By changing the phase temporally, the direction of motion of a rotating particle can be detected.
The Doppler effect is best illustrated as the change in the pitch of the siren of an ambulance as it races toward or away from you – the siren being the source and your ear as the receiver. By using this change in pitch, the relative velocity of the ambulance can be known.
Light, because it is a wave, also experiences an apparent shift in its pitch or frequency when there is a relative velocity between the source and the receiver. Hence, the apparent changing of frequency can also be used to detect motion.
The problem in using the Doppler effect in detecting movement however, rests in the fact that Doppler effect happens only when the relative movement of the source and the receiver are parallel to the line of sight. Any movement that is perpendicular to it cannot be detected.
Recently, two separate groups from Spain (click here for paper) and the United Kingdom (click here for paper) solved this problem by using specially engineered light. The light was made in such a way that its energy flow is twisting. By using this twisted light, one can measure rotation of particles that are perpendicular to the line of sight between the source and the receiver.
One inherent problem that this technique has is the way it is detecting the signal: it detects the signal through interferometry. By doing so, it removes the direction sensitivity of the technique. It cannot distinguish to where the particles are rotating. Are they rotating clockwise or counter-clockwise?
This paper answers that problem (click hereor here for paper). The researchers twist the twisted light in time to either lessen or increase the frequency change of the light. Since the twisting or rotation of the twisted light is controlled and its direction is known, an increase or decrease in the amplitude of the frequency can indicate the direction of the particles’ rotation.
This is a simple technique that does not add to the complexity of the setup.
This technique increases the range of applications of using twisted light in detecting rotating objects.
C. Rosales-Guzmán, N. Hermosa, A. Belmonte, and J. P. Torres, “Direction-sensitive transverse velocity measurement by phase-modulated structured light beams,” Opt. Lett. 39, 5415-5418 (2014). http://dx.doi.org/10.1364/OL.39.005415.