The Fluorescence Upconversion Spectroscopy

Frequency upconversion of 800 nm ultrashort 175 fs optical pulses by two-photon absorption in a stilbenoid compound-doped polymer (PMMA) optical fiber was reported [28]. By the intensity-dependent transmission method, the two-photon absorption cross section was deduced. The combination of a well-designed organic chromophore incorporated into a fiber geometry is appealing for the development of an upconversion blue polymer laser. Upconversion fluorescence and optical power limiting effects based on the two- and three-photon absorption process of a frans-4,4 bis(pyrrolidinyl)stilbene were investigated [29]. The molecular TPA cross section j2 at 550-670 nm and the three-photon absorption (3PA) cross section g3 at 720-1000 nm were measured. The 3PA-induced optical power-limiting properties were also illustrated at 980 nm. 

The isomerization dynamics of cis-stilbene in the first ecdted singlet state was studied by fluorescence upconversion [30]. Lifetime measurements were made with subpicosecond resolution in 2-propanol, decanol, hexane, and tetradecane. The cis-stilbene fluorescence decay curves were single exponential in all solvents except for decanol, where they were adequately described by a double exponential. A picosecond component in the fluorescence anisotropy decay measurements made in 2-propanol suggests that motion along the reaction coordinate was directly measured. 

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