Measurements of two-photon-excited fluorescence (TPF) of fluorescein and Rhodamine 6G in various solvents were performed with a continuous-wave (cw) laser for excitation and an acousto-optic tunable filter for spectral dispersion. Interestingly, the cw laser excitation produced an unwanted thermal-lens effect when the measurements were performed in solvents that absorb the excitation laser light (e.g., alcohols and water, because these solvents absorb the 780-nm excitation light through the overtone and combination transitions of the O—H group). The defocusing effect of the thermal lens leads to a decrease in the TPF signal. Because the strength of the thermal lens depends on the thermo-optical properties (d<i>n</i>/d<i>T</i> and thermal conductivity) of the solvent, its interference makes the effect of solvents on the TPF much different from those on one-photon-excited fluorescence. However, the thermal-lens interference will not limit the application of this cw laser excited TPF technique because, even when measurements were performed in solvents that absorb cw excitation laser light, the thermal-lens interference was observed only in solvents such as nonpolar organic solvents that have relatively better thermo-optical properties. Interference was not observed in water, which is the most widely used solvent for the TPF technique (because water has poor thermo-optical properties).
© 2000 Optical Society of America
(230.0230) Optical devices : Optical devices
(300.2530) Spectroscopy : Fluorescence, laser-induced
(300.6410) Spectroscopy : Spectroscopy, multiphoton
(300.6430) Spectroscopy : Spectroscopy, photothermal
Marc Fischer and Chieu D. Tran, "Thermal-Lens-Induced Anomalous Solvent’s effect on Fluorescence Produced by Two-Photon Continuous-Wave Laser Excitation," Appl. Opt. 39, 6257-6262 (2000)