Experimental Observation of Linear and Rotational Doppler Shifts from Several Designer Surfaces

An orbital angular momentum (OAM) carrying beam has the ability to detect a spinning surface from its rotational Doppler effect. However, a mixture of linear and rotational Doppler effects can occur when an OAM beam is illuminated to a target, with not only spins but also vibrations. In this paper, we experimentally observe using OAM carrying beams, both linear and rotational Doppler effects from several designer surfaces. Specifically, a spinning polarization-independent metasurface, helicoidal reflector and propeller are applied respectively in this study. We demonstrate by the use of two microwave beams with opposite OAM to separate rotational Doppler shift from micro-Doppler shift. The proposed method can also be applied to measure the spinning speed of rotational objects, which have wider applications in intelligent sensing, radar and quantum optics.