In this paper, we propose a new metasurface that is able to reflect a known incoming electromagnetic wave into an arbitrary direction, with perfect power efficiency. This seemingly simple task, which we hereafter call perfect anomalous reflection, is actually highly nontrivial because of the differing wave impedances and complex interference between the incident and reflected waves. Heretofore, proposed metasurfaces that achieve perfect anomalous reflection require complicated, deeply subwavelength and/or multilayer element structures, which allow them to couple to and from leaky and/or evanescent waves. In contrast, we demonstrate that using a bipartite Huygens’ metasurface (BHM)—a passive and lossless metasurface with only two cells per period—perfect anomalous reflection can be achieved over a wide angular and frequency range. Through simulations and experiments at 24 GHz, we show that a properly designed BHM can anomalously reflect an incident electromagnetic wave from ${\theta }_i=50°$ to ${\theta }_r=-22.5°$, with perfect power efficiency to within experimental precision.

In this article, the basic principles and the main applications of Huygens’ metasurfaces (HMSs) are reviewed from microwaves to optics. In general, HMSs comprise a thin layer of orthogonal electric and magnetic dipoles, which form an array of Huygens’ sources. In a refraction setting, these sources radiate mostly in the forward direction and can be used to manipulate an incident electromagnetic wave at will. In the case of passive HMSs, the Huygens’ sources are induced by an incident electromagnetic field. Examples of passive manipulations include reflectionless refraction, perfect anomalous reflection, and arbitrary antenna beam forming. In the case of active HMSs, the Huygens’ sources are impressed active sources. Active HMS manipulations include cloaking and subwavelength spot formation in a cavity environment.

Implementation of abrupt phase discontinuities along a surface has been the theme of recent research on electromagnetic metasurfaces. Simple functionalities such as reflecting, refracting, or focusing plane waves have been demonstrated with devices featuring phase discontinuities, but optical surfaces allowing independent magnitude and phase control on the scattered waves have yet to emerge. In this paper, we propose the first true optical Huygens’ surface, which explicitly utilizes orthogonal electric and magnetic responses to realize total control on an optical surface’s local reflection coefficients. This extends the functionality of metasurfaces to an unprecedented level. We first demonstrate that a nanorod gap-surface plasmon resonator can act as a Huygens’ source. Thereafter, by properly tuning and rotating these resonators, we realize arbitrary reflection optical metasurfaces—surfaces for which the local reflection coefficients can be independently tailored in both magnitude and phase. We demonstrate the versatility of this approach through designs of a metasurface that asymmetrically reflects two copolarized beams and a Dolph-Tschebyscheff optical reflectarray.

Optical microscopy suffers from a fundamental resolution limitation arising from the diffractive nature of light. While current solutions to sub-diffraction optical microscopy involve combinations of near-field, non-linear and fine scanning operations, we hereby propose and demonstrate the optical super-microscope (OSM) – a superoscillation-based linear imaging system with far-field working and observation distances – which can image an object in real-time and with sub-diffraction resolution. With our proof-of-principle prototype we report a point spread function with a spot size clearly reduced from the diffraction limit, and demonstrate corresponding improvements in two-point resolution experiments. Harnessing a new understanding of superoscillations, based on antenna array theory, our OSM achieves far-field, sub-diffraction optical imaging of an object without the need for fine scanning, data post-processing or object pre-treatment. Hence the OSM can be used in a wide variety of imaging applications beyond the diffraction limit, including real-time imaging of moving objects.

This paper investigates the retroreflective properties of a type of periodic metasurface which is aggressively discretized to include only two cells per grating period. The resultant metasurface, which we term the binary Huygens’ metasurface, dramatically simplifies the design towards highly efficient, lowcost and robust metasurface retroreflectors. Using this discretization method, we designed, simulated and measured two binary Huygens’ metasurfaces which retroreflect an incident plane wave at 82.87° from broadside, for TE and TM polarized waves respectively. The simulated results for a 2D infinite structure show power efficiencies of 94% (TE) and 99% (TM). The measured results show a power efficiency of 93% for both TE and TM polarizations. This shows that it is possible to obtain a high retroreflection power efficiency at an angle of choice using a very simple (slot or patch) metasurface design. The proposed design methodology allows one to achieve not only reftroreflection, but also reflection to arbitrary angles, with very high efficiency, which diversifies its uses.