Realization of a Bilayer Elastic Topological Insulator.

Elastic waves and their associated devices offer versatile platforms for sensing, metrology, and information processing. The rise of topological materials has enabled unprecedented control of elastic waves in solids, giving rise to elastic wave topological insulators (EWTIs) that host defect-immune, high-fault-tolerance edge states. However, most existing studies remain confined to monolayer configurations, which limit elastic wave propagation and device performance to 2D planes. In contrast, the more promising and practical regime of multilayer elastic wave devices has received little attention. In this work, we experimentally demonstrate a class of bilayer elastic wave topological insulators (BLEWTIs) with high-fault-tolerance. By introducing the layer degree of freedom, BLEWTIs exhibit four distinct topological phases, in contrast to the two found in monolayer EWTIs. This enables the construction of multiple types of domain walls with diverse transmission behaviors, such as layer beam splitting. Consequently, we realize high-fault-tolerance interlayer converters and beam splitters operating along the normal direction of the 2D plane-features impossible to achieve in monolayer systems. Our findings pave the way for advanced elastic wave applications, including layer-selective emitters and splitters and multi-path topological routing, marking a significant step toward the development of compact, high-performance electromechanical and optomechanical devices.