Planar metal-only omnidirectional retroreflector using transmitarray and blazed grating for TE and TM polarizations

Retroreflector is a device that reflects light along its incident direction over a continuous range of incident angles. The retroreflectors proposed so far are so bulky and uneven that they cannot be mounted on large structures. Recently, two-layer planar retroreflectors have been proposed in the field of optics and acoustics, but at microwave frequencies only one retroreflector with non-planar overall structure has been presented that doesn't cover omnidirectional incidence and only supports TM polarization. The term omnidirectional means the capability of retroreflectivity in all azimuth angles (φ_i) and a wide elevation angle (θ_i) range. In this paper, a novel planar and metal-only retroreflector is proposed at microwave frequencies that efficiently reflects the electromagnetic wave along its incident direction in omnidirectional angle range and supports both TE and TM polarization. The proposed structure consists of two separate cascaded layers. The top layer is a transmitarray structure in which incident waves with different angles are focused at different locations on the bottom layer. The bottom layer reflects back focused waves along its focus direction and two distinct structures are proposed to implement it. The first type is a novel spatially varying blazed grating and the second type is a spatially varying phase gradient metasurface, as used in the optics and aucostics. The first type offers omnidirectional property for both TE and TM polarization while the second type offers higher monostatic RCS for only TE polarization. Simulated and measured results of the proposed retroreflector with the first type bottom layer show an omnidirectional retroreflectivity with half-power (3-dB RCS level) elevation field of view of 60^° (-30^°to 30^°) at frequency of 10 GHz with a normal incidence efficiency of 63%.