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

Abstract

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 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 μm, 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.

Two-photon absorption cross section for Coumarins 102, 153 and 307

“…As mentioned above, it was found in paper [2] that the shape of the TPA cross section coincides with the one-photon absorption shape for the C102 solution in methanol. We measured the shape of the TPA cross sections of solutions C307 and C153 in methanol using C102 as a reference.…”

“…It was found in paper [2] that the shape of the TPA cross section coincides with the one-photon absorption for the Coumarin 102 solution in methanol. This effect is possible in the absence of a central symmetry in the molecule and was observed earlier for certain organic phosphors [3].…”

“…Next, we compare the results of this paper with the previous publications for coumarins. As far as we know, the absolute value of TPA cross-section for C102 in methanol was evaluated only in one work [2]. In work [2], a comparison with bis-MSB at a wavelength of 725 nm was used to estimate the absolute value of the C102 cross-section.…”

“…As far as we know, the absolute value of TPA cross-section for C102 in methanol was evaluated only in one work [2]. In work [2], a comparison with bis-MSB at a wavelength of 725 nm was used to estimate the absolute value of the C102 cross-section. The TPA cross-section for bis-MSB at 725 nm was calculated by extrapolation from data from paper [9], resulting in a  TPE (λ = 725 nm) = 4.8 GM.…”

A New Series of Quadrupolar Type Two‐Photon Absorption Chromophores Bearing 11, 12‐Dibutoxydibenzo[a,c]‐phenazine Bridged Amines; Their Applications in Two‐Photon Fluorescence Imaging and Two‐Photon Photodynamic Therapy

“…Emission spectra were recorded by a Jasco FP-6500 spectrofluorometer. One photon emission quantum yields (F) and 2PA cross section (s 2 ) were obtained by comparative measurement using coumarin 480 in methanol as in reference [16]. The quantum yield of coumarin 480 in methanol was determined by the fluorescence standard, quinine sulfate, which the quantum yield was assumed to be 0.53 AE 0.02 (in 0.1 M H 2 SO 4 ) [37].…”