Abstract
Degenerate two-photon absorption (2PA) of a series of organic fluorophores is measured using femtosecond fluorescence excitation method in the wavelength range, λ2PA = 680-1050 nm, and ~100 MHz pulse repetition rate. The function of relative 2PA spectral shape is obtained with estimated accuracy 5%, and the absolute 2PA cross section is measured at selected wavelengths with the accuracy 8%. Significant improvement of the accuracy is achieved by means of rigorous evaluation of the quadratic dependence of the fluorescence signal on the incident photon flux in the whole wavelength range, by comparing results obtained from two independent experiments, as well as due to meticulous evaluation of critical experimental parameters, including the excitation spatial- and temporal pulse shape, laser power and sample geometry. Application of the reference standards in nonlinear transmittance measurements is discussed.
Highlights
Spectroscopic reference standards facilitate carrying out measurements of molecular absorption- and scattering cross sections, quantum yields, etc. under circumstances, where absolute methods are overly involved or inapplicable [1]
[14] For this purpose, we have constructed two independent experimental setups, using different 76 – 80 MHz pulse repetition rate femtosecond lasers, where one setup is optimized for the measurement of 2-photon absorption (2PA) spectral shapes and the other is optimized for the absolute cross section measurement, and where we have increased the accuracy of characterization of all critical temporal, spatial- and spectral parameters of the excitation beam
The 2PA spectral shapes are presented in Fig. 3 by black symbols, and the corresponding absolute cross section measurements are shown as red squares
Summary
Spectroscopic reference standards facilitate carrying out measurements of molecular absorption- and scattering cross sections, quantum yields, etc. under circumstances, where absolute methods are overly involved or inapplicable [1]. Augmented accuracy is most critical for calibration and optimization of fluorophores used in multi-photon microscopy [11,12,13], as well as in the emerging area of quantitative 2PA spectroscopy for measuring the strength of intra- and intermolecular electric fields [14] For this purpose, we have constructed two independent experimental setups, using different 76 – 80 MHz pulse repetition rate femtosecond lasers, where one setup is optimized for the measurement of 2PA spectral shapes and the other is optimized for the absolute cross section measurement, and where we have increased the accuracy of characterization of all critical temporal-, spatial- and spectral parameters of the excitation beam. In order to cover a sufficiently broad wavelength range, we take advantage of large solvatochromic and fluorosolvatochromic shifts of the 1PA and 2PA spectra of some of the chromophores
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