Spin Orbital Coupling Enabled Rotational Doppler Effect for Enhanced Object Detections

Compared to conventional plane waves, vortex electromagnetic waves exhibit a distinctive capability to detect the spin motion of a target along its rotational axis. By analyzing the frequency shift in the echo signal, the rotational speed of the target can be accurately estimated. However, for objects with low velocities, the frequency shift error becomes more pronounced, imposing stringent requirements on hardware precision. To mitigate this limitation, spin-orbit coupling induced by geometric phase synthesis metasurfaces is utilized to generate hybrid-mode vortex waves for detecting rotating targets. By modulating the number of hybrid modes, the magnitude of the echo frequency shift can be adjusted, thereby enhancing the accuracy of rotational speed detection. A representative frequency of 9.37 GHz within the X-band, commonly employed in shipborne radar systems, was selected for analysis. Experimental results indicate that at a rotational speed of 20 revolutions per second (r/s), the detection error is 5% for single-mode vortex waves and 0.6% for hybrid-mode vortex electromagnetic waves. The proposed method demonstrates high sensitivity in detecting the rotational speed of objects, with potential applications in radar detection and specialized industrial equipment.