SOURCE-CORRECTED 2-PHOTON-EXCITED FLUORESCENCE MEASUREMENTS BETWEEN 700 AND 880 NM

Passively mode-locked titanium:sapphire (Ti:S) lasers are capable of g enerating a high-frequency train of transform-limited subpicosecond pu lses, producing peak powers near 10(5) W at moderate average powers. T he low energy per pulse (<20 nJ) permits low fluence levels to be main tained in lightly focused beams, reducing the possibility of saturatin g fluorescence transitions. These properties, combined with a waveleng th tunability from approximately 700 nm to 1 mu m, provide excellent o pportunities for studying simultaneous two-photon excitation (TPE). Ho wever, pulse formation is very sensitive to a variety of intracavity p arameters, including group velocity dispersion compensation, which lea ds to wavelength-dependent pulse profiles as the wavelength is scanned . This wavelength dependence can seriously distort band shapes and app arent peak heights during collection of two-photon spectral data. Sinc e two-photon excited fluorescence is proportional to the product of th e peak and average powers, it is not possible to obtain source-indepen dent spectra by using average power correction schemes alone. Continuo us-wave, single-mode lasers can be used to generate source-independent two-photon data, but these sources are four to five orders of magnitu de less efficient than the mode-locked Ti:S laser and are not practica l for general two-photon measurements. Hence, a continuous-wave, singl e-mode Ti:S laser has been used to collect a source-independent excita tion spectrum for the laser dye Coumarin 480. This spectrum may be use d to correct data collected with multimode sources; this possibility i s demonstrated by using a simple ratiometric method to collect accurat e TPE spectra with the mode-locked Ti:S laser. An approximate value of the two-photon cross section for Coumarin 480 is also given.