Periodic Metasurface Research Articles

Clarification of surface modes of a periodic nanopatch metasurface.

We study the angle-dependent optical reflectance spectrum of a metasurface consisting of a periodic array of film-coupled plasmonic nanopatch particles. The nanopatch metasurface exhibits a strong, angle-independent absorption resonance at a wavelength defined by the nanopatch geometry and relative density. When the nanopatches are arranged in a regular lattice, a second, sharp absorption dip is present that varies strongly as a function of the incidence angle. This second resonance is a collective effect involving the excitation of surface plasmon modes and relates to a Wood's anomaly. Using an analytical model, we compute the surface modes of the structure and confirm details about the various mechanisms that contribute to the reflection spectra. The measured reflectance spectra are in excellent agreement with both analytical calculations and full-wave numerical simulations.

Polarization-resolved characterization of plasmon waves supported by an anisotropic metasurface

Optical metasurfaces have great potential to form a platform for manipulation of surface waves. A plethora of advanced surface-wave phenomena such as negative refraction, self-collimation and channeling of 2D waves can be realized through on-demand engineering of dispersion properties of a periodic metasurface. In this letter, we report on polarization-resolved measurement of dispersion of plasmon waves supported by an anisotropic metasurface. We demonstrate that a subdiffractive array of strongly coupled resonant plasmonic nanoparticles supports both TE and TM plasmon modes at optical frequencies. With the assistance of numerical simulations we identify dipole and quadrupole dispersion bands. The shape of isofrequency contours of the modes changes drastically with frequency exhibiting nontrivial transformations of their curvature and topology that is confirmed by the experimental data. By revealing polarization degree of freedom for surface waves, our results open new routes for designing planar on-chip devices for surface photonics.

Hybrid unidirectional meta-coupler for vertical incidence to a high-refractive-index waveguide in telecom wavelength.

Unidirectional optical manipulation, especially the coupling from a vertical light beam to a waveguide unidirectionally, is desirable in photonic integration. We first propose a hybrid unidirectional meta-coupler for vertical incidence to a high-refractive-index waveguide in telecom wavelength, a periodic plasmonic metasurface composed of metal-insulator-metal unit cells is used for phase matching. Three designs are given for devices working around wavelengths 0.85, 1.31, and 1.55μm. The simulated coupling efficiencies are all around 70%, and the 1dB coupling bandwidths are 29, 82, and 105nm, respectively. Our approach paves the way for the applications of optical metasurfaces to planar lightwave circuits.

Designer surface plasmon dispersion on a one-dimensional periodic slot metasurface with glide symmetry.

In this Letter, we explore the dispersion of spoof surface plasmons supported by a single-layer glide-symmetric structure. This structure consists of an infinitely long double-notched slot perforated in a metal layer. The presence of a degeneracy of the two lowest-order modes at the Brillouin zone boundary, which have non-zero group velocity is explained and experimentally demonstrated. Further, the dependence of the band structure when glide-symmetric configuration is broken is also explored.

Wave-vector and polarization dependent impedance model for a hexagonal periodic metasurface exemplified through finite-difference time-domain simulations.

An isotropic impedance sheet model is proposed for a loop-type hexagonal periodic metasurface. Both frequency and wave-vector dispersion are considered near the resonance frequency. Therefore both the angle and polarization dependences of the metasurface impedance can be properly and simultaneously described in our model. The constitutive relation of this model is transformed into auxiliary differential equations which are integrated into the finite-difference time-domain algorithm. Finally, a finite large metasurface sample under oblique illumination is used to test the model and the algorithm. Our model and algorithm can significantly increase the accuracy of the homogenization methods for modeling periodic metasurfaces.

Shaping electromagnetic waves using software-automatically-designed metasurfaces

We present a fully digital procedure of designing reflective coding metasurfaces to shape reflected electromagnetic waves. The design procedure is completely automatic, controlled by a personal computer. In details, the macro coding units of metasurface are automatically divided into several types (e.g. two types for 1-bit coding, four types for 2-bit coding, etc.), and each type of the macro coding units is formed by discretely random arrangement of micro coding units. By combining an optimization algorithm and commercial electromagnetic software, the digital patterns of the macro coding units are optimized to possess constant phase difference for the reflected waves. The apertures of the designed reflective metasurfaces are formed by arranging the macro coding units with certain coding sequence. To experimentally verify the performance, a coding metasurface is fabricated by automatically designing two digital 1-bit unit cells, which are arranged in array to constitute a periodic coding metasurface to generate the required four-beam radiations with specific directions. Two complicated functional metasurfaces with circularly- and elliptically-shaped radiation beams are realized by automatically designing 4-bit macro coding units, showing excellent performance of the automatic designs by software. The proposed method provides a smart tool to realize various functional devices and systems automatically.

Polarizability calculation of arbitrary individual scatterers, scatterers in arrays, and substrated scatterers

We develop a semi-analytical approach for calculation of polarizability tensors of individual scatterers, scatterers in arrays, and substrated scatterers. The approach is based on the integration from induced charges and currents on the scatterer. By taking advantage of the present approach we calculate the individual polarizability tensors of isolated scatterers in a homogeneous isotropic medium. Moreover, we present the procedure of obtaining the polarizability tensors of substrated scatterers: scatterers located between two different isotropic media. Furthermore, using the proposed method we determine the surface susceptibility tensors of scatterers in a periodic metasurface. To compare and verify the results of arrays, we also investigate two other methods for calculating surface susceptibilities of scatterers in arrays. We examine the validity of the proposed approach using some examples and compare the results with other approaches. The proposed approach, in comparison to other reported approaches, is simpler, easily implemented, and does not require spherical harmonics expansion or complicated far-field calculations.

Dyadic Green’s Functions for Dipole Excitation of Homogenized Metasurfaces

Dyadic Green’s functions for an electric dipole source over an infinite periodic metasurface are obtained using homogenized, nonlocal, anisotropic, generalized sheet-transition conditions. The homogenized Green’s functions can efficiently model near-field point source excitation of typical metasurface structures. The Green’s functions can be decomposed into discrete and continuous spectral components, providing physical insight into the wave dynamics. Several different metasurfaces are considered, and the results are validated by comparison with a full-wave array-scanning method, demonstrating computational efficiency of the proposed homogenized Green’s function approach.

Plasmonic phase-gradient metasurface for spontaneous emission control

We combine the concept of phase-gradient metasurfaces with fluorescence directionality control of an ensemble of incoherent emitters. We design a periodic metasurface to control the scattering amplitude of the lattice in momentum space. The lattice is embedded in a waveguiding layer doped with organic fluorophores. In contrast to the usual symmetric directionality that plasmonic lattices impart on emission, we find that the phase gradient translates into asymmetric directional emission into the far field, determined by scattering on a subset of the reciprocal lattice vectors. The measured asymmetry is well explained by analytical modeling.

Composite Metamaterial and Metasurface Integrated With Non-Foster Active Circuit Elements: A Bandwidth-Enhancement Investigation

A metamaterial is designed and integrated with non-Foster active elements to investigate wideband negative permeability characteristics. A composite structure made of periodic resonant loops terminated with capacitors is first developed, and demonstrated that the negative permeability can be achieved near the resonance where the material is highly dispersive and the response is narrowband. To overcome this limitation, a negative active inductor is designed and utilized to compensate for the frequency dispersion of the loop and to achieve wideband negative permeability. Transmission-line circuit model analogy is employed to theoretically investigate the behavior of the composite configuration and predict its physical parameters, followed by a full-wave analysis based on the CST Studio Suite to numerically validate the results. A simple periodic metasurface structure is also implemented to demonstrate the ability of negative active elements (e.g., negative C and negative L) to obtain wideband characteristics. As a result, considerable bandwidth enhancement for both composite metamaterial and metasurface integrated with negative active elements is achieved compared to traditional designs. The basic principle of non-Foster elements is discussed and Linvill's topology is used throughout the paper to obtain the negative impedance. The stability of the entire system is then investigated using the eigenvalue technique and, finally, some related issues to the stability of the non-Foster elements are discussed.