Acoustic waves propagating in fluids like gas and liquid can carry angular momentum, similar to the angular momentum of light. Acoustic vortex beams with rotating helical phase front possess orbital angular momentum (OAM), whose radiation torque was applied for the control of particle rotations. The orthogonality between acoustic OAM with different topological charges was exploited to increase information capacity and communication speed of a single pulse. Meanwhile, the circular energy flux obtained in several phononic crystals forms another type of acoustic OAM named pseudospin was implemented for the realization of one-way topological edge states. In acoustic helical waveguides, the geometric phase induced by the curvature of the propagation paths was also observed. The properties of the acoustic OAM are similar to their optical counterparts. Besides OAM, optical waves can also carry spin angular momentum (SAM) induced by the rotation of optical polarization characterized by local electric vector like circular polarized light. The optical spin is an intrinsic quantity that induces many intriguing wave physics including optical spin Hall effect and spin-orbit coupling of light. On the other hand, the existence of acoustic spin is an important question that is still under debating. Because of the longitudinal nature of acoustic waves propagating in fluids like airborne sound, the acoustic waves are usually characterized by scalar pressure fields and are considered to be spinless. While most of acoustic phenomena can be characterized by the scalar pressure field, the local particle velocity vector field is a crucial quantity for the understanding of the complete acoustic physics. In this work, we analytically derive a density quantity that characterize the acoustic spin if exist. The acoustic spin can exist in waves whose local particle velocity vector is rotating about itself. This acoustic spin is experimentally observed in the interference of two beams propagating perpendicular to each other. The spin induced torque is measured by a design acoustic dipole particle that interacts with the local particle velocity field. The acoustic spin also exist in evanescent waves propagating along a periodic groove waveguide, where acoustic spin-momentum locking is observed experimentally.
Acoustic zero index metamaterials such as density-near-zero metamaterials have received increasing attention due to their potential applications on beam forming, cloaking, wave tunneling, and imaging. High transmission resulted by impedance matching of such zero index metamaterials and surrounding media requires the effective density and inverse bulk modulus to be simultaneously zero. Metamaterials possessing this property are called double zero index metamaterials. The design of double zero index metamaterials needs scatterers with sound speed lower than the background medium, which is extremely challenging for air acoustics because the air sound speed is among the lowest. This challenge can be solved for high order waveguide mode by designing structures with larger thickness. An experimental scan of the pressure field inside our design metamaterial excited by a point source reveals the existence of a Dirac cone at the Brillouin zone center. The measured envelope of the propagating wave inside the metamaterial shows double negative, double positive, and double zero properties below, above, and at the Dirac point,respectively. This result is confirmed by the measured acoustic beam out of the metamaterial. A gapless transition between double negative and double positive acoustic metamaterials is realized. The development of this double zero index metamaterial provides new routes to broaden practical applications of acoustic metamaterials.
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