Spatiotemporal coupling of excited state dynamics in time-resolved microscopies.

In the high-density excitation limit, as is often probed with ultrafast spectroscopies, spatial and temporal evolution of photogenerated excited states are strongly coupled, giving rise to artifacts that influence experimentally-determined material parameters. The interplay between spatial and temporal degrees of freedom is especially pronounced in pump-probe microscopy, where small laser spot sizes amplify the effects of spatiotemporal coupling on spectroscopic observables. To quantitatively model these effects, a continuum model is developed that accounts for laser spot size as well as nonlinear excited state decay and diffusion. It is shown that effective excitation densities cannot be used to determine quantitatively correct rate constants. Significant error is introduced unless experimental data is fit with a numerical model that accounts for spatial anisotropy in the excitation density. Furthermore, the quantitative determination of material diffusion coefficients is shown to be highly sensitive to experimental parameters.