Abstract
Chirality is ubiquitous in nature. Acoustic fields can carry chirality in the form of vortex states characterized by quantized topological charges and helical wavefronts. These vortex states carry intrinsic orbital angular momentum (OAM) and provide significant degrees of freedom for sound manipulations for various applications. In addition, acoustic fields can carry nonzero spin density due to the rotation of velocity vectors, giving rise to another form of acoustic chirality. The chiral acoustic fields can enable nontrivial chiral sound-matter interactions with novel applications that cannot be achieved with conventional scalar properties of sound waves. This thesis presents a theoretical study of the two types of chiral acoustic fields (i.e., acoustic fields carrying OAM and acoustic fields carrying spin) in artificial structures. We focus on the absorption and topological properties of the chiral acoustic fields in acoustic resonators and metamaterials.We first investigate the absorption of acoustic vortex states carrying OAM in a one-dimensional (1D) chiral metamaterial lattice. The lossy 1D metamaterial gives rise to different absorption of the vortex states with opposite topological charges (i.e., opposite OAM), corresponding to the acoustic counterpart phenomenon of optical helical dichroism (HD) that has attracted considerable attention in recent years. We find that the acoustic HD is strongly enhanced in the metamaterial lattice with 𝐶2 rotational symmetry, compared with the weak HD effect in the 𝐶4 rotational symmetric system. The underlying physics is related to the non-Hermitian exceptional points and OAM bandgaps of the eigenmodes, which can be well captured by an effective two-band Hamiltonian. The breaking of 𝐶4 symmetry gives rise to exceptional points near the Brillouin-zone center and boundaries and OAM bandgaps, which selectively enhance the absorption of one vortex state.
Then, we extend the study to three-dimensional (3D) chiral metamaterials that support the propagation of transverse sound, which is a new type of airborne sound wave with transverse polarization. In contrast to the conventional longitudinal sound, the transverse sound can be circularly polarized and carry spin angular momentum. We investigate the absorption of the transverse sound in the 3D chiral metamaterials with loss. It is observed that the transverse sound with opposite spin can give rise to different absorption in the metamaterial, corresponding to the circular dichroism (CD) phenomenon in acoustics. We find that the acoustic CD effect is negligible if the metamaterial possesses 𝐶4 rotational symmetry, and it can be significantly enhanced by adding loss in selected domains of the metamaterial to break the 𝐶4 rotational symmetry. We provide a physical explanation based on the exceptional points in the complex band structure, the polarization band gaps, and the quality factors of the eigenmodes.
Last, we find that spin-carrying chiral acoustic fields can also emerge in acoustic resonators in the form of polarization singularities, at which the velocity field is circularly polarized. We investigated these polarization singularities in various acoustic resonators with different geometries and explored their topological properties. We find that these chiral acoustic fields can give rise to intriguing spatial configurations of polarization, such as Möbius strips, the evolutions of which can be understood with the topological invariant characterizing the polarization singularities.
These results in this thesis contribute to the understanding of chiral sound-matter interactions in artificial structures and can generate broad applications in acoustic sensing, acoustic trapping, and underwater acoustic communications. The physics may also be extended to other classical waves, such as electromagnetic waves and water waves.
| Date of Award | 28 Aug 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Shubo WANG (Supervisor) |
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