Source-corrected two-photon excited fluorescence measurements between 700 and 880 nm
- Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6113 (United States)
- Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (United States)
- Coherent Laser Group, Santa Clara, California 95054 (United States)
Passively mode-locked titanium:sapphire (Ti:S) lasers are capable of generating a high-frequency train of transform-limited subpico-second pulses, producing peak powers near 10{sup 5}thinspW at moderate average powers. The low energy per pulse ({lt}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. Continuous-wave, 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 source-independent 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 mode-locked Ti:S laser. An approximate value of the two-photon cross section for Coumarin 480 is also given. {copyright} {ital 1998} {ital Society for Applied Spectroscopy}
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- DOE Contract Number:
- AC05-96OR22464
- OSTI ID:
- 664948
- Journal Information:
- Applied Spectroscopy, Vol. 52, Issue 4; Other Information: PBD: Apr 1998
- Country of Publication:
- United States
- Language:
- English
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