Longitudinal acoustic spin and global spin-orbit interaction in vortex beams

Area of Science:

• Acoustics
• Wave Physics
• Angular Momentum

Background:

• Spin and orbital angular momenta (AM) are crucial in physics, but acoustic waves, typically longitudinal, are considered spin-0.
• While acoustic spin AM density exists, the total longitudinal spin AM often vanishes, limiting understanding of acoustic AM properties.

Purpose of the Study:

• To theoretically establish the spin, orbital, and total AM of acoustic vortex beams.
• To discover and analyze the non-zero integral longitudinal spin AM in acoustic fields.
• To explore new spin-orbit interaction mechanisms and AM conservation laws in acoustics.

Main Methods:

• Development of a self-consistent theoretical framework for acoustic angular momentum.
• Analysis of acoustic vortex beams under compression and expansion.
• Investigation of acoustic canonical-Minkowski and kinetic-Abraham AM.

Main Results:

• Demonstrated that acoustic vortex beams carry a non-zero integral longitudinal spin AM.
• Unveiled a novel spin-orbit interaction mechanism during vortex beam compression/expansion.
• Clarified the conservation properties of acoustic canonical-Minkowski AM versus kinetic-Abraham AM.

Conclusions:

• The study elucidates fundamental aspects of spin and orbital AM in acoustics.
• Findings reveal new insights into acoustic spin-orbit interactions and AM conservation.
• The results offer potential for new AM-based applications in acoustics and other classical wave systems.