Investigation of nondegenerate two-photon absorption in common fluorescent dyes
The nondegenerate two-photon excited fluorescence (ND-TPEF) and nondegenerate two-photon absorption (ND-TPA) cross-sections of two common fluorescent dyes were investigated using the femtosecond pump-probe technique at several wavelength combinations. Time-resolved and laser intensity dependent fluorescence revealed that the fluorescence was originated from the ND-TPA effect. Experimental ND-TPA cross-section exhibited larger values than corresponding degenerate two-photon absorption case. Additionally, an improved essential-state model involving multiple TPA excited states was proposed to evaluate the TPA cross-section of organic molecules, which showed good agreement with experimental results. This study indicates that the investigated fluorescent dyes would play important roles in multicolor two-photon imaging.