Subwavelength focusing by engineered power-flow conformal metamirrors

Abstract

Many advances in reflective metasurfaces have been made during the last few years, implementing efficient manipulations of wave fronts, especially for plane waves. Despite numerous solutions that have been developed throughout the years, a practical method to obtain subwavelength focusing without the generation of additional undesired scattering is a challenge to this day. In this paper, we introduce and discuss lossless reflectors for focusing incident waves into a point. The solution is based on the so-called power-flow conformal surfaces that allow theoretically arbitrary shaping of reflected waves. The metamirror shape is adapted to the power flow of the sum of the incident and reflected waves, allowing a local description of the reflector surface based on the surface impedance. In particular, we present a study of two scenarios. First, we study the scenario when the field is emitted by a point source and focused at an image point (in the considered example, with the λ/20 resolution). Second, we analyze a metasurface capable of focusing the power of an illuminating plane wave. This work provides a feasible strategy for various applications, including detecting biological signals near the skin, sensitive power focusing for cancer therapy, and point-to-point power transfer.

Perfect-Lens Theory Enables Metasurface Reflectors for Subwavelength Focusing

“…In our recent work [36], we have used this power flow-conformal mirror approach to realize subwavelength focusing, requiring that the desired reflected waves converge to a point (or to a line, in the two-dimensional case). However, in that case, the presence of a drain is required to absorb the power of the focused wave after convergence at the focal point.…”

A Lossless Sink Based on Complex Frequency Excitations

“…For example, prior works have shown that a sink can be made from a scatterer whose impedance is perfectly matched to that of a converging wave. [ 8 , 9 , 11 ] This approach requires that loss be introduced into the system. In the context of the scattering matrix formalism relating input and output energy channels, this can be understood as an example of coherent perfect absorption (CPA) with a single radiation channel.…”

Subwavelength Focusing with Reflective Metasurfaces Engineered Using the Concept of Perfect Lens

“…Here, we show that it is possible to design an impenetrable reflecting surface that provides the same functionality. In this work, based on our recent results on subwavelength focusing with power flow-conformal metamirrors [2], we use that approach to emulate the field created by a perfect lens. This approach allows theoretically perfect focusing in the absence of any receiving object (energy sink) at the focal point, because the created power flow is continuous power flow: after converging to the focal point, the power flow again diverges into space behind the focus.…”