LAUSR

Significance The emergence of “negative” frequencies in physical systems is often accompanied by intriguing consequences. For example, supersonic motion between a source and an observer leads to a negative Doppler-shifted frequency, the physical meaning of which is time reversal of the received signal. To our knowledge, the rotational analogue of this situation—the consequences of generating negative rotationally Doppler-shifted waves—has not been studied. Here we show, using an acoustic source, that a negative rotational Doppler shift is associated with a handedness reversal of the orbital angular momentum carried by the wave. We demonstrate that this handedness reversal can occur even at significantly subsonic velocities, making our findings relevant to interactions of ultrafast rotating systems with optical frequency radiation. The linear Doppler shift is familiar as the rise and fall in pitch of a siren as it passes by. Less well known is the rotational Doppler shift, proportional to the rotation rate between source and receiver, multiplied by the angular momentum carried by the beam. In extreme cases the Doppler shift can be larger than the rest-frame frequency and for a red shift, the observed frequency then becomes “negative.” In the linear case, this effect is associated with the time reversal of the received signal, but it can be observed only with supersonic relative motion between the source and receiver. However, the rotational case is different; if the radius of rotation is smaller than the wavelength, then the velocities required to observe negative frequencies are subsonic. Using an acoustic source at ≈100 Hz we create a rotational Doppler shift larger than the laboratory-frame frequency. We observe that once the red-shifted wave passes into the “negative frequency” regime, the angular momentum associated with the sound is reversed in sign compared with that of the laboratory frame. These low-velocity laboratory realizations of extreme Doppler shifts have relevance to superoscillatory fields and offer unique opportunities to probe interactions with rotating bodies and aspects of pseudorelativistic frame translation.