Acoustic Amplified Transmission and Emission through k-Gap

Abstract Floquet time crystals, characterized by momentum band gaps (k-gaps), offer powerful mechanisms for exotic wave control. However, their experimental realization of airborne sound remains a significant challenge. Here, we demonstrate a phononic time crystal by integrating discrete resonant meta-atoms into a one-dimensional acoustic waveguide, forming an effectively homogeneous, time-varying metamaterial. Through dynamic modulation of the effective compressibility, we induce a k-gap and experimentally observe amplified acoustic transmission with a transmittivity reaching 1340%. At half the modulation frequency, we identify an anomalous transmission peak under continuous-wave excitation, with its amplitude sensitively dependent on the phase difference between the incident wave and the modulation cycle. Leveraging the k-gap, we also demonstrate amplified emission by placing a monopole source at the center of the time crystal, achieving an emission enhancement with a Purcell factor exceeding 30. Furthermore, we demonstrate the versatility of our platform by introducing a temporal super-cell, which induces momentum band folding and results in a double k-gap structure. This reconfigurable and programmable approach enables the realization of customized resonant responses, and it also opens new pathways for constructing higher-dimensional phononic time crystals and exploring nontrivial topological dynamics in time-modulated media.