A metalens-based bicycle safety reflector for autonomous vehicle radars

Abstract With the rising number of interactions between autonomous or sensor-assisted vehicles—especially in poor weather conditions—come the need and opportunity for a new class of bicycle safety reflectors designed to enhance cyclist visibility to radars. To this effect, the first retrodirective planar metalens-based tag operating in the millimeter-wave automotive frequency range is proposed. The compact, lightweight design ( $$4.8~\\textrm{g}$$ ) consists of two layers: a metalens layer and a patch antenna pixel layer. The metalens focuses incoming plane waves from different incidence angles onto corresponding patch antenna pixels on the second layer, which re-radiate the signal back through the metalens, enabling retrodirective operation. The proposed tag was thoroughly evaluated, demonstrating reliable detection beyond 70 m and a peak monostatic radar cross section (RCS) of $$3.54~\\textrm{dBsm}$$ with stable retrodirectivity over $$\\pm 40^\\circ $$ , providing an average gain improvement of $$7.58~\\textrm{dB}$$ and an RCS enhancement of $$15.16~\\textrm{dB}$$ relative to a lens-less reference. A realistic deployment scenario on a metallic bicycle demonstrated up to a 110x improvement in its detectability at broadside, as a controlled proof-of-concept demonstration of improved radar detectability. These results highlight the potential of the proposed passive tag to operate as a low-cost, lightweight, and easily integrable bicycle safety reflector for future next-generation autonomous vehicle radar systems.