Metamaterials and metasurfaces for designing metadevices:Perfect absorbers and microstrip patch antennas

In the past twenty years, electromagnetic metamaterials represented by left-handed metamaterials（LHMs） have attracted considerable attention due to the unique properties such as negative refraction, perfect lens, and electromagnetic cloaks. In this paper, we present a comprehensive review of our group＇s work on metamaterials and metasurfaces. We present several types of LHMs and chiral metamaterials. As a two-dimensional equivalent of bulk three-dimensional metamaterials, metasurfaces have led to a myriad of devices due to the advantages of lower profile, lower losses, and simpler to fabricate than bulk three-dimensional metamaterials. We demonstrate the novel microwave metadevices based on metamaterials and metasurfaces： perfect absorbers and microwave patch antennas, including novel transmission line antennas,high gain resonant cavity antennas, wide scanning phased array antennas, and circularly polarized antennas.

Nearly Perfect Absorbers Operating Associated with Fano Resonance in the Infrared Range

We fabricate a three-layer metamaterial of metal patterns/dielectric/metal films. The optical properties associated with Fano resonance of the metamaterials are investigated experimentally and theoretically. The results indicate that the introduction of Fano resonance due to symmetry breaking leads to a much wider absorption range. Furthermore, the amplitude and phase of reflection can be modulated effectively by adjusting various free parameters using the proposed structure.

Tuning the metal filling fraction in metal-insulator-metal ultra-broadband perfect absorbers to maximize the absorption bandwidth

In this paper, we propose a methodology to maximize the absorption bandwidth of a metal-insulator-metal(MIM) based absorber. The proposed structure is made of a Cr-Al_2O_3-Cr multilayer design. At the initial step,the optimum MIM planar design is fabricated and optically characterized. The results show absorption above 0.9 from 400 nm to 850 nm. Afterward, the transfer matrix method is used to find the optimal condition for the perfect light absorption in an ultra-broadband frequency range. This modeling approach predicts that changing the filling fraction of the top Cr layer can extend light absorption toward longer wavelengths. We experimentally proved that the use of proper top Cr thickness and annealing temperature leads to a nearly perfect light absorption from 400 nm to 1150 nm, which is much broader than that of a planar design. Therefore, while keeping the overall process lithography-free, the absorption functionality of the design can be significantly improved. The results presented here can serve as a beacon for future performance-enhanced multilayer designs where a simple fabrication step can boost the overall device response without changing its overall thickness and fabrication simplicity.

Nonlinear terahertz metamaterial perfect absorbers using GaAs[Invited]

We investigate the nonlinear response of terahertz(THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz time-domain spectroscopy. The resonance frequency redshifts 20 GHz and the absorbance is reduced by 30% as the incident peak field is increased from 30 to 300 kV/cm. The nonlinear response arises from THz field driven interband transitions and intervalley scattering in the GaAs. To eliminate the Fresnel losses from the GaAs substrate, we design and fabricate a flexible metamaterial saturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.

A three-band perfect absorber based on a parallelogram metamaterial slab with monolayer MoS_(2)

As a two-dimensional(2D)material,monolayer MoS2which limits its optical applications has a low absorption efficiency.In this paper,we propose a three-band perfect metamaterial absorber in the visible light range based on monolayer MoS_(2).The peak absorptivity of the structure at each resonance wavelength is nearly perfect,moreover,the light absorption of monolayer MoS2is obviously enhanced at the three resonant wavelengths.The dielectric–dielectric–metal structure we designed produces the coupling of Fabry–Perot resonance and high-order diffraction guided-mode resonance at different absorption peaks,which has been proved by the slab waveguide theory.In addition,the multi-modal absorption phenomenon is explained by extracting the equivalent impedance.The results show that we can adjust the absorption peak wavelength by regulating the parameters of the structure.This structure not only provides an idea for enhancing the interaction between light and two-dimensional materials but also has potential applications for optical detection devices.

Narrowband perfect terahertz absorber based on polar-dielectrics metasurface

We theoretically propose a narrowband perfect absorber metasurface(PAMS) based on surface phonon polaritons in the terahertz range. The PAMS has unit cell consisting of a silver biarc on the top, a thin polar-dielectric in the middle and a silver layer at the bottom. The phonon polaritons are excited at the interface between the silver biarc and the polar dielectric, and enhance the absorption of the PAMS. The absorption peak is at 36.813 μm and the full width half maximum(FWHM) is nearly 36 nm, independent of the polarization and incidence angle. The electric fields are located at the split of the biarc silver layer and the quality factor Q is 1150. The FWHM decreases with the decreasing split width. When the thickness of the bottom layer is larger than 50 nm, the narrow band and high absorption are insensitive to the thickness of those layers. The designed absorber may have useful applications in terahertz spectra such as energy harvesting, thermal emitter, and sensing.

Achieving a multi-band metamaterial perfect absorber via a hexagonal ring dielectric resonator

A multi-band absorber composed of high-permittivity hexagonal ring dielectric resonators and a metallic ground plate is designed in the microwave band. Near-unity absorptions around 9.785 GHz, 11.525 GHz, and 12.37 GHz are observed for this metamaterial absorber. The dielectric hexagonal ring resonator is made of microwave ceramics with high permittivity and low loss. The mechanism for the near-unity absorption is investigated via the dielectric resonator theory. It is found that the absorption results from electric and magnetic resonances where enhanced electromagnetic fields are excited inside the dielectric resonator. In addition, the resonance modes of the hexagonal resonator are similar to those of standard rectangle resonators and can be used for analyzing hexagonal absorbers. Our work provides a new research method as well as a solid foundation for designing and analyzing dielectric metamaterial absorbers with complex shapes.

Nonlinear coherent perfect photon absorber in asymmetrical atom–nanowires coupling system

Coherent perfect absorption provides a method of light-controlling-light and has practical applications in optical communications. Recently, a cavity-based nonlinear perfect photon absorption extends the coherent perfect absorber（CPA）beyond the linear regime. As nanowire-based system is a more competitive candidate for full-optical device, we introduce a nonlinear CPA in the single two-level atom–nanowires coupling system in this work. Nonlinear input–output relations are derived analytically, and three contributions of atomic saturation nonlinearity are explicit. The consociation of optical nonlinearity and destructive interference makes it feasible to fabricate a nonlinear monoatomic CPA. Our results also indicate that a nonlinear system may work linearly even when the incoming lights are not weak any more. Our findings show promising applications in full-optical devices.

Solar broadband metamaterial perfect absorber based on dielectric resonant structure of Ge cone array and InAs film

The broadband metamaterial perfect absorber has been extensively studied due to its excellent characteristics and promising application prospect.In this work a solar broadband metamaterial perfect absorber is proposed based on the structure of the germanium(Ge)cone array and the indium arsenide(InAs)dielectric film on the gold(Au)substrate.The results show that the absorption covers the whole ultraviolet-visible and near-infrared range.For the case of A>99%,the absorption bandwidth reaches up to 1230 nm with a wavelength range varied from 200 nm to 1430 nm.The proposed absorber is able to absorb more than 98.7%of the solar energy in a solar spectrum from 200 nm to 3000 nm.The electromagnetic dipole resonance and the high-order modes of the Ge cone couple strongly to the incident optical field,which introduces a strong coupling with the solar radiation and produces an ultra-broadband absorption.The absorption spectrum can be feasibly manipulated via tuning the structural parameters,and the polarization insensitivity performance is particularly excellent.The proposed absorber can possess wide applications in active photoelectric effects,thermion modulators,and photoelectric detectors.

A Perfect Graphene Absorber with Waveguide Coupled High-Contrast Gratings

To achieve the enhancement and manipulation of light absorption in graphene within the visible and near infrared regions, a design consists of high-contrast gratings and two evanescently coupled slabs with graphene monolayer is demonstrated. The operation principle and design process of the proposed structure are analyzed using the coupled mode theory, which is confirmed by the rigorous coupled wave analysis. It is proved that the absorptance of graphene monolayer can be greatly enhanced to unity. The thickness of grating and slab layers can significantly change the line width and resonant mode position in the absorption spectra. Furthermore, high tunability in amplitude and bandwidth of the absorption spectra can be achieved by controlling the structural parameters of the hybrid structure. The proposed devices could be efficiently exploited as tunable and selective absorbers, and could be allowed to realize other two-dimensional materials-based selective photo-detectors.

Theory of complex-coordinate transformation acoustics for non-Hermitian metamaterials

Transformation acoustics(TA)has emerged as a powerful tool for designing several intriguing conceptual devices,which can manipulate acoustic waves in a flexible manner,yet their applications are limited in Hermitian materials.In this work,we propose the theory of complex-coordinate transformation acoustics(CCTA)and verify the effectiveness in realizing acoustic non-Hermitian metamaterials.Especially,we apply this theory for the first time to the design of acoustic parity-time(PT)and antisymmetric parity-time(APT)metamaterials and demonstrate two distinctive examples.First,we use this method to obtain the exceptional points(EPs)of the PT/APT system and observe the spontaneous phase transition of the scattering matrix in the transformation parameter space.Second,by selecting the Jacobian matrix's constitutive parameters,the PT/APT-symmetric system can also be configured to approach the zero and pole of the scattering matrix,behaving as an acoustic coherent perfect absorber and equivalent laser.We envision our proposed CCTAbased paradigm to open the way for exploring the non-Hermitian physics and finding application in the design of acoustic functional devices such as absorbers and amplifiers whose material parameters are hard to realize by using the conventional transformation method.

Recent advances on perfect light absorbers and their promise for high-performance opto-electronic devices[Invited]

Perfect absorbers(PAs)are devices that can efficiently absorb electromagnetic waves.Great attention has been attracted since metamaterial PAs(MPAs)were first proposed in 2008.In recent years,with the development of nanophotonics and the improvement of nanomanufacturing technology,considerable progresses have been achieved in designing MPAs using new materials and new structures.In this review,we summarized first the latest developments of PAs from five directions:dualband,multi-band,wideband,narrow-band,and tunable light absorption.The shortcomings of the previous PAs and the latest improvements were introduced as well.Then,the application of perfect absorption in solar cells,sensors,switches,and structural colors was discussed.Finally,we presented the main challenges and prospects in these fields.Novel PAs for applications in a wide field of opto-electronic devices will continuously progress with breakthrough advances in absorbers related technology and science.

Tunable and multichannel terahertz perfect absorber due to Tamm surface plasmons with graphene

In this paper, we have shown that perfect absorption at terahertz frequencies can be achieved by using a composite structure where graphene is coated on one-dimensional photonic crystal(1 DPC) separated by a dielectric. Due to the excitation of optical Tamm states(OTSs) at the interface between the graphene and 1 DPC, a strong absorption phenomenon occurs induced by the coupling of the incident light and OTSs. Although the perfect absorption produced by a metal–distributed Bragg reflector structure has been researched extensively, it is generally at a fixed frequency and not tunable. Here, we show that the perfect absorption at terahertz frequency not only can be tuned to different frequencies but also exhibits a high absorption over a wide angle range. In addition,the absorption of the proposed structure is insensitive to the polarization, and multichannel absorption can berealized by controlling the thickness of the top layer.

Visible-near infrared ultra-broadband polarization-independent metamaterial perfect absorber involving phase-change materials

We numerically demonstrate a novel ultra-broadband polarization-independent metamaterial perfect absorber in the visible and near-infrared region involving the phase-change material Ge_2Sb_2Te_5(GST).The novel perfect absorber scheme consists of an array of high-index strong-absorbance GST square resonators separated from a continuous Au substrate by a low-index lossless dielectric layer(silica)and a high-index GST planar cavity.Three absorption peaks with the maximal absorbance up to 99.94% are achieved,owing to the excitation of plasmon-like dipolar or quadrupole resonances from the high-index GST resonators and cavity resonances generated by the GST planar cavity.The intensities and positions of the absorption peaks show strong dependence on structural parameters.A heat transfer model is used to investigate the temporal variation of temperature within the GST region.The results show that the temperature of amorphous GST can reach up to 433 K of the phase transition temperature from room temperature in just 0.37 ns with a relatively low incident light intensity of 1.11×10~8W∕m^2,due to the enhanced ultra-broadband light absorbance through strong plasmon resonances and cavity resonance in the absorber.The study suggests a feasible means to lower the power requirements for photonic devices based on a thermal phase change via engineering ultra-broadband light absorbers.

Tunable near-infrared plasmonic perfect absorber based on phase-change materials

A tunable plasmonic perfect absorber with a tuning range of 650 nm is realized by introducing a 20 nm thick phase-change material Ge2Sb2Te5 layer into the metal–dielectric–metal configuration.The absorption at the plasmonic resonance is kept above 0.96 across the whole tuning range.In this work we study this extraordinary optical response numerically and reveal the geometric conditions which support this phenomenon.This work shows a promising route to achieve tunable plasmonic devices for multi-band optical modulation,communication,and thermal imaging.

Polarization-selective nanogold absorber by twisted stacking

Metamaterial absorbers show great promise for applications in optical manipulation,photodetection,solar energy harvesting,and photocatalysis.In this work,we present a twisted stacked metamaterial design that serves as a plasmonic perfect absorber with polarization selectivity.Leveraging effective energy localization,the metamaterial realizes a near-unity absorbance of up to 99.6%for right circularly polarized incidence and 97.2%for left circularly polarized incidence.At a longer wavelength in the visible range,the chiral metamaterial becomes more sensitive to the polarization state of the incident wave,retaining an ultrahigh absorption of light(~94%)for only a given polarization state,that is,light in this polarization state is effectively shielded.A giant circular dichroism signal of up to 7°can be simultaneously observed.Electromagnetic field and charge distribution simulations further reveal that the ultrahigh performance of the design is attributed to the interplay between cavity coupling,magnetic resonances,and plasmonic coupling.Besides switchable and tunable chirality,the plasmonic metamaterial presents a nearperfect absorption band with tunable operational wavelengths.We envision that the high-performance chiral gold metamaterial proposed here can serve as a good candidate for light trapping,chirality sensing,polarized light detection,and polarizationenhanced photocatalysis.

Negative refraction based on purely imaginary metamaterials

By introducing a new mechanism based on purely imaginary metamaterials （PIMs）, we reveal that bidirectional negative refraction and planar focusing can be obtained using a pair of PIM slabs, over- coming the unidirectional limit in parity-time （PT）-symmetric systems. Compared with PT-symmetric systems, which require two different types of materials, the proposed negative refraction can be realized using two identical media. In addition, asymmetric excitation with bidirectional total transmission is observed in our PIM system. Therefore, a new way to realize negative refraction with properties that are unavailable in PT-symmetrie systems is presented.

PTX-symmetric metasurfaces for sensing applications

In this paper,we introduce an ultra-sensitive optical sensing platform based on the parity-time-reciprocal scaling(PT^-symmetric non-Hermitian metasurfaces,which leverage exotic singularities,such as the exceptional point(EP)and the coherent perfect absorber-laser(CPAL)point,to significantly enhance the sensitivity and detectability of photonic sensors.We theoretically studied scattering properties and physical limitations of the PTX-symmetric metasurface sensing systems with an asymmetric,unbalanced gain-loss profile.The PTLY-symmetric metasurfaces can exhibit similar scattering properties as their Pr-symmetric counterparts at singular points,while achieving a higher sensitivity and a larger modulation depth,possible with the reciprocal-scaling factor(i.e.,X transformation).Specifically,with the optimal reciprocalscaling factor or near-zero phase offset,the proposed PTX-symmetric metasurface sensors operating around the EP or CPAL point may achieve an over 100 dB modulation depth,thus paving a promising route toward the detection of small-scale perturbations caused by,for example,molecular,gaseous,and biochemical surface adsorbates.