Spin-orbit interactions of transverse sound

Abstract

Spin-orbit interactions (SOIs) endow light with intriguing properties and applications such as photonic spin-Hall effects and spin-dependent vortex generations. However, it is counterintuitive that SOIs can exist for sound, which is a longitudinal wave that carries no intrinsic spin. Here, we theoretically and experimentally demonstrate that airborne sound can possess artificial transversality in an acoustic micropolar metamaterial and thus carry both spin and orbital angular momentum. This enables the realization of acoustic SOIs with rich phenomena beyond those in conventional acoustic systems. We demonstrate that acoustic activity of the metamaterial can induce coupling between the spin and linear crystal momentum k, which leads to negative refraction of the transverse sound. In addition, we show that the scattering of the transverse sound by a dipole particle can generate spin-dependent acoustic vortices via the geometric phase effect. The acoustic SOIs can provide new perspectives and functionalities for sound manipulations beyond the conventional scalar degree of freedom and may open an avenue to the development of spin-orbit acoustics.

Imaginary spin-orbital coupling in parity-time symmetric systems with momentum-dependent gain and loss

“…And in condensed matter physics, the investigation of SOC has led to fruitful achievements (such as the spin-Hall effect [3], topological insulator [4], just name a few) with potential applications in spintronics [5] and quantum computations [6]. Currently, SOC researches are drastically expanding to the fields of cold atom physics [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], optics [23][24][25][26][27][28][29] and acoustics [30][31][32][33][34]. Cold atom systems have no natural SOC, whereas using the Raman coupling scheme [11] (and also some others, see the review article [12]) artificial SOC has been experimentally realized, furthermore supersolid stripe states [13][14][15][16][17][18][19][20] and momentum-space Josephson oscillations [21,22] can be generated.…”

“…In dual-core waveguides, SOC can be synthesized by dispersively coupling the light field in different cores [26][27][28], and stripe solitons can be produced [29]. In acoustical systems, SOC has also been successfully synthesized [30][31][32][33][34].…”

Metahouse: noise-insulating chamber based on periodic structures

“…Acoustic metasurfaces were also explored as potential platforms for analog computing [17] and, vice versa, advances in computer science and artificial intelligence boost design procedures to achieve desired properties of metamaterials and metasurfaces [18][19][20][21]. Metamaterials can be also used as a platform to explore analogies of the quantum concepts, such as Hall effect [22,23], spin properties [24][25][26][27], skyrmions [28], and twistronics [29].…”

Elastic Spin and Orbital Angular Momenta

“…Introduction.-Recently, there was a great renewed interest in the spin and orbital angular momenta of acoustic waves, i.e., sound waves in fluids or gases [1][2][3][4][5][6][7][8] and elastic waves in solids [9][10][11][12][13][14][15]. Although general fieldtheory principles of the momentum and angular momentum of sound and elastic waves have been known [16][17][18], and the orbital angular momentum of sound waves was extensively studied both theoretically and experimentally [19][20][21][22][23][24][25][26][27][28], new observable phenomena involving spin-polarization properties of sound waves, as well as the spin and orbital angular momenta of elastic waves, prompted a new wave of investigations in these fields.…”

“…In the circular basis, the operator of the z-component of the spin becomes diagonal: diag(1, −1, 0). Substituting the field (7) in the circular-Cartesian basis into Eqs. (5), we obtain the normalized z-components of the momentum, spin, and orbital angular momentum densities: Pz…”