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We report experimental and theoretical studies of the two-photon absorption spectrum of two nitrofuran derivatives: nitrofurantoine, (1-(5-nitro-2-furfurilideneamine)-hidantoine) and quinifuryl, 2-(5′-nitro-2′-furanyl)ethenyl-4-{N-[4′-(N,N-diethylamino)-1′-methylbutyl]carbamoyl} quinoline. Both molecules are representative of a family of 5-nitrofuran-ethenyl-quinoline drugs that have been demonstrated to display high toxicity to various species of transformed cells in the dark. We determine the two-photon absorption cross-section for both compounds, from 560 to 880 nm, which present peak values of 64 GM for quinifuryl and 20 GM for nitrofurantoine (1 GM = 1×10-50cm4.s.photon-1). Besides, theoretical calculations employing the linear and quadratic response functions were carried out at the density functional theory level to aid the interpretations of the experimental results. The theoretical results yielded oscillator strengths, two-photon transition probabilities, and transition energies, which are in good agreement with the experimental data. A higher number of allowed electronic transitions was identified for quinifuryl in comparison to nitrofurantoine by the theoretical calculations. Due to the planar structure of both compounds, the differences in the two-photon absorption cross-section values are a consequence of their distinct conjugation lengths. [ABSTRACT FROM AUTHOR]

The study of excited state properties of chlorophyll a is a subject of foremost interest, given that it plays important roles in biological process and has also been proposed for applications in photonics. This work reports on the excited state absorption spectrum of chlorophyll a solution from 460 to 700 nm, obtained through the white-light continuum Z-scan technique. Saturation of absorption was observed due to the ground state depletion, induced by the white-light continuum region that is resonant with the Q band of chlorophyll a. The authors also observed reverse saturation of absorption related to the excitation from the first excited state to a higher energy level for wavelengths below 640 nm. An energy-level diagram, based on the electronic states of chlorophyll a, was employed to interpret their results, revealing that more states than the ones related to the Q and B bands participate in the excited state absorption of this molecule. [ABSTRACT FROM AUTHOR]

We report a single-beam heterodyne Z-scan method capable of measuring slow response nonlinear refraction of absorbers with excellent sensitivity. By imposing a small sinusoidal modulation on the incoming continuous wave laser beam and recording both the magnitude and phase of the intensity transmitted through a small aperture, we are able to eliminate parasitic linear effects and achieve a sensitivity good enough to determine the absorption coefficient of a 2 mm-thick water sample at 532 nm with moderate power. [ABSTRACT FROM AUTHOR]

Excited state absorption and excited state dynamics of indocyanine‐green (ICG) dissolved in dymethyl sulfoxide were measured using white‐light continuum Z‐scan (WLCZScan) and white‐light continuum pump–probe (WLCPP) techniques. The excited state absorption spectrum, obtained through Z‐scan measurements, revealed saturable absorption (SA) for wavelengths longer than 630 nm, while reverse saturable absorption (RSA) appeared, as indicated by a band at approximately 570 nm. Both processes were modeled by a three‐energy‐level diagram, from which the excited state cross‐section values were determined. SA and RSA were also observed in pump–probe experiments, with a recovery time in the hundreds of picoseconds time scale due to the long lifetime of the first excited state of ICG. Such results contribute to the understanding of ICG optical properties, allowing application in photonics and medicine. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Two-photon absorption spectra of a triarylamine compounds dissolved in toluene were measured using the well-known Z-scan technique, employing 120-fs laser pulse-width. According to the results, an extra band located at around 900nm was observed only for triarylamine with azoaromatic units. On the other hand, a shift in the two-photon absorption band for triarylamine, with and without azoaromatic units, is observed when different electron donor/acceptors groups are changed. The fitting of the spectra, using sum-over-states model, allowed us to obtain the spectroscopic parameters of each molecule, which appears to be in reasonable agreement with molecules presenting similar structural moieties. [ABSTRACT FROM AUTHOR]

Abstract: This work reports on the excited-state absorption spectrum of oxidized Cytochrome c (Fe3+) dissolved in water, measured with the Z-scan technique with femtosecond laser pulses. The excited-state absorption cross-sections between 460 and 560nm were determined with the aid of a three-energy-level model. Reverse saturable absorption was observed below 520nm, while a saturable absorption process occurs in the Q-band, located around 530nm. Above 560nm, a competition between saturable absorption and two-photon absorption was inferred. These results show that Cytochrome c presents distinct nonlinear behaviors, which may be useful to study electron transfer chemistry in proteins by one- and two-photon absorption. In addition, owing to these nonlinear optical features, this molecule may be employed in applications involving photodynamics therapy and saturable absorbers. [Copyright &y& Elsevier]

Abstract: This work reports the degenerate two-photon absorption spectrum for a series of Salen complexes possessing two azo dye units. The observed spectra peak at 1000nm, twice the wavelength of the linear absorption peak, and increase monotonically to the blue due to resonant enhancement of the non linearity. The two-photon absorption properties of these molecules were found to be an additive effect of chromophores and independent of the metal ions. The two-photon absorption spectra were described with a sum-over-states model, from which the transition dipole moment and dipole moment change were determined and compared with quantum chemistry calculation. [Copyright &y& Elsevier]

Abstract: In this work we have measured the nonlinear absorption spectrum of ytterbium bis-phthalocyanine (YbPc2) in chloroform solution employing the Z-scan technique with white-light continuum (WLC) pulses. Two distinct excited state absorption behaviors were observed: a saturable absorption (SA) at the Q-band region and a reverse saturable absorption (RSA) around 530nm. A three-energy-level diagram was used to explain the experimental results, leading to the excited state absorption cross-section determination from 500 up to 675nm. [Copyright &y& Elsevier]

Highlights: [•] We have obtained the excited states spectra of triplet and singlet states. [•] It was employed UV/Vis absorption, flash photolysis and Z-scan techniques. [•] S1 →Sn absorption spectra were obtained by white-light Z-scan technique. [•] T1 →Tm absorption was obtained by flash photolysis and pulse train Z-scan. [•] The vibronic structuration observed is probably caused by hydrogen bonds. [ABSTRACT FROM AUTHOR]