Near GHz Lithium Niobate Higher-Order Topological Nanomechanical Metamaterials.

Precise control over the localization of acoustic waves at microwave frequencies reveals new opportunities in emerging fields like quantum acoustics and spin mechanics. Conventional microwave acoustic resonators, engineered via phonon band structures, are prone to disturbances from fabrication defects, constraining their further development. Acoustic high-order topological insulators, known for their defect robustness and precise localization, have emerged as the preferred approach for developing high-performance resonators. However, the operating frequencies of existing acoustic high-order topological insulators have been limited to relatively low frequencies. Here, we present on-chip acoustic higher-order topological insulators (700-750 MHz) operating in the ultrahigh-frequency band, using lithium niobate nanomechanical metamaterials with smooth surfaces and sharp corners. By breaking the inversion symmetry of honeycomb lattices, higher-order valley Hall topological insulators featuring both odd-type and even-type corner states are constructed. Together, these advances promote the practical application of topological acoustics.