UC San Diego

Guiding phononic waves with low loss is attractive from both scientific and technological points of view, where a well-confined, robust, and unidirectional phononic waveguide with little reflection is desired. However, phonons are magnetically inert, making it challenging to implement related unidirectional/chiral transport in phononic systems. Reciprocal topological waveguides are proven to be capable of guiding waves unidirectionally over defects and disorders in electronic and photonic systems. They can be constructed by tuning the lattice symmetry, which is a very intriguing approach to be extended to guide phonon propagations. In this dissertation, unidirectional phononic waveguides from 2D to 3D systems, that are mainly based on topological physics, are investigated theoretically and experimentally. Starting with a most fundamental 2D spring-mass theoretical model, we prove that pseudospins can be introduced to a phononic system by relative variations of the inter- and intra-unit cell spring constants. These pseudospins correspond to a pair of topologically protected counter-propagating edge states at the boundary of such a system, which are unidirectional and immune to backscattering. Next, a phononic waveguide constitutes of Helmholtz resonators is proposed to further improve the confinement and robustness of phonon propagations. We also propose a highly robust guiding principle, based on a line defect within a true triangular phononic lattice. Such waveguiding mechanism is experimentally demonstrated for surface acoustic waves (SAWs), that overcomes obstacles involving beam steering and lateral diffraction. Lastly, we extend the idea of creating a defect-line waveguide from 2D to 3D, and develop a helical waveguide with a screw dislocation in a hexagonal close packed (hcp) phononic crystal. Our simulations and measurements prove the directionality of the helical modes confined at the screw dislocation.