Mini review of ultrafast fluorescence polarization spectroscopy [invited]

A mini review is presented on the theory, experiment, and application of the ultrafast fluorescence polarization dynamics and anisotropy with examples of two important medical dyes, namely Indocyanine Green and fluorescein. The time-resolved fluorescence polarization spectra of fluorescent dyes were measured with the excitation of a linearly polarized femtosecond laser pulse, and detected using a streak camera. The fluorescence emitted from the dyes is found to be partially oriented (polarized), and the degree of polarization of emission decreases with time. The decay of the fluorescence component polarized parallel to the excitation beam was found to be faster than that of the perpendicular one. Based on the physical model on the time-resolved polarized emission spectra in nanosecond range first described by Weber [J. Chem. Phys.52, 1654 (1970)], a set of first-order linear differential equations was used to model fluorescence polarization dynamics and anistropy of dye in picoseconds range. Using this model, two important decay parameters were identified separately: the decay rate of total emission intensity and the decay rate of the emission polarization affected by the rotation of fluorescent molecules causing the transfer of emission polarization from one orthogonal component to another. These two decay rates were separated and extracted from the measured time-resolved fluorescence polarization spectra. The emission polarization difference among dyes arising from different molecular volumes was used to enhance the image contrast.