Deep-subwavelength phononic beams with topological interface states

We propose deep-subwavelength phononic beams that can support topological interface states (TISs) at low frequencies. To achieve this, we employ unit cells with acoustic black hole (ABH) configurations, enabling the creation of the first band gap in a low-frequency range where the unit cell size is much smaller than the wavelength of the controlled wave. Connecting two distinct beams with different topological properties results in TISs within their low band gaps. Through numerical simulations and experimental validation, we demonstrate the realization of TISs in the frequency range of 4.3–8.5 Hz, utilizing the designed phononic beams with lattice constants less than λ/20.