Structured environment controls dynamics of light-matter interaction processes via modified local density of electromagnetic states. In typical scenarios, where nanosecond-scale fluorescent processes are involved, mechanical conformational changes of the environment during the interaction processes can be safely neglected. However, slow decaying phosphorescent complexes (e.g., lanthanides) can efficiently probe micro- and millisecond scale motion via near-field interactions with nearby structures. As the result, lifetime statistics can inherit information about nanoscale mechanical motion. Here we study light-matter interaction dynamics of phosphorescent dyes, diffusing in a proximity of a plasmonic nanoantenna. The interplay between time-varying Purcell enhancement and stochastic motion of molecules is considered via a modified diffusion equation, and collective decay phenomena is analysed. Fluid properties, such as local temperature and diffusivity, are mapped on phosphorescent lifetime distribution and then extracted with the help of inverse Laplace transformation. The presented photonic platform enables performing contactless all-optical thermometry and diffusion measurements, paving a way for a range of possible applications. In particular, detailed studies of nanofluidic processes in lab-on-a-chip devices, challenging for analysis with other optical methods, can be performed with time-dependent phosphorescence.

• Received 1 October 2019

DOI:https://doi.org/10.1103/PhysRevB.101.035420