Tunable Metadevices Research Articles

Reconfigurable High-Efficiency metadevice using Kirigami-Inspired phase gradient metasurfaces

Kirigami-inspired metasurface has attracted great attention in electromagnetic (EM) wave manipulations, due to its unprecedented and tailorable structural transformations via mechanical approaches. However, it is still challenging for wavefront control because of its unfeasibility in forming a phase gradient at different folding angle β. Here, we report a strategy of kirigami-inspired reconfigurable phase gradient metasurfaces to efficiently control beam steering of scattering wave through mechanically changing β, where the control range of β is theoretically derived based on EM diffraction and generalized Snell’s law. For demonstration, reconfigurable anomalous reflectors with high efficiency are demonstrated in both dual-element and tri-element tunable metadevices. In former case, the reflection angle varies within 37°∼47° under polarization along y axis by changing β within the range of 10°∼40°, whereas it varies within 36°∼50° under that along x axis when β is changed within 10°∼39°, and numerical and experimental results show measured efficiency of beam steering over 78.6%. In latter case, the reconfigurable tri-element metadevice with various folding states are also experimentally characterized to further verify our strategy. Compared with available metasurfaces, our reconfigurable strategy using maneuverable structures to control EM wave is more versatile and shows great potential in engineering applications.

Switchable magnetic dipole assisted double QBICs in an asymmetry four-leaf clover-shaped Ge2Sb2Te5 metasurface

Dynamically tunable metasurface resonators with high Q factors and excellent sensing performance still lack research. This paper proposes a dual-band symmetry broken induced quasi bound state in the continuum (QBIC) sensor in the near-infrared region based on an asymmetric four-leaf clover-shaped Ge2Sb2Te5 (GST) metasurface. The high-quality transmission spectrum can be tuned by switching the GST crystalline state. Two QBIC magnetic dipole resonances are excited at the amorphous GST state, QBIC Ⅰ (intracavity MD mode) and QBIC Ⅱ (intercavity high-order MD mode). Based on theoretical simulation, the refractive index sensitivity of the QBIC Ⅱ mode (318.05 nm/RIU) is enhanced higher than twice of the QBIC Ⅰ mode (134.03 nm/RIU) owing to the unique electromagnetic field properties. The ultra-high Q factors generated by the two MD resonance modes are 1.1 × 105 (QBIC Ⅰ) and 1.71 × 105 (QBIC Ⅱ) respectively for a small perturbation of the air hole offset distance D = 5 nm. By discussing the effect of the main structure parameters and the analyte thickness on the sensitivity, the optimal parameters are determined. The two strong QBICs supported by the a-GST metasurface are stable when the air hole’s offset from the unit center is at different directions and distances, indicating a good tolerance to fabrication requirements. These results provide a competitive switchable dual-band QBIC metasurface sensor, providing a potential route for tunable metadevices for functional sensing and dynamic control over signals.

A functionality switchable meta-device: from perfect reflection to perfect absorption

Tunable meta-devices trigger immense interest in many applications like biosensing, thermal camouflage, radiative cooling, thermal emitting, infrared imaging, and energy harvesting. This paper presents a polarization-independent broadband tunable absorbing and reflecting meta-device using a phase change material Germanium-Antimony-Tellurium (GST). The proposed design manifests high absorptivity >80% over a broadband range of wavelength spanning the mid-IR regime. Furthermore, by changing the phase of GST material from crystalline to amorphous with the help of external stimuli, the absorbance of the device was reduced to almost zero in the range from 4 μm to 12 μm. To previse the angular constancy of the meta-absorber, the absorptivity was scrutinized under different incidence angles in both transverse magnetic (TM) and transverse electric (TE) polarizations which manifested the robustness of the proposed meta-absorber. As a result, the proposed absorber has tunable broadband absorbance and angular stability, which proclaim great potential in applications such as thermal camouflage, radiation detection, thermal emitter, and stealth technology, etc.

Interplay of bound states in the continuum empowers spectral-lineshape manipulation in all-dielectric metasurfaces

The bound states in the continuum (BICs) in optical metasurfaces have latterly received prominent attention for their ultrahigh quality factors and the promise that they hold for realizing lowthreshold nanolasers, high-harmonic generation, and slow-light devices. In this scenario, the flexible tuning of the BIC is at the heart of multifunctional and tunable metadevices underpinning the central role in the modulation of optical spectral responses. Most BIC-inspired works are limited to single types of BICs modes only affected by the structural perturbation without tunable functionality, while different types of BIC modes have received little attention in manipulating the spectral line shapes in optical metasurface. Here, we show that the simultaneous generation and tailored interplay of two types of BIC modes can empower the realization of tailorable spectral lineshape manipulation in all-dielectric metasurfaces. In particular, the magnetic symmetry-protected BICs and accidental BIC result from the preserved reflection symmetry and geometrical parameter perturbation, respectively. The varied asymmetric parameters and the polarization angle of the incident light are responsible for the generation and interplay of the symmetry-protected and accidental quasi-BIC modes. Additionally, the interplaying quasi-BICs enable the implementation of the dual-wavelength passive optical switches throughout the related telecom bands, and can exhibit the tunable spectral features of different types of Fano resonances and EIT analog-based slow-light effect. These results could offer exploration potential for the development of multifunction and high-performance metadevices.

Optically controlled dielectric metasurfaces for dynamic dual-mode modulation on terahertz waves

Dynamically controlling terahertz (THz) waves with an ultracompact device is highly desired, but previously realized tunable devices are bulky in size and/or exhibit limited light-tuning functionalities. Here, we experimentally demonstrate dynamic modulation on THz waves with a dielectric metasurface in mode-selective or mode-unselective manners through pumping the system at different optical wavelengths. Quasi-normal-mode theory reveals that the physics is governed by the spatial overlap between wave functions of resonant modes and regions inside resonators perturbed by pump laser excitation at different wavelengths. We further design/fabricate a dielectric metasurface and experimentally demonstrate that it can dynamically control the polarization state of incident THz waves, dictated by the strength and wavelength of the pumping light. We finally numerically demonstrate pump wavelength-controlled optical information encryption based on a carefully designed dielectric metasurface. Our studies reveal that pump light wavelength can be a new external knob to dynamically control THz waves, which may inspire many tunable metadevices with diversified functionalities.

High quality factor resonant metasurface with etchless lithium niobate

Actively tunable metasurfaces have widespread applications in optics from light modulation, beam steering to LiDAR and many others. Recently, lithium niobate (LN) based metasurfaces have attracted significant attention for their great potentials in integrated electro-optically tunable metadevices. However, the hardness and chemical inactivity of LN make it difficult to process, which severely limits the quality (Q) factors and performance of LN based metasurfaces. In this paper, a type of high Q etchless LN resonant metasurface is experimentally demonstrated. The metasurface shows a measured Q factor of 563 around the telecom wavelength and even higher Q factors are expected to be realized using similar designs in the future. This work represents an important progress for LN resonant metasurfaces and paves the way for the development of high performance electro-optically tunable LN metadevices.

Modulation of Surface Plasmonic Bending Beam via Nanoslit Interactions

The discussion of resonance mechanisms for artificial structural units has always been a key to producing highly efficient, active and tunable meta-devices in the fields of controlling surface plasmon polaritons (SPPs) to generate surface plasmonic bending beams (SPBs). In this study, an array of 20 antisymmetric double V-shaped structures was designed to generate an SPB. The arms of the double V-shaped structures were panned to control the electric field intensity distributions of the SPB. The influence of the polarization states (such as polarization angles, linearly polarized (LP), left-circularly polarized (LCP) and right-circularly polarized (RCP) light) of the incident light on electric field intensity of SPB is discussed. These results can be well explained by the theory of dipole radiation. The numerical simulation results are in good agreement with the theoretical analysis. It is hoped that these results will help guide subsequent work in optimizing SPB generators.

Multifield nested metafilters for wave propagation control

The present work proposes a novel class of multifield nested tunable metadevices that serve as high performance acoustic metafilters. The designed metafilter is characterized by a multiscale, hierarchical structure. At the mesoscale, the metamaterial consists of a sequence of two different periodically alternating layers: a polymeric homogeneous layer, which exhibits a viscoelastic constitutive response, and a microstructured one. The latter is based on the periodic repetition of a multiphase microscale cell that is composed by a stiff elastic external coating, a viscoelastic phase and an internal disk of piezoelectric material shunted by an external electrical circuit having a tunable impedance/admittance This tuning parameter affects the constitutive elastic properties of the piezoelectric phase and, in turn, the overall response of the microscale cell, thereby ultimately enabling to achieve an optimal filtering performance for the metadevice. Due to the periodicity of the multiphase cell at the microscale, a two-scale variational-asymptotic homogenization technique is exploited in the frequency domain in order to obtain the frequency-dependent overall constitutive properties of the microstructured layer. Subsequently, in-plane free wave propagation inside the periodic multilayered metamaterial at the mesoscale is investigated by means of Floquet–Bloch theory, together with the transfer matrix method. By triggering the shunting effect, a stiffening of the piezoelectric phase can be achieved, which is demonstrated to open low frequency band gaps in the metamaterial frequency spectrum. The filtering capability of the metadevice has been assessed as a function of its geometrical features and the tuning parameter, thus paving the way towards the design of sophisticated and topologically optimized acoustic filters.

Multifunctional and Transformative Metaphotonics with Emerging Materials.

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Switching the second harmonic generation by a dielectric metasurface via tunable liquid crystal.

Optical modulators are key ingredients in optoelectronics applications ranging from energy harvesting, sensor and imaging devices. In this framework, nonlinear photon conversion mechanisms constitute an attractive opportunity to add logic capabilities to these apparatuses. Here, we investigate the directionality of the emitted second harmonic signal generated in a dielectric metasurface consisting of AlGaAs nanocylinders embedded into a liquid crystal matrix. We numerically demonstrate that, by switching the liquid crystal orientation with a realistic voltage bias, it is possible to modulate the total power and the emission pattern of the SH signal coming from the proposed metasurface. Our results open important opportunities for tunable metadevices such as nonlinear holograms and dynamic displays.

A Tunable Metasurface with Switchable Functionalities: From Perfect Transparency to Perfect Absorption

AbstractA thin screen exhibiting dynamically switchable transmission/absorption functionalities is highly desired in practice. However, a trilayer transmissive metasurface with active elements controlled in a uniform manner cannot exhibit independently controlled transmission and absorption properties due to intriguing interplays between the scattering and absorbing properties in systems exhibiting inversion symmetries. This motivates to employ the coupled‐mode theory to establish a generic phase diagram for such transmissive metasurfaces with active elements loaded in different layers tuned independently and to guide researchers design tunable metadevices with completely decoupled transmission/absorption responses. Based on such a phase diagram, a microwave metasurface is designed/fabricated with PIN diodes incorporated, and it is experimentally demonstrated that its functionality can switch from perfect transparency to perfect absorption, controlled by external voltages applied across the diodes. In addition to finding immediate applications in practice (e.g., smart radomes), the results of this study also provide a new type of tunable meta‐atom for building metasurfaces with flexible wave‐front control abilities.

Multifunction Tunable Metasurface for Entire-Space Electromagnetic Wave Manipulation

Metasurfaces trigger great interests due to their potential ability to manipulate the propagation of electromagnetic (EM) waves. Plenty of fascinating EM phenomena have been generated using metasurface. However, two defects are hindering their development. The first one is that most metasurfaces only perform a specific functionality once designed, which means they do not possess tunable properties. The second defect is that most of them work in either reflection mode or transmission mode, leaving half of the EM space completely unexplored. Therefore, it is challenging to design metasurface with tunability and entire-space working modes simultaneously. Here, a tunable metasuface is presented to solve these problems. The proposed metasurface element is a multilayer structure which can manipulate EM waves in the entire space. Concurrently, p-i-n switches are loaded on each element which can dynamically control transmission and reflection phases. Thus, some intriguing EM modulation functions in the entire space are realized, such as beam forming, anomalous reflection, and diffusion. The simulated and experimental results prove the tunable metasurface. The proposed metasurface offers a feasible strategy to design tunable metadevices and for other related practical applications.

All-silicon reconfigurable metasurfaces for multifunction and tunable performance at optical frequencies based on glide symmetry

Dielectric metasurfaces have opened promising possibilities to enable a versatile platform in the miniaturization of optical elements at visible and infrared frequencies. Due to high efficiency and compatibility with CMOS fabrication technology, silicon-based metasurfaces have a remarkable potential for a wide variety of optical devices. Adding tunability mechanisms to metasurfaces could be beneficial for their application in areas such as communications, imaging and sensing. In this paper, we propose an all-silicon reconfigurable metasurface based on the concept of glide symmetry. The reconfigurability is achieved by a phase modulation of the transmitted wave activated by a lateral displacement of the layers. The misalignment between the layers creates a new inner periodicity which leads to the formation of a metamolecule with a new sort of near-field interaction. The proposed approach is highly versatile for developing multifunctional and tunable metadevices at optical frequencies. As a proof of concept, in this paper, we design a bifunctional metadevice, as well as a tunable lens and a controllable beam deflector operating at 1.55 μm.

Tunable near-infrared perfect absorber based on the hybridization of phase-change material and nanocross-shaped resonators

Ge2Sb2Te5 (GST) is a kind of non-volatile chalcogenide phase-change material, which has a significant difference in permittivity between its amorphous and crystalline states in the infrared range. On account of this remarkable property, the combination of GST and metamaterials has great potential in tunable meta-devices. In this paper, a perfect absorber based on a nanocross-resonator array stacked above a GST spacer layer and an Au mirror (i.e., a metal-dielectric-metal configuration) is designed and experimentally demonstrated. A thin indium tin oxide (ITO) protective layer is inserted between the GST spacer and the Au resonator to avoid heat-induced oxidation of the GST layer during phase transition. We found that the ITO layer not only can protect the GST layer from deterioration, but also allows a significant blue shift in the absorption peak from 1.808 μm to 1.559 μm by optimizing the thickness of the two dielectric layers without scaling down the size of the metal structure, which provides a more feasible idea in pushing the absorption peak to higher frequency. The LC circuit model is presented to explain this blue-shift phenomenon, which is mainly attributed to the engineering of the dielectric environment of the parallel plate capacitance. In addition, such good performance in dynamitic modulation makes this perfect absorber a robust candidate for optical switching and modulating in various situations.

Photoexcited Graphene Metasurfaces: Significantly Enhanced and Tunable Magnetic Resonances

Artificially constructed metamaterials or metasurfaces with tailored resonant elements provide a revolutionary platform for controlling light at the subwavelength scale. Switchable or frequency-agile meta-devices are highly desirable in achieving more flexible functionalities and have been explored extensively by incorporating various materials, which respond to external stimuli. Graphene, a two-dimensional material showing extraordinary physical properties, has been found very promising for tunable meta-devices. However, the high intrinsic loss of graphene severely obstructs us from achieving high-quality resonance in various graphene metamaterials and metasurfaces, and the loss compensation can be considered as a straightforward strategy to take further advantages of enhanced light–graphene interactions. Here, we demonstrate that the photoexcited graphene, in which the quasi-Fermi energy of graphene changes corresponding to optical pumping, can boost the originally extremely weak magnetic resonance in a...

Electrically tunable all-dielectric optical metasurfaces based on liquid crystals

We demonstrate electrical tuning of the spectral response of a Mie-resonant dielectric metasurface consisting of silicon nanodisks embedded into liquid crystals. We use the reorientation of nematic liquid crystals in a moderate applied electric field to alter the anisotropic permittivity tensor around the metasurface. By switching a control voltage “on” and “off,” we induce a large spectral shift of the metasurface resonances, resulting in an absolute transmission modulation of up to 75%. Our experimental demonstration of voltage control of dielectric metasurfaces paves the way for new types of electrically tunable metadevices, including dynamic displays and holograms.