Reversal of orbital angular momentum arising from an extreme Doppler shift

Abstract

SignificanceThe 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.

A compact acoustic spanner to rotate macroscopic objects

“…This frequency was chosen based on the frequency response of the loudspeakers enabling maximum conversion efficiency of electrical power into acoustic power. Previous works involved with acoustic OAM relied on a PC, one or more sound-cards, or bulky electronics in order to generate the required audio signals 10,12,14 . In order to improve the affordability and compactness of the acoustic OAM generating equipment, an Arduino Due board was used in conjunction with a custom-designed filter/amplifier printed circuit board (PCB).…”

“…Many research groups world-wide have studied the transfer of optical momentum but comparatively fewer have looked at the acoustic transfer. First to show the transfer of an acoustic OAM was Volke-Sepiúlveda and co-workers 10 , and subsequent work by others 11–14 having demonstrated various acoustic analogies to optical angular momentum effects. In most of this previous work the helically-phased acoustic beam has been created using a ring of loudspeakers, each driven at the same angular frequency ω s , but with an azimuthally changing phase delay given by .…”

Doppler effect and micro‐Doppler effect of vortex‐electromagnetic‐wave‐based radar

“…Some experiments with optical vortex EM waves have been carried out to validate the angular Doppler effect [8, 9]. Recently, the angular Doppler effect has also been observed in an OAM‐based acoustic experiment [19]. This means that, even when the relative motion between radar and target is perpendicular to the direction of LOS, we can also detect a frequency shift of the echo, which is independent of the carrier frequency of transmitted signal.…”

Measuring the phase velocity of twisted wavefronts

“…Such arrays offer the ability to pattern the wavefront of sound dynamically in a manner similar to the way spatial light modulators do for light, and phased array antennae do for radio waves (15,16). Specifically, recent experimental demonstrations have shown that large arrays of acoustic transducers can shape an acoustic wavefront to impart orbital angular momentum (OAM), as well as a wavefront that can be dynamically controlled (17)(18)(19)(20)(21)(22).…”