Abstract

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.

Measurement of two-photon excitation cross sections of molecular fluorophores with data from 690 to 1050 nm

Chris Xu and Watt W. Webb
J. Opt. Soc. Am. B 13(3) 481-491 (1996)

Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm

Marius A. Albota, Chris Xu, and Watt W. Webb
Appl. Opt. 37(31) 7352-7356 (1998)

Determination of absolute two-photon excitation cross sections by in situ second-order autocorrelation

Chris Xu, Jeffrey Guild, Watt W. Webb, and Winfried Denk
Opt. Lett. 20(23) 2372-2374 (1995)

Absolute two-photon excitation spectra of red and far-red fluorescent probes

Mary Grace M. Velasco, Edward S. Allgeyer, Peng Yuan, Jaime Grutzendler, and Joerg Bewersdorf
Opt. Lett. 40(21) 4915-4918 (2015)

Two-photon-excited absolute emission cross-sectional measurements calibrated with a luminance meter

Rakesh Kapoor, Christopher S. Friend, and Amitava Patra
J. Opt. Soc. Am. B 20(7) 1550-1554 (2003)