Metasurface Sample Research Articles

Arbitrarily rotating polarization direction and manipulating phases in linear and nonlinear ways using programmable metasurface

Independent controls of various properties of electromagnetic (EM) waves are crucially required in a wide range of applications. Programmable metasurface is a promising candidate to provide an advanced platform for manipulating EM waves. Here, we propose an approach that can arbitrarily control the polarization direction and phases of reflected waves in linear and nonlinear ways using a stacked programmable metasurface. Further, we extend the space-time-coding theory to incorporate the dimension of polarization, which provides an extra degree of freedom for manipulating EM waves. As proof-of-principle application examples, we consider polarization rotation, phase manipulation, and beam steering at linear and nonlinear frequencies. For validation, we design, fabricate, and measure a metasurface sample. The experimental results show good agreement with theoretical predictions and simulations. The proposed approach has a wide range of applications in various areas, such as imaging, data storage, and wireless communication.

Broadband Quantum Efficiency Enhancement of Al0.3InAsSb p–i–n Photodiodes with All‐Dielectric Amorphous Germanium Metasurfaces

Transparent amorphous germanium (a‐Ge) has emerged as a promising material for engineering nanostructures and metasurfaces, offering significant potential for enhancing the performance of photonic devices in the short‐wavelength infrared (SWIR) spectrum. Herein, the successful application of a‐Ge metasurfaces with a truncated pyramid profile to enhance the external quantum efficiency (EQE) of a digital alloy Al0.3InAsSb p–i–n photodiode across a broad‐wavelength range in the SWIR is presented. The experimental findings demonstrate a broadband enhancement in EQE. Two metasurface samples are designed to emphasize different‐wavelength ranges. Notably, 51% improvement in EQE at 1550 nm and 125% enhancement at 2000 nm is achieved. Finite‐difference time domain simulations show that the observed EQE improvement originates from the reduction of reflection and electromagnetic field enhancement. This study underscores the promising role of a‐Ge metasurfaces in advancing the capabilities of SWIR photodetectors. It lays the groundwork for further exploration in optoelectronic device enhancements.

Polarization-frequency multiplexing holograms employing Bi-layered patterned coding metasurfaces

A surge in interest surrounding metasurfaces has been witnessed in recent years, driven by their capability to autonomously manipulate electromagnetic wavefront (EM) in three-dimensional space. While the initial focus in metasurface research has often been on manipulating electromagnetic waves using single polarization and frequency schemes, significant advancements have been made to overcome these limitations. Recent developments have introduced multifunctional metasurfaces that enable simultaneous control over multiple frequencies and polarizations, substantially enhancing their capacity for information processing and communication. Building on these advancements, our work introduces a novel three-channel full-space metasurface that leverages double-face copper-cladded patterns on a single-layer substrate, significantly increasing channel capacity through both frequency and polarization multiplexing. In order to achieve efficient control of the EM wavefront in multi-frequency and polarization scenarios, a super unit cell-based approach is utilized, wherein high-performance meta-atoms are combined to form super unit cells with minimal interferences. Specifically, different functionalities can be achieved in full space under different wave polarizations and frequencies. To validate the concept, a metasurface sample is fabricated and experimentally tested. The experimental results agree with both theoretical predictions and simulations, confirming the effectiveness of a three-channel metasurface holography. The metasurface achieves independent control of the incident EM wave's polarization and frequency, thus enhancing information storage capacity and encryption security.

Multi‐Wavelength Metasurface Empowered Cryptography for Heightened Security and Improved Fidelity

AbstractInformation security holds paramount importance in the contemporary information society. Here, a novel cryptography method is proposed based on the multi‐wavelength complex‐amplitude metasurface, which can achieve a high security level by generating a frequency‐selective key as an additional layer of protection. Moreover, the quality of the decrypted holographic images can be significantly improved by employing the complex‐amplitude encoding method and the modified visual secret sharing scheme, which adopts a new superposition operator to prevent fidelity loss. As a proof‐of‐concept demonstration, a prototype metasurface sample is designed, fabricated, and characterized, where the measured results agree very well with the numerical ones and the design goals. The proposed method can reinforce security level and improve fidelity of the decrypted message, rendering it appealing for applications in communication and anti‐counterfeiting.

Magnetic toroidal dipole resonance terahertz wave biosensor based on all-silicon metasurface

Optical sensing is crucial in many applications and has received widespread attention in recent years. Transmission spectroscopy with narrow linewidth resonances is indispensable for improving metasurface sensing performance. However, the narrower linewidth results in a smaller resonant modulation range, which seriously hinders the application of metasurfaces in sensing detection. We designed an all-silicon metasurface sensor. The structural layer of the metasurface works together with the substrate layer. Multiple magnetic ring dipoles are generated in the structural layer and multiple magnetic dipoles are generated in the substrate layer. The two work together to stimulate sharp Fano resonance, with a quality factor of up to 272.84. Based on the characteristics of metasurface, when applied to sensing and detection, the theoretical sensitivity can reach 59.64 GHz/RIU. Metasurface samples were successfully prepared using photolithography and dry etching. After characterizing the transmission spectrum of the prepared metasurface, the actual quality factor reached 76.8. At the same time, the metasurface sensor was used to detect bovine serum albumin (BSA) solutions of different concentrations. Based on the test results, it is calculated that the detection limit of the metasurface sensor for detecting BSA can reach 0.027 mg/mL, and the detection sensitivity can reach 9.7 GHz/(ng/mm2). The results of theoretical simulation and biological experiments show that the sensor has good sensing performance. This work is of great significance to the research of all-silicon metasurfaces and the application of metasurface sensors.

Multi-mode vortex beams generation with single-layer transmissive metasurface

Orbital angular momentum (OAM) is a phenomenon of vortex phase distribution in free space, the infinity of whose modes and the orthogonality between modes allow it to possess great potential in enhancing channel capacity. A transmissive metasurface consisting of a single-layer substrate and a two-layer metallic structure is proposed to enable vortex beam generation based on the concept of the Pancharatnam-Berry (PB) phase. The phase response is achieved by continuous phase modulation under circularly polarised (CP) wave incidence due to the simultaneous rotation of the upper and lower metal structures. Besides, through the introduction of both phase compensation on the metasurface and phase superposition of the beams to the array design, multi-beam generation is expected to be realized. A metasurface sample composed of 30 30 elements (450 mm 450 mm) is fabricated and measured in a microwave darkroom on the basis of the simulation results. Both the simulation and measurement results provide firm demonstration that the designed metasurface is capable of simultaneously generating four beams carrying OAM mode numbers of and different beam directions at the same aperture. The designed metasurface in this paper utilized for generating multi-mode vortex multi-beams will be of wide application in multi-channel transmission in wireless communication systems.

Enhanced laser-induced damage performance ofall-glass metasurfaces for energetic pulsed laserapplications.

To fabricate optical components with surface layers compatible with high-power laser applications that may operate as antireflective coatings, polarization rotators, or harness physical anisotropy for other uses, metasurfaces are becoming an appealing candidate. In this study, large-beam (1.05cm diameter) 351-nm laser-induced damage testing was performed on an all-glass metasurface structure composed of cone-like features with a subwavelength spacing of adjacent features. These structures were fabricated on untreated fused silica glass and damage tested, as were structures that were fabricated on fused silica glass that experienced a preliminary etching process to remove the surface Beilby layer that is characteristic of polished fused silica. The laser-induced damage onset for structures on untreated fused silica glass was 19.3J⋅c m -2, while the sample that saw an initial pretreatment etch exhibited an improved damage onset of 20.4J⋅c m -2, only 6% short of the reference pretreated glass damage onset of 21.7J⋅c m -2. For perspective, the National Ignition Facility operational average fluence at this wavelength and pulse length is about 10J/c m 2. At a fluence of 25.5J⋅c m -2, the reference (pretreated) fused silica initiated 5.2 damage sites per m m 2, while the antireflective metasurface sample with a preliminary etching process treatment initiated 9.8 damage sites per m m 2. These findings demonstrate that substrate-engraved metasurfaces are compatible with high energy and power laser applications, further broadening their application space.

Twofold optical display and encryption of binary and grayscale images with a wavelength-multiplexed metasurface

Abstract The remarkable capability in regulating light polarization or amplitude at the nanoscale makes metasurface a leading candidate in high-resolution image display and optical encryption. Diverse binary or grayscale meta-images were previously shown concealed in a single metasurface, yet they are mostly stored at same encryption level and share an identical decryption key, running the risk of exposing all images once the key is disclosed. Here, we propose a twofold optical display and encryption scheme demonstrating that binary and grayscale meta-images can be concurrently embedded in a nonspatially multiplexed silicon metasurface, and their decryptions demand for drastically different keys. Unlike previous metasurfaces relying on isolated transmission or phase manipulations upon orthogonal linear polarization incidences, this is made possible by exploiting silicon meta-atoms featuring joint transmission amplitude and polarization control at two wavelengths. In detail, the selected two meta-atoms exhibit large polarization-independent transmission difference (∼85 %) at a wavelength of 800 nm, while functioning as the nano-quarter-wave plate at wavelength of 1200 nm. Through elaborate design in simulation, a binary image can be witnessed when the metasurface is merely illuminated by an unpolarized light of wavelength 800 nm or under white light illumination. However, a distinct binary or grayscale image will come into view by inspecting the metasurface with an analyzer and when the incident light is circularly polarized at the wavelength of 1200 nm. Two metasurface samples are fabricated and successfully verified the claims experimentally. The proposed approach is expected to bring new insights to the field of optical display and encryption.

Dual-Band Metasurface With Extreme Angular-Asymmetric Transmission and Frequency Selection Based on Resonant Coupling Effect

Asymmetric transmission plays an important role in many electromagnetic systems. Different to the asymmetric transmissions in full space, angular-asymmetric transmissions in half space support different characteristics for symmetrical oblique incidences, which has great potentials for antenna designs and wireless systems. By introducing guided wave modes and resonant coupling effect into metasurface design, this paper designs and demonstrates a dual band multi-functional metasurface with angular-asymmetric transmission and frequency selection under oblique incidences. At 8.7 GHz, the electromagnetic wave under 30° TM oblique incidence is absorbed, and the wave under -30° TM oblique incidence transmits, which presents an angular-asymmetric transmission. At 10.4GHz, both ±30° TM oblique incident waves can be transmitted, which presents a frequency selection function. The metasurface sample with optimized geometry parameters has been fabricated and measured, and the measurement agree well with the simulation results.

A metacontinuum model for phase gradient metasurfaces

Acoustic metamaterials and metasurfaces often present complex geometries and microstructures. The development of models of reduced complexity is fundamental to alleviate the computational cost of their analysis and derivation of optimal designs. The main objective of this paper is the derivation and validation of a metacontinuum model for phase gradient-based metasurfaces. The method is based on the transformation acoustics framework and defines the metasurface in terms of anisotropic inertia and bulk modulus. Thermal and viscous dissipation effects in the metacontinuum are accounted for by introducing a complex-valued speed of sound. The model is implemented in a commercial FEM code, and its predictions are compared with numerical simulations on the original geometry and also using an equivalent boundary impedance approach. The results are examined for an exterior acoustics benchmark and for an in-duct installation in terms of transmission coefficient with the four-pole matrix method. The metacontinuum model gives solid results for the prediction of the acoustic properties of the examined metasurface samples for all the analyzed configurations, as accurate as the equivalent impedance model on which it is based and outperforming it in some circumstances.

Switchable Dual-Functional Metasurface for THz Absorption and Electromagnetically Induced Transparency

The metasurfaces based on nanostructure film play an important role in many fields. Usually, the properties and functions of metasurfaces are limited by their structure. Once the metasurface samples are processed, their functions have already been restricted. The dual-function device designed in this work utilizes the phase transition characteristic of vanadium dioxide (VO2). The entire layer of VO2 film is inserted between the double metal micro-nano structure. When VO2 film is in the metallic state after phase change, an isotropic narrow absorber is obtained in the terahertz (THz) region, which consists of a top Z-shaped meta-atom, a middle dielectric layer, and a bottom VO2 film. By adjusting structure parameters of VO2 film, perfect absorption is realized at the frequency of 0.525 THz with the overall absorption beyond 91%. When VO2 is in insulating state, the top Z-shaped meta-atom will interact with the bottom Z-shaped structure, and the resonance coupling leads to the appearance of electromagnetically induced transparency (EIT). The designed metal-VO2 hybrid metamaterial opens possible avenues for switchable functionalities in a single device.

A 4D‐Printed Electromagnetic Cloaking and Illusion Function Convertible Metasurface

AbstractThe appearance of metasurfaces provides a new way for the realization of cloaking and illusion devices. However, most proposed metasurface functional devices only display a single cloaking and illusion function. Herein, a dynamic metasurface device with switchable electromagnetic cloaking and illusion functions is fabricated based on a 4D‐printing process. Owing to the shape memory polymer's characteristic of shape change above the glass transition temperature, under the stimulation of temperature, it can control the structural shape change of the metasurface, change the phase distribution law, and realize dynamic cloaking and illusion functions. As proof, the metasurface samples with changeable structure are designed and fabricated by fused deposition modeling of 4D printing combined with liquid metal injection molding. Both the simulation and experimental results confirm that the designed metasurface has excellent cloaking and illusion performance at microwave frequency. This work provides a new idea for the realization of integrated metasurface devices with electromagnetic cloaking and illusion functions.

Design and experiment of terahertz unidirectional transmission structure based on neural network

In metasurface design, massive meta-atoms have to be optimized to produce the desired properties, which are time-consuming and sometimes prohibitive. In this paper, neural network model composed forward modeling and inverse design network is proposed to realize rapid, efficient, and automatic metasurface design of THz unidirectional transmission. Specifically, massive metasurface samples are generated by Python-CST co-simulation to train the neural network model and make it robust to predict the THz responses and vice versa. The consistency of simulation and prediction results imply that the model can accurately capture the nonintuitive complex relationship between structural parameters and electromagnetic spectra. To further verify the model, retrieved structural parameters for unidirectional transmission are deduced due to inverse design network and double L-shape arrays metasurface is experimental fabricated. It is shown that the experimental asymmetric transmission responses by retrieved structural parameters are identicalwith the simulations by design parameter. Such results effectively verify the design and model, providing a paradigm for functional metasurface design and the development of applications in optoelectrical fields.

Metasurfaces Excited by an Evanescent Wave for Terahertz Beam Splitters with a Tunable Splitting Ratio

In terahertz (THz) photonics, a beam splitter is an important component. Although various THz beam-splitting devices using several principles have been proposed, the splitting ratio of existing designs is not adjustable. Here, a THz beam splitter with an adjustable splitting ratio is demonstrated using a metasurface integrated onto a prism. The metasurface excited by an evanescent wave can convert part of a linearly polarized incident wave into a cross-polarized wave and manipulate its phase simultaneously. As a result, the cross-polarized wave can pass through the interface, even if the incident angle is larger than the total internal reflection angle, while the co-polarized wave is reflected by the prism. The splitting ratio of the device can be adjusted from 4.56:1 to 0.63:1 by tuning the resonant response of the metasurface and varying the interval distance between the metasurface and the prism. The metasurface samples are fabricated using low-cost standard printed circuit technology, and the experimental results are consistent with the simulations. Therefore, the proposed beam splitter with a tunable splitting ratio is promising as a key component in the THz system.

Large‐Area and Flexible Plasmonic Metasurface for Laser–Infrared Compatible Camouflage

AbstractDue to interminable surveillance and reconnaissance through various sophisticated multispectral detectors, the need for multispectral compatible camouflage is now more than ever. Here, a flexible plasmonic metasurface is proposed to simultaneously realize low reflection at representative lasers (i.e., 1.06, 1.55, and 10.6 µm) and low emission in the atmosphere windows of both 3–5 and 8–14 µm. High absorption for both 1.06 and 1.55 µm lasers is realized by the destructive‐interference design of the multilayer Au/Ge/Ti/Ge films, while low reflection for the 10.6 µm laser results from the coding metasurface design, and low emission is attributed to ultrahigh reflection of the continuous Au/Ge/Ti/Ge films in the atmosphere windows. As a proof of concept, a flexible metasurface sample (10 cm × 10 cm) is prepared by the soft‐lithography technology. The measured specular reflectivities are 0.017, 0.13, and 0.17 at wavelengths 1.06, 1.55, and 10.6 µm, respectively. Meanwhile, the average emissivities are 0.19 and 0.11 in 3–5 and 8–14 µm, respectively. Additionally, the flexible metasurface also exhibits integrated advantages including easy mass fabrication, good mechanical flexibility and robustness, super‐hydrophobic characteristic, unambiguously demonstrating the success of the design strategy for promoting multispectral compatible camouflage.

Reprogrammable Metasurface Controlled by 2D Thermal Fields.

The combination of thermal field sensing and microwave operation is an innovative topic in metamaterials. Although there exists research on modulating electromagnetic waves by controlling each column of the metasurface elements for programmable metasurfaces, the regulation is not flexible. In view of this, this paper proposes a metasurface based on distributed thermal sensing that can be independently modulated by each element. In this paper, the metasurface adopts a 1-bit coding metasurface, which is combined with PIN diodes to modulate the phase response. The voltage control circuit feeds back the change in the thermistors to the switching state of the PIN diode. Each metasurface unit contains thermistors, which are used to sense thermal stimulation and can be independently modulated. The metasurface composed of these elements can feel the field generated via heat energy. We can control electromagnetic waves by controlling this field. In order to prove the feasibility of this scheme, a metasurface sample of 8 × 8 elements was designed. Three patterns were used for the design, fabrication, and measurement of the samples. Meanwhile, printed circuit board (PCB) technology was applied. The results show that the simulated results are highly consistent with the experimental results, which verifies that this scheme is practicable.

Temperature tunable electromagnetically induced transparency in terahertz metasurface fabricated on ferroelectric platform

The integration of active materials in terahertz (THz) metasurfaces is pivotal for the realization of functional device applications in diverse fields like sensing, imaging, communication, etc. In this context, ferroelectric materials endowed with tunable electro-optic properties have recently emerged as a novel candidate for achieving actively tuned THz metasurfaces. Here, we experimentally investigate temperature tuning of electromagnetically induced transparency (EIT) effects in a THz metasurface based on ferroelectric barium titanate (BaTiO3 (BTO)) thin film. We characterize tunable dielectric properties of the BTO thin film under variable temperatures (25 °C–100 °C) at THz frequencies by utilizing THz-time domain spectroscopy technique. Based on this aspect, we design a THz metasurface capable of displaying the EIT effect. THz transmissions through the metasurface sample are then probed for different applied temperatures. The EIT features undergo frequency shifts along with amplitude modulations owing to the temperature induced variations of the dielectric properties of the BTO thin film. A total red shift ∼27 GHz in EIT resonance dip is observed experimentally as the temperature increases from 25 °C to 100 °C. Therefore, we demonstrate utilities of ferroelectric platform toward the development of temperature tunable EIT metasurfaces.

Design and measurement of a single-band metasurface sample in the terahertz band

A single-band metasurface sample in the terahertz band is designed, simulated, and measured. This terahertz metasurface sample reveals a transmission peak. And the resonance frequency of this peak is 2.06THz (amplitude is 0.74, half-bandwidth is 0.7THz). In experiments, the diameter is reduced, which leads to the amplitude is enahcned and this peak move to low frequencies. The parameter T3 is reduced, which results in the amplitude of the transmission peak strengthened, while the resonance frequency stable. When the incident angle of the electromagnetic wave is increased, this transmission peak is reduced and the resonance frequency moved to the low frequencies. The polarization angle is increased, the metasurface sample exhibits similar resonance behaviors.

Design and experimental measurements of single/dual mode tunable metasurfaces

The development of optoelectronic devices based on metasurfaces is becoming an emerging field. However, the performance of conventional metasurfaces based on metal array are determined by the structural design strategy, resulting in the optical response behaviors being solidified during the sample preparation process. Therefore, designing and developing metasurfaces with reconfigurable and actively modulated properties is an important way to expand the performance of optoelectronic devices. The tunability of phase change materials is an important component in the development of similar devices. A tunable metasurface that containing Si and GST arrays is proposed and measured. Based on the phase-change modulability of GST arrays, this metasurface shows the properties of single/dual-mode control, multi-level control, and continuous control of reflection. Finally, the effect of the incident angle of electromagnetic waves on the reflective properties of the metasurface is measured. Under the TE polarization condition, the increase of the incident angle has a weak effect on the reflection performance of the metasurface sample. Under TM polarization conditions, the MD resonance reflection peak is splited into two new reflection peaks. And the resonant properties (amplitude and resonant wavelength) of the new reflection peaks are incident angle tunable. Therefore, this all-dielectric metasurface reveals the potential to be applied in reconfigurable, high-efficiency, and multi-level controlled optoelectronic devices.

Polarization-independent tunable terahertz slow light with electromagnetically induced transparency metasurface

Tunable slow light systems have gained much interests recently due to their efficient control of strong light–matter interactions as well as their huge potential for realizing tunable device applications. Here, a dynamically tunable polarization independent slow light system is experimentally demonstrated via electromagnetically induced transparency (EIT) in a terahertz (THz) metasurface constituted by plus and dimer-shaped resonators. Optical pump-power dependent THz transmissions through the metasurface samples are studied using the optical pump THz probe technique. Under various photoexcitations, the EIT spectra undergo significant modulations in terms of its resonance line shapes (amplitude and intensity contrast) leading to dynamic tailoring of the slow light characteristics. Group delay and delay bandwidth product values are modulated from 0.915 ps to 0.42 ps and 0.059 to 0.025 as the pump fluence increases from 0 to 62.5 nJ cm−2. This results in tunable slow THz light with group velocities ranging from 2.18 × 105 m s−1 to 4.76 × 105 m s−1, almost 54% change in group velocity. The observed tuning is attributed to the photo-induced modifications of the optoelectronic properties of the substrate layer. The demonstrated slow light scheme can provide opportunities for realizing dynamically tunable slow light devices, delay lines, and other ultrafast devices for THz domain.