Coupled acoustic resonator systems with novel acoustic properties

• Main Author: Yang, Yahui
• Other Authors: Zhang Baile
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Physics::Acoustics
• Online Access: https://hdl.handle.net/10356/146052
• Institution: Nanyang Technological University

The discovery of topological phases is a remarkable development with profound impact in condensed matter physics. Such systems host robust unidirectional edge states immune to backscattering from local defects or disorders and there exist topological invariants characterizing the global properties. Inspired by the potential applications based on their unique properties, tremendous explorations to extend the topological concept to acoustic systems have been witnessed in recent years. Analogues of topological phases of matter have been rapidly developed in acoustic systems, which acts as an excellent platform to study the topological phenomena. Among various kinds of acoustic systems to explore novel acoustic properties, coupled acoustic resonator systems have drawn much attention. The acoustic resonator systems are always of large size in space, making the sample fabrications and experimental measurements easier, and they could be analyzed with tight-binding model in theoretical investigations. In this thesis, three projects to investigate novel properties in sound waves based on coupled acoustic resonator systems are discussed. At first, the topological valley Hall edge states for sound waves are demonstrated. In our design, a two-dimensional periodic acoustic resonator system which directly simulates a gapped graphene monolayer is adopted. The inversion symmetry of the lattice is broken by differing the heights of two resonators within a unit cell, leading to the analogue of valley Hall effect. Similar to a gapped graphene, gapless topological valley edge states are shown at a zigzag domain wall separating different domains with opposite valley Chern numbers, while an armchair domain wall hosts no gapless edge states. Next, the observation of acoustic pseudo-Landau levels is demonstrated. Our system consists of the acoustic resonators specifically arranged according to a triaxial strain field, which can effectively generate a uniform magnetic field in the acoustic system. The acoustic transmission spectra for different strain strengths have been measured, which exhibit transmission gaps near the Dirac frequency due to the formation of acoustic Landau levels. The pressure profiles in the resonators are displayed to demonstrate the changes between spreading and localizing, as the frequency is tuned continuously among different discrete spectral components. Thirdly, the experimental realization of acoustic two-dimensional higher-order topological insulators in acoustic resonator systems is introduced. A second-order triangular-shaped topological insulator on kagome lattice is demonstrated. The nontrivial bulk topology for the lattice is characterized by quantized Wannier centers. Through measuring transmission spectra and acoustic pressure distributions, the topological corner states at three corners are observed. Then, the lattice was extended to three-dimensional and a third-order topological insulator on an anisotropic diamond lattice was implemented, leading to topological corner states at two corners of a rhombohedron-like sample. The work in this thesis has theoretically studied and experimentally investigated topological related novel properties in acoustic resonator systems, ranging from the strain field induced magnetic-like effects, to the bulk polarization. Introducing topological concepts to acoustic systems benefits both two fields; as shown in our work, acoustic resonators system acts as a highly tunable platform for exploring the topological phases, which build on larger scales and thus pose only little fabrication challenges in practical compared to photonic and electronic systems. Meanwhile,topological properties also contribute to the robust control of sound waves, which is desirable for practical wave transport applications and may prompt innovative sound device such as sound lasing.