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

Abstract

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.

Practical two‐photon‐absorption cross sections and spectra of eosin and hematoxylin

“…The details of the employed experimental setup, procedure and data elaboration are described in a previous paper [36]. Such approach has the peculiar feature of exploiting a transversally line-shaped beam thus yielding a very high signal-to-noise ratio [13,36].…”

“…The details of the employed experimental setup, procedure and data elaboration are described in a previous paper [36]. Such approach has the peculiar feature of exploiting a transversally line-shaped beam thus yielding a very high signal-to-noise ratio [13,36]. While the method has been originally conceived for the measurement of relative spectra, we now extend its use to the evaluation of absolute spectra by means of a reference fluorochrome for which the values of the cross section are available.…”

“…The cuvette has been placed on a microscope slide lying on a suitable micrometric actuator. Further details on the apparatus can be found in [36][37][38][39][40]. The detection setup exploits a Nikon oil-immersion 60× objective with a numerical aperture of 1.4 and a working-distance of '130 μm.…”

“…In our configuration the twophoton fluorescence is excited by a wavelength-tunable mode-locked Ti:Sapphire laser source in a microscope setup. The properly shaped beam illuminates a specific sample of dye according to an innovative measurement approach [36]. Such a technique belongs to the class of the so-called indirect methods for TPA assessment and has been originally conceived to evaluate the relative TPA spectrum [13,36].…”

“…The properly shaped beam illuminates a specific sample of dye according to an innovative measurement approach [36]. Such a technique belongs to the class of the so-called indirect methods for TPA assessment and has been originally conceived to evaluate the relative TPA spectrum [13,36]. In this work the method is upgraded to the evaluation of the absorption cross section as well.…”