Quasi-omnibearing retro-reflective metagrating protected by reciprocity

Abstract

Reciprocity is ubiquitous in antennas for receiving and radiating electromagnetic (EM) waves, i.e., if an antenna has good receiving performance at a given direction, it also has good radiation performance in that direction. Inspired by this, we propose a method of designing a quasi-ominibearing retro-reflective metagrating (RRMG) protected by the reciprocity of antennas. Based on the second-order mode around 15.0 GHz of a short-circuited structured patch antenna (SPA), incident transverse magnetic waves can be received, channeled into the coaxial lines, reflected by the shortened end, and finally re-radiated into free space with a reversed wave vector. RRMGs are contrived consisting of this identical SPA, with a grating constant allowing ± 2 nd-, ± 1 st-, and zeroth-order diffractions. Oblique incidence, plus the tilted nulls of the re-radiation pattern, can eliminate − 1 st, zeroth, + 1 st, and + 2 nd orders, and only the − 2 nd order is left to achieve retro-reflections. Prototypes were fabricated and measured. Simulated and measured results show that the RRMGs maintain only − 2 nd-order diffraction for incident angles 32.2 ° ≤ θ i < 90.0 ° in four quadrants, and that RRMGs can achieve quasi-omnibearing retro-reflections for θ i = 50.0 ° . The use of higher-order diffraction brings more degrees of freedom in manipulating EM waves, and this strategy can be readily extended to millimeter waves, THz wave, or even optical regimes.