  We measure the rotational and translational velocity components of particles
moving in helical motion using the frequency shift they induced to the
structured light beam illuminating them. Under Laguerre-Gaussian mode
illumination, a particle with a helical motion reflects light that acquires an
additional frequency shift proportional to the angular velocity of rotation in
the transverse plane, on top of the usual frequency shift due to the
longitudinal motion. We determined both the translational and rotational
velocities of the particles by switching between two modes: by illuminating
with a Gaussian beam, we can isolate the longitudinal frequency shift; and by
using a Laguerre-Gaussian mode, the frequency shift due to the rotation can be
determined. Our technique can be used to characterize the motility of
microorganisms with a full three-dimensional movement.
