  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 sense micro- and millisecond scale
motion via near-field interactions. As the result, lifetime statistics can
inherit information about nano-scale mechanical motion. Here we study
light-matter interaction dynamics of phosphorescent dyes, diffusing in a
proximity of a plasmonic nanoantenna. The interplay between the 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 diffusion coefficient are mapped on
phosphorescent lifetime distribution extracted with the help of inverse Laplace
transform. We present rather simple photonic platform enabling contactless
all-optical thermometry and diffusion measurement and paving a way for a
plethora of possible applications. Among them, the proposed analysis can be
used for detailed studies of nanofluidic processes in lab-on-a-chip devices,
which are extremely hard or even impossible to analyse with other optical
methods.
