Abstract
We present an experimental technique to determine the degenerate two-photon absorption (2PA) spectra by performing a single Z-scan using a high-spectral-irradiance white light continuum (WLC) generated by a hollow core fiber. The hollow fiber was filled with Argon (Ar) gas at a pressure of 0.6 bar and was pumped with 500 mJ, 30 fs, and 800 nm pulses. The broadband WLC pulses with 350 nm bandwidth in the range of 600–950 nm were compressed to sub-8 fs pulses. To characterize and interpret the data obtained from this method, the spectral, temporal and spatial characteristics of the WLC were first analyzed. The WLC emerging from the compressor was dispersed using a prism pair and then focused into the sample by a cylindrical lens. Since different spectral components are spatially separated, any part of the sample in the beam cross section is irradiated with almost single wavelength pulses leading to only a degenerate 2PA process. The nonlinear transmittance was then measured by a charge-coupled-device (CCD) line camera as a function of the sample position while the sample was moved along the beam direction by a motorized translation stage. In this way the Z-scans at different wavelengths in the WLC spectral range can be measured and thus the wavelength-resolved degenerate 2PA spectra can be obtained by performing a single scan using dispersive WLC. This method was verified on a well-characterized dye Rhodamine B and yield a reasonable agreement with the data found in the literature. We used this method to determine the 2PA spectra of some two-photon initiators (2PIs) developed for two-photon polymerization (2PP) based 3D micro-structuring.
Highlights
Two-photon absorption (2PA) is an optical nonlinear process in which two photons are simultaneously absorbed to excite the molecule to a higher energy level
We generated a high-spectral-irradiance white light continuum (WLC) by slightly focusing femtosecond pulses into a hollow core fiber filled with Ar gas
This WLC can be used as an ultra-broadband source in Z-scan experiments for rapid and simultaneous measurements of the degenerate 2PA spectrum of verity of nonlinear mediums
Summary
Two-photon absorption (2PA) is an optical nonlinear process in which two photons are simultaneously absorbed to excite the molecule to a higher energy level. By comparing the transmission of the sample at the focus to that of the pure solvent, the attenuation of different spectral components can be determined [28,29,30] In this method the obtained 2PA spectrum is of degenerate nature, it represents the relative, but not the absolute value of the 2PA cross section. The transmitted narrow band laser beam can be utilized to carry out the Z-scan In this way, by changing the position of the slit and repeating the Z-scan for the new wavelength the 2PA cross section can be measured over the entire WLC spectra via point-by-point determination scheme. By changing the position of the slit and repeating the Z-scan for the new wavelength the 2PA cross section can be measured over the entire WLC spectra via point-by-point determination scheme This is a feasible method and similar to method (b) it has not been reported in literature. To check the validity of our technique over the WLC spectrum the 2PA cross section of a well-characterized organic dye, Rhodamine B [38, 39], was determined and the results are presented in this paper
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