Quantum Sensing of Motion in Colloids via Time-Dependent Purcell Effect

Light–matter interaction dynamics is governed by the strength of local coupling constants, tailored by surrounding electromagnetic structures. Characteristic decay times in dipole-allowed fluorescent transitions are much faster than mechanical conformational changes within an environment and, as a result, the latter can be assumed static during the emission process. However, slow-decaying compounds can break this commonly accepted approximation and introduce new interaction regimes. Here, slow-decaying phosphorescent compounds are proposed to perform quantum sensing of the nearby structure's motion via observation of collective velocity-dependent lifetime distributions. In particular, characteristic decay of an excited dye molecule, being comparable with its passage time next to a resonant particle, is modified via time-dependent Purcell enhancement, which leaves distinct signatures on properties of emitted light. Velocity mapping of uniformly moving particles within a fluid solution of phosphorescent dyes is demonstrated via the analysis of modified lifetime distributions. The proposed interaction regime enables performing studies of a wide range of phenomena, where time-dependent light–matter interaction constants can be utilized for extraction of additional information about a process.