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Applied Spectroscopy

Applied Spectroscopy


  • Vol. 52, Iss. 4 — Apr. 1, 1998
  • pp: 536–545

Source-Corrected Two-Photon Excited Fluorescence Measurements between 700 and 880 nm

W. G. Fisher, E. A. Wachter, Fred E. Lytle, Michael Armas, and Colin Seaton

Applied Spectroscopy, Vol. 52, Issue 4, pp. 536-545 (1998)

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Passively mode-locked titanium:sapphire (Ti:S) lasers are capable of generating a high-frequency train of transform-limited subpicosecond pulses, producing peak powers near 105 W at moderate average powers. The low energy per pulse ( 20 nJ) permits low fluence levels to be maintained in tightly focused beams, reducing the possibility of saturating fluorescence transitions. These properties, combined with a wavelength tunability from approximately 700 nm to 1 mu m, provide excellent opportunities for studying simultaneous two-photon excitation (TPE). However, pulse formation is very sensitive to a variety of intracavity parameters, including group velocity dispersion compensation, which leads to wavelength-dependent pulse profiles as the wavelength is scanned. This wavelength dependence can seriously distort band shapes and apparent peak heights during collection of two-photon spectral data. Since two-photon excited fluorescence is proportional to the product of the peak and average powers, it is not possible to obtain source-independent spectra by using average power correction schemes alone. Continuouswave, single-mode lasers can be used to generate source-independent two-photon data, but these sources are four to five orders of magnitude less efficient than the mode-locked Ti:S laser and are not practical for general two-photon measurements. Hence, a continuous-wave, single-mode Ti:S laser has been used to collect a sourceindependent excitation spectrum for the laser dye Coumarin 480. This spectrum may be used to correct data collected with multimode sources; this possibility is demonstrated by using a simple ratiometric method to collect accurate TPE spectra with the modelocked Ti:S laser. An approximate value of the two-photon cross section for Coumarin 480 is also given.

W. G. Fisher, E. A. Wachter, Fred E. Lytle, Michael Armas, and Colin Seaton, "Source-Corrected Two-Photon Excited Fluorescence Measurements between 700 and 880 nm," Appl. Spectrosc. 52, 536-545 (1998)

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