Broadband geometric phase within topological complementary pairs for reconfigurable acoustic metalens

Geometric phase, arising from the cyclic evolution of orbital angular momentum (OAM) transformation, provides an effective phase manipulation mechanism for acoustic wavefront control. However, the potential of this geometric phase for broadband applications is largely unexplored, limiting the development of acoustic device applications. Here, we reveal the broadband mechanism of geometric phase generated by twisted topological complementarity pairs (TCPs) and demonstrate its capability to enable reconfigurable broadband metasurfaces. We find that the phase resolution (m) within the TCP structure serves as a key control parameter, increasing it significantly enhances the operational bandwidth, as experimentally verified with a maximum bandwidth of approximately 1800 Hz. Leveraging this broadband response of the geometric phase, we design and fabricate an acoustic geometric-phase metalens utilizing TCP array, which exhibits versatile focusing functionalities, including on/off-axis focusing, needle-like focusing, and double-point focusing. This reconfigurable metalens with high-efficiency performance is observed across a wide frequency range from 3100 to 3900 Hz. By decoupling functional reconfigurability from structural complexity, this passive mechanical tuning scheme establishes a highly reliable manufacturing route for smart acoustic systems, demonstrating significant application potential for extreme environments.