Planar Metasurface Research Articles

Controlling asymmetric transmission phase in planar chiral metasurfaces

AbstractMetasurfaces with ultrathin artificial structures have attracted much attention because of their unprecedented capability in light manipulations. The recent development of metasurfaces with controllable responses opens up new opportunities in various applications. Moreover, metasurfaces composed of twisted chiral structures can generate asymmetric responses for opposite incidence, leading to more degrees of freedom in wave detections and controls. However, most past studies had focused on the amplitude responses, not to mention using bi-directional phase responses, in the characterization and light manipulation of chiral metasurfaces. Here, we report a birefringent interference approach to achieve a controllable asymmetric bi-directional transmission phase from planar chiral metasurface by tuning the orientation of the metasurface with respect to the optical axis of an add-on birefringent substrate. To demonstrate our approach, we fabricate planar Au sawtooth nanoarray metasurface and measure the asymmetric transmission phase of the metasurface placed on a birefringent sapphire crystal slab. The Au sawtooth metasurface-sapphire system exhibits large oscillatory behavior for the asymmetric transmission phase with the tuning parameter. We confirm our experimental results by Jones matrix calculations using data obtained from full-wave simulations for the metasurface. Our approach in the characterization and light manipulation of metasurfaces with controllable responses is simple and nondestructive, enabling new functionalities and potential applications in optical communication, imaging, and remote sensing.

On the broadband vibration isolation performance of nonlocal total-internal-reflection metasurfaces

The concept of a nonlocal elastic metasurface has been recently proposed and experimentally demonstrated in Zhu et al. (2020). When implemented in the form of a total-internal-reflection (TIR) interface, the metasurface can act as an elastic wave barrier that is impenetrable to deep subwavelength waves over an exceptionally wide frequency band. The underlying physical mechanism capable of delivering this broadband subwavelength performance relies on an intentionally nonlocal design that leverages long-range connections between the units forming the fundamental supercell. This paper explores the design and application of a nonlocal TIR metasurface to achieve broadband passive vibration isolation in a structural assembly made of multiple dissimilar elastic waveguides. The specific structural system comprises shell, plate, and beam waveguides, and can be seen as a prototypical structure emulating mechanical assemblies of practical interest for many engineering applications. The study also reports the results of an experimental investigation that confirms the significant vibration isolation capabilities afforded by the embedded nonlocal TIR metasurface. These results are particularly remarkable because they show that the performance of the nonlocal metasurface is preserved when applied to a complex structural assembly and under non-ideal incidence conditions of the incoming wave, hence significantly extending the validity of the results presented in Zhu et al. (2020). Results also confirm that, under proper conditions, the original concept of a planar metasurface can be morphed into a curved interface while still preserving full wave control capabilities.

Stability of active photonic metasurface pairs

Adding active components to a photonic device may dramatically enrich and improve its performance but, at the same time, creates the risk of instability, namely, occurrence of unwanted self-oscillations. Stability considerations are not always given the attention they deserve when setups employing gain media are investigated; thus, the desired effects or reported regimes may not be achievable. In this work, a generic electromagnetic configuration comprising a pair of planar impedance metasurfaces is examined and analytical stability conditions for its operation are derived. The obtained results for the analyzed basic module can shed light on the stability conditions of more complex active systems that incorporate such components and serve a broad range of applications from imaging and polarization engineering to invisibility cloaking and wavefront transformations.

Quantum metasurfaces of arrays of Λ-emitters for photonic nano-devices

We address exotic optical response of a planar metasurface comprising a monolayer of regularly spaced quantum three-level emitters with a doublet in the ground state (the so-called Λ-emitters). All emitters are coupled by the retarded dipole field which depends on the current state of all emitters. This coupling introduces a feedback into the system. Complex interplay of the latter with the intrinsic nonlinearity of a three-level system results in several remarkable effects in the monolayer’s optical response, such as multistability, self-oscillations, and chaos. The peculiarity of the considered system is that some of the predicted nonlinear effects manifest themselves at very low excitation field intensities (on the order of 1 W/cm2), which is advantageous for possible applications: the monolayer can operate as a perfect reflector, a bistable mirror, and a THz or noise generator. It is argued therefore that the proposed system is a promising candidate for a building block for various photonic nano-devices.

Multiband ultrathin chiral metasurface for polarization conversion, symmetric and asymmetric transmission

This paper presents a planar chiral metasurface with two layers of omega-shaped unit cells placed on opposite sides of a substrate. The metasurface achieves linear-to-circular polarization conversion within a single band and cross-polarized transmission of linearly polarized incident waves in five bands. Due to the asymmetry and anisotropy of the metasurface in its plane, the linear-to-circular and cross-polarization conversion transmission bands are asymmetric in nature. The structure also exhibits a symmetric co-polarized transmission band for both x- and y-directed incident E-field orientations. The proposed structure gives good response for incident angles up to 600 from 3 GHz to 11.5 GHz and up to 300 from 11.5 GHz to 13.5 GHz for both transverse-electric and transverse-magnetic incidence. It is also ultrathin and has a thickness of λ0/64.7, where λ0 is the free space wavelength at the lowest frequency of operation. The proposed MS can be used for S, C, X as well as Ku microwave band applications.

Excellent sensing based on dual-plasmon induced transparency in graphene metasurface

Focusing on the realization of excellent sensing for graphene-based metasurface in the terahertz band. A planar monolayer graphene metasurface is proposed for sensing. Dual plasmon induced transparency (dual-PIT) phenomenon is realized in our proposed metasurface by use of the finite-difference time-domain (FDTD) method. This dual-PIT transmission spectrum can be effectively modulated by Fermi energies and positional parameters, and triple-PIT phenomenon can be realized. Interestingly, the excellent sensing performance can be achieved in terahertz band. The maximum sensitivity and Figure of Merit (FOM) are of 4.1884 THz /RIU and 43.31, respectively. These results may play an important role for designing excellent plasmonic graphene-based modulators and sensors in terahertz.

Topologically Enhanced Circular Dichroism from Metasurfaces

Obtaining strong circular dichroism (CD) from planar chiral metasurfaces has been hotly pursued, to realize ultracompact circular polarizers for applications in photography, spectroscopy, and biological sensing. Many attempts have failed, due to insufficient thickness of the structures. Here the authors present efficient enhancement of the CD response from a metasurface by utilizing its topological edge state. One can foresee a direct impact of this result on engineering solutions for the applications mentioned above.

An ultra-broadband microwave absorber based on hybrid structure of stereo metamaterial and planar metasurface for the S, C, X and Ku bands

Broadband electromagnetic (EM) wave absorbers are persistently desired in the past decades due to their extensive applications in different fields. In this paper, we proposed and demonstrated a metamaterial absorber (MA) based on a hybrid structure of stereo and planar resistive metamaterial elements. In principle, this hybrid MA is a combination of the Dällenbach layer absorber and circuit analog (CA) absorber. By adjusting its dispersion feature, the stereo metamaterial based on standing resistive patch can work as a broadband artificial Dällenbach layer absorber for C, X and Ku bands. Such stereo metamaterial was also utilized as a dispersive dielectric spacer and a metasurface consisting of resistive circle rings was deployed on the top of it to implement an improved CA absorber for S and C bands. By combing them together, the proposed hybrid MA can achieve absorption over 90% in an ultra-broad bandwidth from S to Ku band (2.59–16.80 GHz). Since our design takes full advantage of the physical space, the hybrid MA achieves an actual thickness as only 1.075 times of the ultimate thickness corresponding to its absorption performance. Furthermore, the hybrid MA has comprehensive advantages in terms of stability against oblique incidence, easy processing and light-weight. The MA prototype was fabricated assisted by the silk screen printing process and its ultra-broadband absorption performance was experimentally validated by the measured results. The new concept of hybrid MA and specific design scheme proposed here provide a promising approach for broadband, low-profile and light weight EM absorber.

Wavefront Control with Nanohole Array-Based Out-of-Plane Metasurfaces.

Planar metasurfaces provide exceptional wavefront manipulation at the subwavelength scale by controlling the phase of the light. Here, we introduce an out-of-plane nanohole-based metasurface design with the implementation of a unique self-rolling technique. The photoresist-based technique enables the fabrication of the metasurface formed by nanohole arrays on gold (Au) and silicon dioxide (SiO2) rolled-up microtubes. The curved nature of the tube allows the fabrication of an out-of-plane metasurface that can effectively control the wavefront compared to the common planar counterparts. This effect is verified by the spectral measurements of the fabricated samples. In addition, we analytically calculated the dispersion relation to identify the resonance wavelength of the structure and numerically calculate the phase of the transmitted light through the holes with different sizes. Our work forms the basis for the unique platform to introduce a new feature to the metasurfaces, which may find many applications from stacked metasurface layers to optical trapping particles inside the tube.

Near field microwave subsurface imaging using metasurface loaded planar antenna and synthetic aperture radar (SAR) technique

A novel near field subsurface microwave imaging scheme is proposed by using a high gain planar metasurface (MS) loaded slot antenna integrated with the synthetic aperture radar (SAR) algorithm. The antenna structure here comprises a slot antenna integrated with the proposed MS designed using the epsilon very large (EVL) unit cell. The proposed MS-based antenna structure has a very low profile of 0.033λ, with a measured peak gain of 5.5 dB at 6.47 GHz. After adding two more additional MS layers, the peak gain of the resultant antenna is 12.1 dB. The broadside gain and efficiency are significantly enhanced with the incorporation of the EVL MS. Afterward, the antenna structure, along with the vector network analyzer and the 2-D automated scanner is employed for the near field imaging of test media by recording the scattering data along the target area. A spatial subwavelength resolution with a typical value of about 0.43λ is obtained at 1λ standoff distance after applying the single frequency SAR algorithm to the measured raw scattering data. The overall applicability of the proposed low-profile MS loaded antenna integrated with the SAR algorithm is demonstrated for the near field subsurface microwave imaging of various metallic and dielectric targets with subwavelength spatial resolution in free space and lossy media.

Efficient coupling of evanescent waves in rectangular waveguides based on ultrathin planar capacitive metasurfaces

Efficient coupling of evanescent waves in rectangular waveguides based on ultrathin planar capacitive metasurfaces

Radiation pattern direction control of THz antenna with applying planar graphene metasurface

In this paper an antenna with dynamically controllable radiation pattern direction, in the low THz frequency band, is designed and investigated. The radiating element is composed of a rectangular patch antenna on a grounded dielectric while, radiation pattern control is made possible through applying a superstrate, inspired by metasurface structures, with gap of 25μm from the antenna surface. The radiating patch as well as the ground plane are designed with gold while the metasurface consists of graphene loops and patches which are placed on the either sides of a dielectric with the same material of the antenna substrate. Tunable radiation pattern direction is provided through activating appropriate columns of the metasurface unit cells. With applying an electrostatic bias voltage the graphene surface conductivity increases and the corresponding unit cells resemble properties of metals and impose parasitic effects on the antenna radiation pattern, which through wise selection of the metasurface unit cells the antenna radiation pattern is tilted in the desired direction. It is shown that the antenna radiation pattern can be dynamically controlled in the wide range of ±35degrees, symmetrically, in the H-plane.

Large and efficient unidirectional plane-wave–surface-wave metasurface couplers based on modulated reactance surfaces

Planar metasurfaces for efficient unidirectional conversion of a plane wave into a surface wave are presented. They are designed as a reciprocal device of a decoupler or leaky-wave antenna based on spatially modulated reactance surfaces. In a surface wave-to-leaky wave conversion, the surface reactance is determined from optimization of the complete aperture field over the propagating and evanecent spectra such that the conversion surface becomes locally and globally passive and lossless, while the desired wave conversion is achieved. The coupling efficiency is given by the aperture efficiency of the reciprocal antenna, which can closely approach unity for large apertures. The optimized surface reactance is spatially discretized and translated into physical metasurfaces, comprising an array of subwavelength inclusions. In TM polarization, three lossless metasurface design examples of 40 and 80 wavelengths are provided with performance validation using full-wave simulations: a metallic groove array, a thickness-modulated grounded dielectric substrate, and a width-modulated grounded dielectric ridge array. Deviations of the surface reactance from the optimized distribution reduce the conversion efficiency, but high coupling efficiencies in excess of 98% for the metallic structure and 60% for dielectric structures are predicted.

Realizing transmitted metasurface cloak by a tandem neural network

Being invisible at will has been a long-standing dream for centuries, epitomized by numerous legends; humans have never stopped their exploration steps to realize this dream. Recent years have witnessed a breakthrough in this search due to the advent of transformation optics, metamaterials, and metasurfaces. However, the previous metasurface cloaks typically work in a reflection manner that relies on a high-reflection background, thus limiting the applications. Here, we propose an easy yet viable approach to realize the transmitted metasurface cloak, just composed of two planar metasurfaces to hide an object inside, such as a cat. To tackle the hard-to-converge issue caused by the nonuniqueness phenomenon, we deploy a tandem neural network (T-NN) to efficiently streamline the inverse design. Once pretrained, the T-NN can work for a customer-desired electromagnetic response in one single forward computation, saving a great amount of time. Our work opens a new avenue to realize a transparent invisibility cloak, and the tandem-NN can also inspire the inverse design of other metamaterials and photonics.

Analysis and Synthesis of Cascaded Cylindrical Metasurfaces Using a Wave Matrix Approach

In recent years, there has been extensive research on planar metasurfaces capable of arbitrarily controlling scattered fields. However, rigorous studies on conformal metasurfaces, such as those that are cylindrical, have been few in number likely due to their more complex geometry. Here, wave propagation in cascaded cylindrical structures consisting of layers of dielectric spacers and azimuthally-varying metasurfaces (subwavelength patterned metallic claddings) is investigated. A wave matrix approach, which incorporates the advantages of both ABCD matrices and scattering matrices (S matrices), is adopted. Wave matrices are used to model the higher order coupling between metasurface layers, overcoming fabrication difficulties associated with previous works. The proposed framework provides an efficient approach to synthesize the inhomogeneous sheet admittances that realize a desired cylindrical field transformation. Design examples are reported to illustrate the power and potential applications of the proposed method in antenna design and stealth technology.

Generalized Brewster effect using bianisotropic metasurfaces.

We show that a properly designed bianisotropic metasurface placed at the interface between two arbitrary different media, or coating a dielectric medium exposed to the air, provides Brewster (reflectionless) transmission at arbitrary angles for both the TM and TE polarizations. We present a rigorous derivation of the corresponding surface susceptibility tensors based on the generalized sheet transition conditions and demonstrate by full-wave simulations the system with planar microwave metasurfaces designed for polarization-independent and azimuth-independent operations. The proposed bianisotropic metasurfaces provide deeply subwavelength matching solutions for initially mismatched media. The reported generalized Brewster effect represents a fundamental advance in optical technology, where it may both improve the performance of conventional components and enable the development of novel devices.

Hybrid Toroidal Resonance Response in Planar Core-Shell THz Metasurfaces

Toroidal resonance of planar structure is feasible and interesting for many appealing applications. We numerically and experimentally investigated the toroidal resonances in a planar metamaterial comprising core-shell structures and its constituent core and shell components at THz frequencies. The investigated structure demonstrated sharp toroidal and hybrid toroidal resonance modes in 0.2–0.3 THz range. Our analysis showed that these modes could be explained by the interaction of resonance toroidal modes of the shell and core components. The response of the investigated planar core-shell toroidal metasurface is notably geometry dependent and can be easily tuned by tailoring the device geometry. Presented work can be used for advanced THz photonics applications, including precise bio-sensing, narrow-band filters, fast-switching, and modulation.

Robust analogue of electromagnetically induced transparency for stable meta-devices

Recently, studying metasurface structures for mimicking the quantum phenomena of electromagnetically induced transparency (EIT) has drawn continuous attention, which promotes a new generation of functional electromagnetic devices, including slow-light devices, optical sensors, and so on. In some cases, metasurfaces which can realize stable EIT effect is of critical importance in the real applications. Here, we propose a novel planar terahertz metasurface which exhibits strong EIT behavior owing to the strong coupling between the dark mode and bright mode. The EIT effect is very robust, which is stable to various variations of the geometric parameters, opening up a new avenue to improve the device stability based on metasurfaces.

Planar efficient metasurface for generation of Bessel beam and super-resolution focusing

In this paper, we propose a device generating Bessel beam that can assist microscope to enhance resolution of microscope to λ/3. As the conventional optical devices of generating Bessel beam have some drawbacks, such as low efficiency, and bulky volume, we design plane-axicons for generating Bessel beam using the principle of phase superposition. The designed plane-axicons can generate Bessel beams with zero-order and higher-order by changing topological charge n. We also demonstrate the FWHMs of plane-axicons about as small as λ/3 and have FWHMs with weak fluctuations independent of wavelength across the visible spectrum, which means the Bessel beam can be used to increase resolution of microscope. These performances make the plane-axicons can also enable other advanced research and applications related to Bessel beams ranging from laser fabrication to optical manipulation.

Fan‐beam antenna design based on metasurface lenses

A fan-beam antenna system based on metasurface lenses is proposed for backhaul antenna applications at 10.4 GHz. The planar metasurface lens is composed of three pairs of subwavelength unit cells with high transmission and opposite phase. These phase shifters are arranged to transform the Gaussian distributed near field from a linear patch antenna array into the sin c-function distributed aperture field, which in turn brings a sector radiation pattern in the azimuth plane and a pencil beam in the elevation plane. The simulated and measured results demonstrate that the presented lens antenna offers a sharp roll-off fan beam with the azimuth beamwidth 60°, elevation beamwidth 10°, and the gain 18.2 dB with the flat-top ripple less than 2.0 dB.