Functional Metamirrors Using Bianisotropic Elements

Abstract

Conventional mirrors obey Snell's reflection law: 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 (for example, creating a parabolic reflector) or position some phase-correcting elements on top of a mirror surface (for example, designing a reflectarray antenna). Here we show, both theoretically and experimentally, that full-power reflection with general control over reflected wave phase is possible with a single-layer array of deeply sub-wavelength 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 sub-wavelength 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, multi-frequency reflector antennas for radio astronomy, low-profile conformal antennas for Conventional mirrors, known since the dawn of civilization 1 , obey the simple law of reflection: the reflection angle is equal to the incidence angle. This follows from the fact that the total tangential electric field at the ideal mirror surface is zero, thus, the phase of the electric field in the reflected wave is the opposite to that in the incident wave. If a reflector can be engineered to enable general control over the reflection phase, it is possible to change the direction of the reflected waves at will 2 . Developments in the field of antennas enabled creation of layers with any desired phase of reflection at microwaves. Conceptually, these devices are conventional mirrors, modified by some additional phase-shifting elements positioned close to fully reflecting surfaces. Such artificial layers, in particular conventional reflectarrays 3 and high-impedance surfaces 4 , are realized as arrays of resonant metal patches over a metal ground plane. All the patches of the array usually have identical shape but different dimensions, so that the resonance frequency of an individual patch varies to ensure the desired variations of the reflection phase over the array surface. Controlling the phase variation spanning a 2π range allows one to arbitrarily tune the shape and orientation of the reflected wavefront. Since the conventional reflectarrays incorporate a metal ground plane, the transmission through them is completely blocked and reflection amplitude can be very high if low-loss materials are used. On the other hand, the presence of a metal ground plane forbids transmission at all practically interesting frequencies and limits the application possibilities....

Bifacial Metasurface with Quadrupole Optical Response

“…Metasurfaces are therefore more likely to find widespread technological applications. Recently demonstrated examples include holographic elements [23], lenses and axicons [24], antireflection coatings [25], mirrors [26], and polarization converters [27,28]. To our knowledge, however, a metasurface or a metamaterial with an electric-dipole-free optical response [29] and with a strong dependence of optical reflection on the illumination side [19] has not yet been demonstrated experimentally in the visible spectral range.…”

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

“…The vast majority of these geometries have been based on the generalized laws of reflection and refraction [7], based on which we can engineer the local reflection and/or transmission coefficients so that the impinging wave acquires the tangential momentum necessary to be locally rerouted towards the desired direction [7]- [17]. As research in this area has progressed, a few papers [18]- [20] have pointed out how this approach is inherently limited in efficiency, especially when extreme wave manipulation is considered.…”

Analysis and synthesis of cascaded metasurfaces using wave matrices

“…Adding a magnetic response to metasurfaces has provided additional degrees of freedom in terms of phase coverage for wavefront manipulation, and reflection control [2]. Metasurfaces with magneto-electric coupling have provided control over the reflections [5,23]. A simple approach to realizing metasurfaces with electric, magnetic, and bianisotropic responses is to cascade electric sheet admittances [3][4][5].…”