Angular‐Adaptive Spin‐Locked Retroreflector Based on Reconfigurable Magnetic Metagrating

Abstract

control, [6,[16][17][18] and cloaking. [19][20][21][22] Specifically, a phase-gradient metasurface has been widely used as a ubiquitous tool to control the reflection and transmission of a known incidence wave. [18,[23][24][25][26][27] The majority of these metasurfaces have been based on the generalized laws of reflection and refraction [26] which provide the gradient surface impedance so that the impinging wave acquires the tangential momentum necessary to be locally rerouted toward the desired direction.One fascinating wavefront control for EM wave is retroreflection that is defined as the reflection of the EM wave propagating back along its incident direction. It is very important for compact optical path setup to achieve ultraflat retroreflector in laser fabrication. Heretofore, a conventional cat's-eye retroreflector is composed of a focusing lens and a concave mirror, which behaves like a cat's-eye device in optics. [28,29] Figure 1a shows this conventional conceptual design schematics for achieving retroreflection. This volumetric retroreflector is bulky and not planar, thus not natively compatible for integration and miniaturization. Shortly afterward, utilizing phase-gradient metasurface, low-weight planar ultrathin retroreflector can be implemented. However, as illuminated in Figure 1b, the tangential momenta added by the gradient metasurface have negligible dependence on the incident angle. [11] As a consequence, the gradient metasurface reflects photons back at a particular incident angle, but Retroreflectors made of gradient metasurfaces have recently attracted intense interests due to their ability in reflecting incident light back to its source. So far, the current retroreflectors can only flip the transverse momenta of incident photons in specific incidence angles and thus have limitations in wide-angle applications. Here, a switchable metagrating based retroreflector is proposed for high-efficient spin-locked retroreflection and suppression of undesired diffractions. Upon reflection, the handedness of the waves is kept the same as the incidence. Furthermore, by mechanically altering the folding state of the reconfigurable retroreflector, adaptive tangential momenta could be imparted to the incidence photons, providing a high-performance retroreflection over a continuous range of incidence angles from 27.3° to 52.5°. As a proof of concept, a magnetic metagrating based retroreflector is fabricated at microwave frequencies and experimental measurements show consistent behaviors at various incidence angles. The proposed retroreflector is compact (overall thickness of 0.204 of the wavelength) and inherently insensitive to the illumination angle. As the design concept introduced in the paper could be extended to terahertz and optical frequencies, the design may serve as a promising platform toward reconfigurable spin-based retroreflection devices for not only interface electromagnetics but also ultraflat photonics.For a long time, the ability to manipulate the flow of electromagnetic wave (EM wave) at...

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