Multiple Rotational Doppler Effect Induced by a Single Spinning Meta-Atom

The rotational Doppler effect (RDE) is of fundamental importance in the detection of rotating systems, ranging from molecules to macroscopic bodies. Metasurfaces, which are potential substitutes for traditional optical devices, have been reported to induce a RDE. However, the mechanism of the interaction between an electromagnetic wave and a spinning artificial meta-atom is not yet fully understood. Here, we report a multiple RDE based on a time-varying generalized Pancharatnam-Berry phase, obtained from a single spinning meta-atom placed in a rectangular waveguide. Twofold and sixfold rotational Doppler shifts with respect to the rotational velocity are obtained from meta-atoms with C2 and C3 rotational symmetry, respectively. In contrast to full-size devices, a single meta-atom has minimal rotational inertia and size, enabling its application in integrated systems. This work represents the interaction of an electromagnetic wave with a single spinning meta-atom, which may have potential applications in rotation-detection systems, frequency-gradient metasurfaces, and frequency modulators.