Measurement of two-photon-absorption spectra through nonlinear fluorescence produced by a line-shaped excitation beam

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We propose an innovative experimental approach to estimate the two-photon absorption (TPA) spectrum of a fluorescent material. Our method develops the standard indirect fluorescence-based method for the TPA measurement by employing a line-shaped excitation beam, generating a line-shaped fluorescence emission. Such a configuration, which requires a relatively high amount of optical power, permits to have a greatly increased fluorescence signal, thus avoiding the photon counterdetection devices usually used in these measurements, and allowing to employ detectors such as charge-coupled device (CCD) cameras. The method is finally tested on a fluorescent isothiocyanate sample, whose TPA spectrum, which is measured with the proposed technique, is compared with the TPA spectra reported in the literature, confirming the validity of our experimental approach.

Lay description:

We present a newly conceived experimental technique to measure the two-photon-absorption spectra of fluorescent materials. The proposed method is based on the two-photon fluorescence, but it permits to overcome the typical drawback of such a technique, consisting of the very low signal level, which usually is maintained in a regime of photon counting. In our proposed method the optical excitation of the nonlinear fluorescence is produced by employing a line-shaped laser beam. This configuration, which permits to maintain the intrinsic high selectivity on two-photon-absorption signal which is typical of fluorescence-exploiting techniques enables to simplify the detection apparatus and to increase the signal allowing to not use a photon counter as a detector, which is usually necessary in such a kind of experiments. In the paper we discuss the principia which our system is based on, the experimental procedure, and, finally, we present test measurements carried out on fluorescein isothiocyanate (FITC) sample, a colorant whose two-photon-absorption spectrum is known in literature. The proposed procedure can be classified as an ”indirect method” for the two-photon absorption detection, because it exploits the phenomenon of two-photon-induced fluorescence emission. The employed line-shaped illumination, with an optical configuration similar to what recently proposed in the nonlinear microscopy domain, is demonstrated to provide the advantage of requiring an optical system easier with respect to the standard nonlinear fluorescence-based methods; moreover, the exciting beam with such a shape illuminates a great volume of sample, enabling to obtain a higher nonlinear fluorescence signal: this allows the fluorescence signal to overcome the regime of photon counting usually involved in such a kind of measurements and to not use a phototube for the detection, but a CCD camera. The experiments on FITC samples confirm the validity of such an approach, making our method promising for developing relatively easy and cheap setups for two-photon-absorption measurements.

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