  Topological phononic crystals (PCs) offer a novel approach to control the
behavior of acoustic or elastic waves. In this paper, we design the gradient PC
structures with coupled interfaces to realize topological rainbow trapping and
broadband acoustic energy harvesting. Based on the geometric symmetry of PC
unit cells, we construct the coupled topological interfaces by amalgamating two
PCs with distinct topological phases. Also, the gradient modulation of
structural parameters along the coupled interfaces is employed for the
generation of rainbow trapping. Subsequently, a polyvinylidene fluoride (PVDF)
film is mounted along the coupled interfaces of the gradient PC structures,
facilitating the conversion of acoustic energy into electrical energy. The
numerical and experimental results show that the acoustic waves at different
frequencies stop and are amplified at different positions of the coupled
interfaces. Compared with the harvester in a bare structure, the topological PC
energy harvester significantly enhances the output power at different excited
frequencies. In the experiments, the maximum amplification ratio of the output
power reaches 91. Besides, it is demonstrated that the topological rainbow
trapping of acoustic waves exhibits robustness against random structural
disorders. Notably, the coupled interfaces have broadband and multi-mode
features, which are promising for other applications such as selective
filtering and sensing enhancement.
