Direct field-to-pattern monolithic design of holographic metasurface via residual encoderdecoder convolutional neural network

Complex-amplitude holographic metasurfaces(CAHMs)with the flexibility in modulating phase and amplitude profiles have been used to manipulate the propagation of wavefront with an unprecedented level,leading to higher image-reconstruction quality compared with their natural counterparts.However,prevailing design methods of CAHMs are based on Huygens-Fresnel theory,meta-atom optimization,numerical simulation and experimental verification,which results in a consumption of computing resources.Here,we applied residual encoder-decoder convolutional neural network to directly map the electric field distributions and input images for monolithic metasurface design.A pretrained network is firstly trained by the electric field distributions calculated by diffraction theory,which is subsequently migrated as transfer learning framework to map the simulated electric field distributions and input images.The training results show that the normalized mean pixel error is about 3%on dataset.As verification,the metasurface prototypes are fabricated,simulated and measured.The reconstructed electric field of reverse-engineered metasurface exhibits high similarity to the target electric field,which demonstrates the effectiveness of our design.Encouragingly,this work provides a monolithic field-to-pattern design method for CAHMs,which paves a new route for the direct reconstruction of metasurfaces.

Single-beam leaky-wave antenna with wide scanning angle and high scanning rate based on spoof surface plasmon polariton

We propose a single-beam leaky-wave antenna(LWA) with a wide-scanning angle and a high-scanning rate based on spoof surface plasmon polariton(SSPP) in this paper. The SSPP transmission line(TL) is etched with periodically arranged circular patches, which converts the slow-wave mode into the fast-wave region for radiation. The proposed LWA is designed, fabricated, and tested. The simulated results imply that the proposed LWA not only achieves a high radiation efficiency of about 81.4%, and a high scanning rate of 12.12, but also has a large scanning angle of 176° over a narrow operation bandwidth of 8.3-9.6 GHz(for |S_(11)| <-10 dB). In addition, the simulated average gain of the LWA can reach as high as 10.9 d Bi. The measured scanning angle range is 175° in the operation band of 8.2-9.6 GHz, and the measured average gain is 10.6 dBi. The experimental results are consistent with the simulation, validating its performance. An antenna with high radiation efficiency, wide scanning angle range, and high scanning rate has great potential for application in radar and wireless communication systems.

Shape memory alloy-based 3D morphologically reconfigurable chiral metamaterial for tailoring circular dichroism by voltage control

Three-dimensional chiral materials with intrinsic chirality play a crucial role in achieving a strong chiral response and flexible light manipulation.Reconfigurable chirality through the 3D morphological transformation of chiral materials is significant for greater freedom in tailoring light but remains a challenge.Inspired by the unique 3D morphological memory capability of shape memory alloys(SMAs),we demonstrate and discuss a chiral resonator in the microwave regime that can realize reconfigurable chirality through 3D morphological transformation.The introduction of heating film realizes voltage control of SMA’s morphology for utilizing the temperature sensitivity of SMA better,enabling arbitrary control of circular dichroism(CD)flip and CD intensity.The qualitative and quantitative analysis of the surface current distribution of chiral enantiomers reveals that the chirality of metaatoms originates from the surge of electric dipole pxand electric quadrupole Q.It is worth mentioning that the proposed strategy to achieve reconfigurable chirality using 3D morphological transformations can be directly extended to other higher frequencies,such as visible,infrared,and terahertz bands.Significantly,our paradigm to study the relationship between complex 3D morphology and chirality holds potential for application in biosensing,spin detection,and spin-selective devices.

Ultrawideband metamaterial absorber for oblique incidence using characteristic mode analysis

An ultrawideband,polarization-insensitive,metamaterial absorber for oblique angle of incidence is presented using characteristic mode analysis.The absorber consists of conductive meander square loops and symmetric bent metallic strips,which are embedded with lumped resistors.With the aid of modal currents and modal weighting coefficients,the positions of the lumped resistors are determined.After that,the equivalent circuit(EC)model and admittance formula are proposed and analyzed to further understand the working principle and ultrawide bandwidth.The proposed absorber measures an absorption bandwidth of 4.3–26.5 GHz(144.1%in fractional bandwidth)for 90%absorptivity under normal incidence.At the oblique angle of incidence of 45°,the bandwidth of 90%absorptivity is still 5.1–21.3 GHz(122.72%)for transverse electric(TE)polarization,and 6.8–29.5 GHz(125.07%)for transverse magnetic(TM)polarization.The good agreement among simulation,measurement,and EC calculation demonstrates the validity of the proposed method and indicates that the method can be applied to other microwave and optical frequency bands.The proposed metamaterial absorber can be widely applied in electromagnetic compatibility,electromagnetic interference,radar stealth,and biomedical detection.

Miura origami based reconfigurable polarization converter for multifunctional control of electromagnetic waves

Polarization is one of the basic characteristics of electromagnetic(EM)waves,and its flexible control is very important in many practical applications.At present,most of the multifunction polarization metasurfaces are electrically tunable based on PIN and varactor diodes,which are easy to operate and have strong real-time performance.However,there are still some problems in them,such as few degrees of freedom of planar structure control,complex circuit,bulky sample,and high cost.In view of these shortcomings,this paper proposes a Miura origami based reconfigurable polarization conversion metasurface for multifunctional control of EM waves.The interaction between the electric dipoles is changed by adjusting the folding angleθ,thereby tuning the operating frequency of the polarization conversion and the polarization state of the reflected wave.This mechanical control method brings more degrees of freedom to manipulate EM waves.And the processed sample is with lightweight and low cost.To verify the performance of the proposed origami polarization converter,a Miura origami structure loaded with metal split rings is designed and fabricated.The operating frequency of the structure can be tuned in different folding states.In addition,by controlling the folding angleθ,linear-to-linear and linear-to-circular polarization converters can be realized at different folding states.The proposed Miura origami polarization conversion metasurface provides a new idea for reconfigurable linear polarization conversion and multifunctional devices.

Completely spin-decoupled geometric phase of a metasurface

Metasurfaces have provided an unprecedented degree of freedom(DOF)in the manipulation of electromagnetic waves.A geometric phase can be readily obtained by rotating the meta-atoms of a metasurface.Nevertheless,such geometric phases are usually spin-coupled,with the same magnitude but opposite signs for left-and right-handed circularly polarized(LCP and RCP)waves.To achieve independent control of LCP and RCP waves,it is crucial to obtain spin-decoupled geometric phases.In this paper,we propose to obtain completely spin-decoupled geometric phases by engineering the surface current paths on meta-atoms.Based on the rotational Doppler effect,the rotation manner is first analyzed,and it is found that the generation of a geometric phase lies in the rotation of the surface current paths on meta-atoms.Since the induced surface current paths under the LCP and RCP waves always start oppositely and are mirror-symmetrical with each other,it is natural that the geometric phases have the same magnitude and opposite signs when the meta-atoms are rotated.To obtain spin-decoupled geometric phases,the induced surface current under one spin should be rotated by one angle while the current under the other spin is rotated by a different angles.In this way,LCP and RCP waves can acquire different geometric phase changes.Proof-of-principle prototypes were designed,fabricated,and measured.Both the simulation and experiment results verify spin-decoupled geometric phases.This work provides a robust means to obtain a spindependent geometric phase and can be readily extended to higher frequency bands such as the terahertz,IR,and optical regimes.

Leaky cavity modes in metasurfaces:a route to low-loss wideband anomalous dispersion

Metasurfaces have provided unprecedented degrees of freedom in manipulating electromagnetic waves upon interfaces.In this work,we first explore the condition of wide operating bandwidth in the view of reflective scheme,which indicates the necessity of anomalous dispersion.To this end,the leaky cavity modes(LCMs)in the metaatom are analyzed and can make effective permittivity inversely proportional to frequency.Here we employ the longitudinal Fabry-Perot(F-P)resonances and transverse plasmonic resonances to improve the LCMs efficiency.It is shown that the order of F-P resonance can be customized by the plasmonic modes,that is,the F-P cavity propagation phase should match the phase delay of surface currents excited on the meta-atom.The nth order F-P resonance will multiply the permittivity by a factor of n,allowing larger phase accumulation with increasing frequencies and forming nonlinear phase distribution which can be applied in weak chromatic-aberration focusing design.As a proof-of-concept,we demonstrate a planar weak chromatic-aberration focusing reflector with a thickness ofλ_(0)∕9 at 16.0-21.0 GHz.This work paves a robust way to advanced functional materials with anomalous dispersion and can be extended to higher frequencies such as terahertz,infrared,and optical frequencies.

Switchable chiral mirror based on PIN diodes

Chiral mirrors can produce spin selective absorption for left-handed circularly polarized(LCP) or right-handed circularly polarized(RCP) waves. However, the previously proposed chiral mirror only absorbs the designated circularly polarized(CP) wave in the microwave frequency band, lacking versatility in practical applications.Here, we propose a switchable chiral mirror based on a pair of PIN diodes. The switchable chiral mirror has four working states, switching from the handedness-preserving mirror to the LCP mirror, RCP mirror, and perfect absorber. The basis of these advances is to change the chirality of two-dimensional(2D) chiral metamaterials and the circular conversion dichroism related to it, which is the first report in the microwave frequency band.Surface current distributions shed light on how switchable chiral mirrors work by handedness-selective excitation of reflective and absorbing electric dipole modes. Energy loss distributions verify the working mechanism. The thickness of the switchable chiral mirror is one-tenth of the working wavelength, which is suitable for integrated manufacturing. The measurement results are in good agreement with the simulation results.

Virtual metasurfaces:reshaping electromagnetic waves in distance

Metasurface has provided unprecedented freedoms in manipulating electromagnetic(EM) waves, exhibiting fascinating functions. Conventionally, these functions are implemented right on metasurfaces, where spatial modulations on EM wave amplitudes or phases are achieved by meta-atoms. This study proposes the concept of virtual metasurface(VM), which is formed by arrays of foci away from the entity metasurface. Unlike conventional metasurfaces, spatial modulations on the amplitudes or phases of EM waves occur in the air, with a focal length distance from the entity metasurface. As a proof of concept, we demonstrated a transmissive VM. The entity metasurface consists of transmissive focusing metasurface tiles(TFMTs) with the same focal length. Two TFMTs were designed with phase difference π to enable the most typical checkerboard configuration. The TFMTs were assembled to form the entity metasurface, whereas their foci formed the VM. Due to the π phase difference among adjacent foci, EM propagation along the normal direction was cancelled, leading to four tilted far-field beams. The concept of VM can be readily extended to higher frequencies from terahertz to optical regimes and may find wide applications in communication, camouflage, and other fields.

Six-channel programmable coding metasurface simultaneously for orthogonal circular and linear polarizations

Metasurfaces have intrigued long-standing research interests and developed multitudinous compelling applications owing to their unprecedented capability for manipulating electromagnetic waves,and the emerging programmable coding metasurfaces(PCMs)provide a real-time reconfigurable platform to dynamically implement customized functions.Nevertheless,most existing PCMs can only act on the single polarization state or perform in the limited polarization channel,which immensely restricts their practical application in multitask intelligent metadevices.Herein,an appealing strategy of the PCM is proposed to realize tunable functions in co-polarized reflection channels of orthogonal circularly polarized waves and in co-polarized and cross-polarized reflection channels of orthogonal linearly polarized waves from 9.0 to 10.5 GHz.In the above six channels,the spindecoupled programmable meta-atom can achieve high-efficiency reflection and 1-bit digital phase modulation by selecting the specific ON/OFF states of two diodes,and the phase coding sequence of the PCM is dynamically regulated by the field-programmable gate array to generate the desired function.A proof-of-concept prototype is constructed to verify the feasibility of our methodology,and numerous simulation and experimental results are in excellent agreement with the theoretical predictions.This inspiring design opens a new avenue for constructing intelligent metasurfaces with higher serviceability and flexibility,and has tremendous application potential in communication,sensing,and other multifunctional smart metadevices.

Machine-learning-empowered multispectral metafilm with reduced radar cross section,low infrared emissivity,and visible transparency

For camouflage applications,the performance requirements for metamaterials in different electromagnetic spectra are usually contradictory,which makes it difficult to develop satisfactory design schemes with multispectral compatibility.Fortunately,empowered by machine learning,metamaterial design is no longer limited to directly solving Maxwell’s equations.The design schemes and experiences of metamaterials can be analyzed,summarized,and learned by computers,which will significantly improve the design efficiency for the sake of practical engineer-ing applications.Here,we resort to the machine learning to solve the multispectral compatibility problem of metamaterials and demonstrate the design of a new metafilm with multiple mechanisms that can realize small microwave scattering,low infrared emissivity,and visible transparency simultaneously using a multilayer back-propagation neural network.The rapid evolution of structural design is realized by establishing a mapping between spectral curves and structural parameters.By training the network with different materials,the designed network is more adaptable.Through simulations and experimental verifications,the designed architecture has good accuracy and robustness.This paper provides a facile method for fast designs of multispectral metafilms that can find wide applications in satellite solar panels,aircraft windows,and others.

Digital coding transmissive metasurface for multi-OAM-beam

Orbital angular momentum(OAM)is a phenomenon of vortex phase distribution in free space,which has attracted enormous attention in theoretical research and practical application of wireless communication systems due to its characteristic of infinitely orthogonal modes.However,traditional methods generating OAM beams are bound to complex structure,large device,multiple layers,complex feed networks,and limited beams in microwave range.Here,a digital coding transmissive metasurface(DCTMS)with a single layer substrate and the bi-symmetrical arrow is proposed and designed to generate multi-OAM-beam based on Pancharatnam–Berry(PB)phase principle.The 3-bit phase response can be realized by encoding the geometric phase into rotation angle of unit cell for DCTMS.Additionally,the phase compensation of the metasurface is introduced to achieve the beam focusing and the conversion from spherical wave to plane wave.According to the digital convolution theorem,the far-field patterns and near-field distributions of multi-OAM-beam with l=–2 modes are adequately demonstrated by DCTMS prototypes.The OAM efficiency and the purity are calculated to demonstrate the excellent multiOAM-beam.The simulated and experimental results illustrate their performance of OAM beams.The designed DCTMS has profound application in multi-platform wireless communication systems and the multi-channel imaging systems.

Quasi-omnibearing retro-reflective metagrating protected by reciprocity

Reciprocity is ubiquitous in antennas for receiving and radiating electromagnetic(EM) waves, i.e., if an antenna has good receiving performance at a given direction, it also has good radiation performance in that direction.Inspired by this, we propose a method of designing a quasi-ominibearing retro-reflective metagrating(RRMG)protected by the reciprocity of antennas. Based on the second-order mode around 15.0 GHz of a short-circuited structured patch antenna(SPA), incident transverse magnetic waves can be received, channeled into the coaxial lines, reflected by the shortened end, and finally re-radiated into free space with a reversed wave vector. RRMGs are contrived consisting of this identical SPA, with a grating constant allowing ±2nd-, ±1st-, and zeroth-order diffractions. Oblique incidence, plus the tilted nulls of the re-radiation pattern, can eliminate -1st, zeroth,+1st, and +2nd orders, and only the -2nd order is left to achieve retro-reflections. Prototypes were fabricated and measured. Simulated and measured results show that the RRMGs maintain only -2nd-order diffraction for incident angles 32.2° ≤ θ_(i)< 90.0° in four quadrants, and that RRMGs can achieve quasi-omnibearing retro-reflections for θ_(i)= 50.0°. The use of higher-order diffraction brings more degrees of freedom in manipulating EM waves, and this strategy can be readily extended to millimeter waves, THz wave, or even optical regimes.