Thermomechanical Noise Dilution by Low‐Q Cavity for Nanoparticles Detection

Phonon laser sensors offer powerful label‐free detection, yet their sensitivity remains bounded by noise for which robust suppression strategies are still lacking. Here we introduce a technique for diluting thermomechanical noise that counter‐intuitively selects lower‐Q optical modes to suppress thermal fluctuations of phonon‐laser frequency, followed by active feedback locking for frequency stabilization. The combined approach enables real‐time detection of single 10 nm‐radius polystyrene particles based on a microbubble cavity with a 38 MHz phonon mode, estimating a mass limit of . We further apply the platform to study iron–oxide uptake and reaction dynamics in cells.