Functional Metamirrors Using Bianisotropic Elements

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.

Figure 1

The results of full-wave simulations of the individual inclusions of a metamirror which totally reflects normally incident waves at an angle $\theta ={45}^{\circ }$ from the normal. (a) Field distribution of an incident wave normally impinging on the metamirror surface. (b) Distribution of the copolarized electric field of the scattered waves from each inclusion. The forward scattered waves from the individual inclusions have identical phases opposite to the phase of the incident wave, which yields zero transmission. In the backward direction the inclusions radiate with discrete phase shifts from 0 to $5\pi /3$, clearly revealing anomalous reflection.

Figure 2

(a) The period of a metamirror consisting of 6 subwavelength copper inclusions that provide a linear phase variation of the reflection spanning a $2\pi$ range. The blue box denotes the dielectric substrate. (b) Magnetic field distribution (normalized to the magnetic field of the incident wave) of the transmitted (the $-z$ half-space) and the reflected (the $+z$ half-space) waves. (c) A metalens composed of 6 concentric arrays of the designed subwavelength inclusions. The dielectric substrate is denoted by the blue box. (d) Power density distribution (normalized to the incident power density) of the transmitted (the $-z$ half-space) and the reflected (the $+z$ half-space) waves.

Figure 3

Distribution of the measured (a) incident and (b) reflected copolarized electric field in the waveguide for a metamirror reflecting normally incident waves at an angle $\theta =45°$. The source in (a) and (b) is located at point $z=14\lambda$. Distribution of the measured (c) incident and (d) reflected copolarized electric field for a metalens in the waveguide. The metalens in (c) and (d) is positioned at point $z=0$.