Graded metasurfaces exploit the local momentum imparted by an impedance gradient to mold the impinging wave front. This approach suffers from fundamental limits on the overall conversion efficiency, and it is challenged by fabrication limitations on the spatial resolution. Here, we introduce the concept of metagratings, formed by periodic arrays of carefully tailored bianisotropic inclusions and show that they enable wave front engineering with unitary efficiency and significantly lower fabrication demands. We employ this concept to design reflective metasurfaces for wave front steering without limitations on efficiency. A similar approach can be extended to transmitted beams and arbitrary wave front transformation, opening opportunities for highly efficient metasurfaces for extreme wave manipulation.

Metagratings: Beyond the Limits of Graded Metasurfaces for Wave Front Control.

A new metasurface design offers a simple, practical approach to redirecting electromagnetic waves in an arbitrary manner with near perfect power efficiency over a wide range of angles and frequencies.

https://link.aps.org/pdf/10.1103/PhysRevX.8.011036

Perfect Anomalous Reflection with a Bipartite Huygens’ Metasurface

Radiation patterns and the resonance wavelength of a plasmonic antenna are significantly influenced by its local environment, particularly its substrate. Here, we experimentally explore the role of dispersive substrates, such as aluminum- or gallium-doped zinc oxide in the near infrared and 4H-silicon carbide in the mid-infrared, upon Au plasmonic antennas, extending from dielectric to metal-like regimes, crossing through epsilon-near-zero (ENZ) conditions. We demonstrate that the vanishing index of refraction within this transition induces a “slowing down” of the rate of spectral shift for the antenna resonance frequency, resulting in an eventual “pinning” of the resonance near the ENZ frequency. This condition corresponds to a strong backward emission with near-constant phase. By comparing heavily doped semiconductors and undoped, polar dielectric substrates with ENZ conditions in the near- and mid-infrared, respectively, we also demonstrate the generality of the phenomenon using both surface plasmon and phonon polaritons, respectively. Furthermore, we also show that the redirected antenna radiation induces a Fano-like interference and an apparent stimulation of optic phonons within SiC.

Role of epsilon-near-zero substrates in the optical response of plasmonic antennas

Conventional mirrors obey the simple reflection law that a plane wave is reflected as a plane wave, at the same angle. To engineer spatial distributions of fields reflected from a mirror, one can either shape the reflector or position some phase-correcting elements on top of a mirror surface. Here we show, both theoretically and experimentally, that full-power reflection with general control over the reflected wave phase is possible with a single-layer array of deeply subwavelength inclusions. These proposed artificial surfaces, metamirrors, provide various functions of shaped or nonuniform reflectors without utilizing any mirror. This can be achieved only if the forward and backward scattering of the inclusions in the array can be engineered independently, and we prove that it is possible using electrically and magnetically polarizable inclusions. The proposed subwavelength inclusions possess desired reflecting properties at the operational frequency band, while at other frequencies the array is practically transparent. The metamirror concept leads to a variety of applications over the entire electromagnetic spectrum, such as optically transparent focusing antennas for satellites, multifrequency reflector antennas for radio astronomy, low-profile conformal antennas for telecommunications, and nanoreflectarray antennas for integrated optics.

/pdf/functional-metamirrors-using-bianisotropic-elements-4y2sum28aq.pdf

Functional metamirrors using bianisotropic elements.

Manipulating the properties of the isofrequency contours (IFCs) of materials provides a powerful means of controlling the interaction between light and matter. Hyperbolic metamaterials (HMMs), an important class of artificial anisotropic materials with hyperbolic IFCs, have been intensively investigated. Because of their open dispersion curves, HMMs support propagating high-k modes and possess an enhanced photonic density of states. As a result, HMMs can be utilized to realize hyperlenses breaking the diffraction limit, metacavity lasers with subwavelength scale, high-sensitivity sensors, long-range energy transfer, and so on. Aimed at those who are about to enter this burgeoning and rapidly developing research field, this tutorial article not only introduces the basic physical properties of HMMs but also discusses dispersion manipulation in HMMs and HMM-based structures such as hypercrystals. Both theoretical methods and experimental platforms are detailed. Finally, some potential applications associated with hyperbolic dispersion are introduced.

Hyperbolic metamaterials: From dispersion manipulation to applications

This paper introduces for the first time a quad-mode dielectric resonator filter, using a simple cylinder resonator A four-pole single cavity filter is designed, simulated, and fabricated based on this quadruple mode resonator Additionally, a new type of dual-mode dielectric resonator filters is introduced, using the same cylindrical resonator cut in half along its axis Center frequency control, intra/inter/input-coupling mechanisms, tuning, and spurious improvement methods are discussed, showing versatility of the proposed structures to realize different filtering functions and specifications Measured results are presented for practical filters, employing the proposed quad-mode and dual-mode resonators The dielectric resonator filters presented in this paper offer a significant size and mass reduction in comparison with conventional dielectric resonator filters They promise to be useful for both wireless and satellite applications

Quad-mode and dual-mode dielectric resonator filters

Directive and scannable radiation patterns beyond the microwave region are desirable but leaky-wave antennas in the terahertz and optical range are unavailable. Here, Memarian and Eleftheriades demonstrate continuously scanned leaky-wave radiation from the interface of a photonic crystal with a Dirac-type dispersion.

Dirac leaky-wave antennas for continuous beam scanning from photonic crystals

Blazed gratings can reflect an oblique incident wave back in the path of incidence, unlike mirrors and metal plates that only reflect specular waves. Perfect blazing (and zero specular scattering) is a type of Wood's anomaly that has been observed when a resonance condition occurs in the unit-cell of the blazed grating. Such elusive anomalies have been studied thus far as individual perfect blazing points. In this work, we present reflective blazed surfaces that, by design, have multiple coupled blazing resonances per cell. This enables an unprecedented way of tailoring the blazing operation, for widening and/or controlling of blazing bandwidth and incident angle range of operation. The surface can thus achieve blazing at multiple wavelengths, each corresponding to different incident wavenumbers. The multiple blazing resonances are combined similar to the case of coupled resonator filters, forming a blazing passband between the incident wave and the first grating order. Blazed gratings with single and multi-pole blazing passbands are fabricated and measured showing increase in the bandwidth of blazing/specular-reflection-rejection, demonstrated here at X-band for convenience. If translated to appropriate frequencies, such technique can impact various applications such as Littrow cavities and lasers, spectroscopy, radar, and frequency scanned antenna reflectors.

/pdf/wide-band-angle-blazed-surfaces-using-multiple-coupled-1mhzlg67z3.pdf

Wide-band/angle Blazed Surfaces using Multiple Coupled Blazing Resonances.

A thin, flat metamaterial junction that concentrates incident light to a local spot can potentially outperform a conventional lens. The design, as reported by Mohammad Memarian and George Eleftheriades at the University of Toronto in Canada, consists of two slabs of low-permittivity anisotropic metamaterials with rotated optical axes. The resulting flat, low-profile structure is capable of collecting and guiding light to an intense spot much closer to the structure than that achievable with a traditional lens. Such functionality could aid the collection of light for applications involving solar cells, photodetectors and sensors. Simulations with a silver–glass metamaterial design suggest that the structure’s operation is broadband and can cover a wavelength window of at least 200 nm.

/pdf/light-concentration-using-hetero-junctions-of-anisotropic-1vje449u7r.pdf

Light concentration using hetero-junctions of anisotropic low permittivity metamaterials

Dynamic control of a laser’s output polarization state is desirable for applications in polarization sensitive imaging, spectroscopy, and ellipsometry. Using external elements to control the polarization state is a common approach. Less common and more challenging is directly switching the polarization state of a laser, which, however, has the potential to provide high switching speeds, compactness, and power efficiency. Here, we demonstrate a new approach to achieve direct and electrically controlled polarization switching of a semiconductor laser. This is enabled by integrating a polarization-sensitive metasurface with a semiconductor gain medium to selectively amplify a cavity mode with the designed polarization state, therefore leading to an output in the designed polarization. Here, the demonstration is for a terahertz quantum-cascade laser, which exhibits electrically controlled switching between two linear polarizations separated by 80°, while maintaining an excellent beam with a narrow divergence of ∼3°×3° and a single-mode operation fixed at ∼3.4  THz, combined with a peak power as high as 93 mW at a temperature of 77 K. The polarization-sensitive metasurface is composed of two interleaved arrays of surface-emitting antennas, all of which are loaded with quantum-cascade gain materials. Each array is designed to resonantly interact with one specific polarization; when electrical bias is selectively applied to the gain material in one array, selective amplification of one polarization occurs. The amplifying metasurface is used along with an output coupler reflector to build a vertical-external-cavity surface-emitting laser whose output polarization state can be switched solely electrically. This work demonstrates the potential of exploiting amplifying polarization-sensitive metasurfaces to create lasers with desirable polarization states—a concept which is applicable beyond the terahertz and can potentially be applied to shorter wavelengths.

Mohammad Memarian

Papers

Quad-mode and dual-mode dielectric resonator filters

Dirac leaky-wave antennas for continuous beam scanning from photonic crystals

Wide-band/angle Blazed Surfaces using Multiple Coupled Blazing Resonances.

Light concentration using hetero-junctions of anisotropic low permittivity metamaterials

Metasurface quantum-cascade laser with electrically switchable polarization