Flat Engineered Multichannel Reflectors

Recent advances in engineered gradient metasurfaces have enabled unprecedented opportunities for light manipulation using optically thin sheets, such as anomalous refraction, reflection, or focusing of an incident beam. Here, we introduce a concept of multichannel functional metasurfaces, which are able to control incoming and outgoing waves in a number of propagation directions simultaneously. In particular, we reveal a possibility to engineer multichannel reflectors. Under the assumption of reciprocity and energy conservation, we find that there exist three basic functionalities of such reflectors: specular, anomalous, and retroreflections. Multichannel response of a general flat reflector can be described by a combination of these functionalities. To demonstrate the potential of the introduced concept, we design and experimentally test three different multichannel reflectors: three- and five-channel retroreflectors and a three-channel power splitter. Furthermore, by extending the concept to reflectors supporting higher-order Floquet harmonics, we forecast the emergence of other multichannel flat devices, such as isolating mirrors, complex splitters, and multi-functional gratings.

Popular Summary

People have been able to manipulate light using mirrors and lenses for thousands of years. The discovery of diffractive optics, which makes use of interference among light waves traveling along slightly different paths, led to novel optical components that are much thinner than traditional devices. Today, diffractive components play a fundamental role in industry, communications, and even our everyday life, resulting in devices as varied as diffraction gratings, lenses, antenna arrays, holograms, and filters. These devices, however, are designed to work best when light hits them at a specific angle. If radiation hits the component at a different angle, it responds differently than intended. We have designed reflective diffractive components that respond to electromagnetic radiation from multiple directions in specified and fundamentally different ways.

We introduce the concept of multichannel metasurfaces, which control incoming and outgoing radiation in a number of directions simultaneously. These devices combine functionalities of several optical components in a single flat structure at the same frequency. The characterization of these reflectors is beyond the conventional apparatus of diffraction optics and requires additional formulations adapted from the theory of microwave circuits. We explore the fundamental limitations on the response of multichannel reflectors as well as fabricate and experimentally characterize three different devices.

The concept of flat multichannel surfaces opens up ample opportunities for modern optics. We forecast the emergence of various novel devices such as flat isolating mirrors, where an observer might see a reflection of only himself and not of anyone standing nearby.

Figure 1

(a) Illustration of a periodic metasurface illuminated by a plane wave impinging from an angle ${\theta }_i$. (b) A periodic metasurface illuminated at ${\theta }_i=0°$. The three propagating harmonics of the metasurface are analogous to a three-port network.

Figure 2

(a) The real part of the normal wave number for different Floquet harmonics versus the periodicity of the reflector. (b) The reflection angle of the corresponding Floquet harmonics. The incident angle is ${\theta }_i=0°$.

Figure 3

(a) Operation of the anomalous reflector proposed in Ref. [23]. Reflection angle versus incident angle for different Floquet harmonics. All the harmonics are considered. The circles show at what angle ${\theta }_r$ all the incident energy is reflected from the considered anomalous reflector when it is excited from the corresponding port. (b) The flat anomalous reflector appears to an external observer at different angles as differently tilted mirrors. At ${\theta }_i=0°$ and ${\theta }_i=-70°$ angles, it appears as a mirror tilted at 35°. However, at a ${\theta }_i=70°$ angle, the observer would see himself as in a mirror tilted at 70°.

Figure 4

Numerical simulations of the three-channel mirror modeled as an inhomogeneous sheet with surface impedance calculated from Eq. (5). Real part (instantaneous value) and magnitude of the scattered normalized electric field when the reflector is illuminated from port 3 [(a) and (d)], port 2 [(b) and (e)], and port 1 [(c) and (f)].

Figure 5

(a) Schematic representation of one unit cell of the designed three-channel isolating mirror (retroreflector). (b) A photograph of the fabricated prototype.

Figure 6

(a) Simulated results of the three-channel retroreflector made of a patch array. Distribution of the normalized reflected power across Floquet harmonics propagating at different angles ${\theta }_r$ versus incident angle ${\theta }_i$. (b) Appearance of the flat isolating mirror for an external observer. At ${\theta }_i=0°$ and ${\theta }_i=\pm 70°$ angles, the observer would see himself as in a mirror normally oriented with respect to him.

Figure 7

(a) Illustration of the experimental setup (top view). Measured reflection efficiency of the metasurface when illuminated (b) normally, (c) at angles near ${\theta }_i=-70°$, and (d) at angles near ${\theta }_i=+70°$.

Figure 8

A $50:0:50$ power splitter. (a) Surface impedance dictated by Eq. (7) shown with solid lines and optimized surface impedance shown with circles. (b) A snapshot of the scattered normalized electric field (real value) in the proximity of the splitter located at $z=0$. The splitter is modeled by an ideal sheet, with the optimized surface impedance profile shown in Fig. 8.

Figure 9

Schematic representation of a single unit cell of a designed (a) three-channel power splitter and (b) five-channel retroreflector.

Figure 10

(a) Efficiency of retroreflection from the power splitter under normal illumination. The splitter is matched for normal incidence at 8.2 GHz. (b) Illustration of the experimental setup for the power splitter (top view). (c) Signals measured by the receiving antenna versus the orientation angle of the metasurface ${\theta }_i$ for two positions of the receiving antenna. (d) Signal from a reference metal plate (${\theta }_a=+70°$).

Figure 11

The reflection angles of different Floquet harmonics for a five-channel retroreflector. Here, the reflector periodicity $D\approx 1.064\lambda$ is chosen. Illumination is at (a) ${\theta }_i=70°$ and (b) ${\theta }_i=28°$. The grey dashed line indicates the periodicity of the reflector. For clarity, only seven harmonics are shown.

Figure 12

Measured reflection efficiency of the five-channel isolating mirror when illuminated from (a) port 3, (b) port 4, (c) port 2, (d) port 5, and (e) port 1. The dashed line shows the operating frequency.

Figure 13

(a) $N$-port isolating mirror. The wave impinging from different angles (grey arrows) is reflected back to the source (colored arrows). (b) Multifunctional reflector. Normally incident light is split between ports 2 and 4. Ports 1 and 5 are isolated.