Polarised stimulated emission depletion studies of two-photon excited states

Stimulated emission depletion (STED) population and polarisation dynamics following two-photon excitation are investigated for rhodamine 6G in ethylene glycol. Time resolved fluorescence intensity and polarisation measurements were made using picosecond time-correlated single photon counting (TCSPC). Cross-sections for the stimulated transition were measured between 614nm (2.32 X 10(-16) cm(2)) and 663.5nm (6.05 X 10(-17) cm(2)), ground state vibrational lifetimes were found to vary between 314fs and 467fs. A collinear (180degrees) excitation-detection geometry was employed to investigate re-polarisation of the excited state array yielding fluorescence anisotropies above the two-photon limit. The circumvention of single-photon selection rules is demonstrated allowing the measurement of higher order parameters and correlation functions that are wholly inaccessible to 'conventional' (spontaneous) time resolved fluorescence techniques