Discrete Ultra-Broadband Perfect Anomalous Reflection in Depth Gradient Metasurfaces

Perfect anomalous reflections have been demonstrated in optical phase gradient metasurfaces (PGMs), but suffer from single-frequency (narrow-band) response due to the intrinsic limitation of natural geometric periodicity. Here, we provide both numerical and analytical evidence that a depth gradient metasurface (DGM) can achieve discrete ultra-broadband perfect anomalous reflection in the microwave range in the absence of geometric periodicity. Remarkably, by adjusting the operating frequency of the incident wave, the same effect can be steadily obtained via a physically-equivalent phase periodicity in the PGM. Based on this mechanism, a perfect retroreflector with a broadband response ranging from 1 GHz to 40 GHz is realized. Our work has promising applications in communication, source tracking, and military satellites.