A design of frequency-isolated full-k-space flatband in phononic crystals for ultrasensitive detection and high-Q cavities

Flatbands in artificial lattices hold significant potential for novel applications. They are generally constrained to specific regions in momentum space and can be obtained by tight-binding approximation. This study demonstrates an innovative band engineering strategy that successfully constructs a frequency-isolated flatband spanning the entire Brillouin zone in a two-dimensional phononic crystal. Notably, the acoustic pressure field in this flatband locates mainly in water background rather than steel scatters, which indicates that the strategy to achieve flatbands is beyond the picture of tight-binding approximation. Characterized by vanishing group velocity and singular density of states, phononic crystals with such flatbands can be used as ultrasensitive detectors of background and boundary-free acoustic cavities with high quality factors.