Frequency-tunable high-efficiency retroreflection from a magnetically biased ferrite metagrating

Retroreflection is critical for localization, sensing, and wireless communication. Though the conventional 3D retroreflectors, gratings, and metasurfaces are applied to achieve retroreflection, the reconfiguration of operating frequency is still a challenge. Here, we demonstrate a high-efficiency retroreflector whose operating frequency can be reconfigured without any structural modification. The device consists of a magnetic metagrating backed by a metal plate, where the unit cell of the metagrating is a ring of yttrium-iron-garnet (YIG) ferrite rods. Simulation demonstrated the retroreflector could continuously tune working frequency with efficiency 95% over a frequency range 210 MHz (9.90-10.11 GHz) by varying the applied DC bias field. We revealed that the retroreflection arises from coherent multiple scattering between the metagrating and the backplate, and its frequency is set by the bias-field-dependent anisotropic permeability of the ferrite. Experimental measurements in a parallel-plate waveguide confirm both the high retroreflection efficiency and the frequency tunability of the designed retroreflector. This work establishes a simple, structurally invariant route to tunable retroreflection.