices are numerically presented: a multifunctional deflector that offers continuous beam steering and long-wave pass filtering simultaneously, and a large-area (1 cm × 1 cm) broadband (11-14 µm) varifocal metalens with the ability of achromatic imaging (12.5-13.5 µm). In particular, the metalens features high FOMop values over 16 dB in the achromatic band, with the average focusing efficiency approximating 70% (60%) in amorphous (crystalline) state and a spectral switching contrast ratio surpassing 25 dB. Our design scheme provides an additional degree of freedom for dynamic modulation and offers a novel approach for achieving high-efficiency mid-infrared compact optical devices.

Nanoelectromechanical Tuning of High-Q Slot Metasurfaces.

Nanoelectromechanical devices have been used widely in many applications across photonics, electronics, and acoustics. Their incorporation into metasurface systems could be beneficial in designing new types of active photonic devices. Here, we propose a design of active metasurfaces using a nanoelectromechanical system (NEMS) composed of silicon bars which operates under CMOS-level voltage and achieves phase modulation with wavelength-scale pixel pitch. By introducing a perturbation to the slot mode propagating between the silicon bars, the device operates in a high-Q regime, making the optical mode highly sensitive to mechanical movement. An over 12 dB reflection modulation is observed by full-wave simulation, and over 10% is achieved in the proof-of-concept experiment under CMOS-level voltage. We also simulate a device with 1.8π phase response using a bottom gold mirror. Based on this device, a 3-pixel optical beam deflector is shown to have 75% diffraction efficiency.

Active Thermal Metasurfaces for Remote Heating/Cooling by Mimicking Negative Thermal Conductivity.

Remote temperature control can be obtained by a long-focus thermal lens that can focus heat fluxes into a spot far from the back surface of the lens and create a virtual thermal source/sink in the background material, around which the temperature field distribution can be remotely controlled by varying the parameters of the thermal lens. However, because of the lack of negative thermal conductivity, existing thermal lenses have extremely short focal lengths and cannot be used to remotely control the temperature field around the virtual thermal source/sink. In this study, a general approach is proposed to equivalently realize materials with negative thermal conductivity using elaborately distributed active thermal metasurfaces (ATMSs). Subsequently, the proposed ATMS is used to implement a novel thermal lens with a long focal length designed using transformation thermodynamics, and finally realize the ATMS with realistic materials and experimentally verify the performance of the designed long-focus thermal lens (measured focal length of 19.8mm) for remote heating/cooling. The proposed method expands the scope of the thermal conductivity and provides new pathways to realize unprecedented thermal effects with effective negative thermal conductivity, such as "thermal surface plasmon polaritons," a thermal superlens, the thermal tunneling effect, and the thermal invisible gateway.

Directional terahertz holography with thermally active Janus metasurface

Dynamic manipulation of electromagnetic (EM) waves with multiple degrees of freedom plays an essential role in enhancing information processing. Currently, an enormous challenge is to realize directional terahertz (THz) holography. Recently, it was demonstrated that Janus metasurfaces could produce distinct responses to EM waves from two opposite incident directions, making multiplexed dynamic manipulation of THz waves possible. Herein, we show that thermally activated THz Janus metasurfaces integrating with phase change materials on the meta-atoms can produce asymmetric transmission with the designed phase delays. Such reconfigurable Janus metasurfaces can achieve asymmetric focusing of THz wave and directional THz holography with free-space image projections, and particularly the information can be manipulated via temperature and incident THz wave direction. This work not only offers a common strategy for realizing the reconfigurability of Janus metasurfaces, but also shows possible applications in THz optical information encryption, data storage, and smart windows.

Wideband Linear-to-Multi-Polarization Converter Based on Active Metasurface

This paper proposes a transmission-type linear-to-multi-polarization converter based on cascading four-layer metasurface integrated with PIN diodes. By controlling the states of the PIN diodes, the polarization maintaining (PM) which means transmission without conversion, linear to linear (LTL), linear to right-hand circular polarization (LTRCP), linear to left-hand circular polarization (LTLCP) conversions can be realized. When the PM is realized, the incident wave will pass through the polarizer with low loss over wideband, the isolation is higher than 30 dB. When the linear-to-circular polarization conversion is realized, the incident linearly polarized (LP) wave can be converted to left-hand circularly polarized (LCP) or right-hand circularly polarized (RCP) wave with a relative bandwidth over 40%. When the LTL polarization conversion is realized, the polarization conversion ratio (PCR) is greater than 0.9 from 2.58 to 3.89 GHz. The overlapping bandwidth of the four operating states is 36%. The detailed design process and equivalent circuit of the proposed polarizer are analyzed in this paper. A sample consisting of 10 × 10 elements is fabricated. The measured and simulated results are in good agreement. To the authors’ best knowledge, this is the first reported reconfigurable polarizer capable of PM, LTLCP, LTRCP, and LTL polarization conversions within wideband. The proposed device can play an important role in polarization diversity and RF-integrated aperture of wideband channel multiplexing technology.

Infrared Resonance Tailoring of Individual Split‐Ring Resonators with Phase‐Change Materials by Locally Changing the Dielectric Surrounding of the Antenna Hotspots

AbstractFor miniaturized active metasurfaces, resonance tuning of nanoantennas is a key ingredient. Phase‐change materials (PCMs) have been established as prime candidates for non‐volatile resonance tuning enabled by a change in the refractive index around nanoantennas. Conventionally, this tuning is induced by annealing the entire sample equally and does not allow changes on a meta‐atom level. Recently, it is demonstrated that individual rodantenna resonances can be adjusted by addressing each meta‐atom locally with precise laser pulses and switching the PCM there. However, simultaneously controlling several different modes remains elusive. In this work, PCM‐covered aluminum split‐ring resonators (SRRs) are switched locally to tune both the electric dipole resonances as well as the magnetic dipole resonances. By selectively switching the PCM at different hotspots of the SRRs, both resonances can be tuned individually. Finally, the field enhancement in the magnetic resonance allows continuous tuning of surface‐enhanced infrared absorption of native SiO2. This work serves as a proof of principle for sophisticated resonance tuning via changes in the refractive index at the hotspots of the selected antennas enabling fine‐tuning functionalities on a meta‐atom level and allows for post‐fabrication adjustments of metasurfaces.

Optical Properties of Electrochemically Gated La1−xSrxCoO3−δ as a Topotactic Phase‐Change Material

AbstractMaterials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion‐gel‐gated epitaxial films of the perovskite cobaltite La1−xSrxCoO3−δ (LSCO) with 0.00 ≤ x ≤ 0.70 offer a new route to significant, voltage‐tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig‐consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As‐grown perovskite films are high‐index insulators for x &lt; 0.18 but lossy metals for x &gt; 0.18, due to a percolation insulator‐metal transition. Positive‐voltage gating of LSCO transistors with x &gt; 0.18 reveals a metal‐insulator transition from the metallic perovskite phase to a high‐index (n &gt; 2.5), low‐loss insulating phase, accompanied by a perovskite to oxygen‐vacancy‐ordered brownmillerite transformation at high x. At x &lt; 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower‐index, lower‐loss insulating perovskite state with Δn &gt; 0.6. In simulations of plasmonic metasurfaces, these metal‐insulator and insulator‐insulator transitions support significant, varied mid‐infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room‐temperature phase‐change materials for applications including dynamic thermal imaging, camouflage, and optical memories.