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.

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.

General strategy for ultrabroadband and wideangle absorbers via multidimensional design of functional motifs

Developing wide-angle,polarization-independent,and effective electromagnetic absorbers that endow devices with versatile characteristics in solar,terahertz,and microwave regimes is highly desired,yet it is still facing a theoretical challenge.Herein,a general and straightforward strategy is proposed to surmount the impedance mismatching in the ultrabroadband and wide-angle absorber design.A vertical atom sticking on N×N horizontal meta-atoms with conductive film is proposed as the functional motif,exhibiting the strong ohmic dissipation along both vertical and horizontal directions.Assisted by the intelligent optimization strategy,the structure dimension,location,and film distribution are designed to maintain absorbing performance under different incident angles.As a demonstration,an absorber was designed and proved in both simulation and experiment.Significantly,the over 10 d B absorption from 5 to 34 GHz is achieved in the range of 0°to 70°for both TE and TM,and even 3 to 40 GHz from 60°to 70°for the TE wave.Meanwhile,the proposed multidimensional design of functional motifs can be attached with optical transparency function at will.That is to say,our effort provides an effective scheme for expanding matching area and may also be made in optical,infrared,and terahertz regimes.

A reflective-backing-free metamaterial absorber with broadband response

In this paper,we propose a polarization-independent and broadband perfect infrared(IR)metamaterial absorber(MA)without reflective backing.The proposed absorber is a periodic meta-atom array consisting of metal-dielectric-multilayer truncated cones which can absorb 80%EM wave from 50.70 to 81.87 THz,while transmit 80%EM wave from 0 to 37.71 THz.With the decreasing of frequency,the transmissivity increases,which is close to 100%from 0 to 5 THz.We can broaden the absorption bandwidth of the MA by cascading multi-layers truncated cones.Furthermore,the proposed IR MA promises to be one desirable stealth material for radar-IR compatibility.