Engineering the Nanoscale Acoustic Purcell Effect

Controlling spontaneous emission through environment-dependent density of states, the Purcell effect, has profoundly influenced many areas of wave physics, from quantum systems to metamaterials. However, its realization in acoustics has remained largely unexplored, particularly at the nanoscale, where impedance mismatch and viscous damping severely limit emission efficiency. These constraints impose fundamental trade-offs between output intensity, device compactness, and biocompatibility, hindering the development of efficient, miniaturized ultrasound technologies for imaging, sensing, and therapy. Here, we demonstrate the nanoscale acoustic Purcell effect, in which engineered nanodroplets act as tunable resonant cavities that locally modify the acoustic density of states (DOS) to amplify reradiated sound. A theoretical framework for this effect is developed and experimentally verified through ultrasound emission measurements, yielding a normalized acoustic Purcell factor (APF/D) of 1.6×105 ± 2.6×102 m-1, over two orders of magnitude greater than previously reported acoustic designs. The enhancement arises from nanoscale geometric confinement and interfacial elasticity, providing direct control over emission rate, reradiation efficiency, and bandwidth. Passive-listening imaging confirms >40 dB amplification under identical drive conditions, establishing strong agreement between model and experiment. This work introduces a general framework for nanoscale acoustic emission control, extending the Purcell principle beyond photonic and electronic domains into soft-matter acoustics. By uniting tunable nanostructures with density-of-states engineering, we establish the foundation for nanophononic control of sound–matter interactions, enabling high-contrast, energy-efficient ultrasound sources for biomedical and soft-device applications.