Observation of the rotational Doppler effect from an optically trapped micro-particle

The linear Doppler shift forms the basis of various sensor types for the measurement of linear velocity, ranging from speeding cars to fluid flow. Recently, a rotational analogue was demonstrated, enabling the measurement of angular velocity using light carrying orbital angular momentum (OAM). If measurement of the light scattered from a spinning object is restricted to a defined OAM state, then a frequency shift is observed that scales with the rotation rate of the object and the OAM of the scattered photon. In this work we measure the rotational Doppler shift from micron-sized calcite particles spinning in an optical trap at tens of Hz. In this case the signal is complicated by the geometry of the rotating particle, and the effect of Brownian motion. By careful consideration of these influences, we show how the signal is robust to both, representing a new technique with which to probe the rotational motion of micro-scale particles.