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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]

This work employs the Z-scan technique with 120 fs pulses to investigate the singlet excited state absorption spectra of tetrapyridyl and tetrasulfonatophenyl porphyrins. We have used a three-energy-level model to adjust Z-scan curves in order to obtain the singlet excited absorption cross section from 460 to 800 nm. Starting from these values, we determine the spectra of the ratio between excited and ground singlet state absorption cross sections, whose values are as good as the best found in the literature for reverse saturable absorbers. The results obtained point these porphyrins as good candidates for applications in optical limiting of ultrashort pulses. [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: 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]

Abstract: This work demonstrates that the detuning of the fs-laser spectrum from the two-photon absorption band of organic materials can be used to reach further control of the two-photon absorption by pulse spectral phase manipulation. We investigate the coherent control of the two-photon absorption in imidazole–thiophene core compounds presenting distinct two-photon absorption spectra. The coherent control, performed using pulse phase shaping and genetic algorithm, exhibited different growth rates for each sample. Such distinct trends were explained by calculating the two-photon absorption probability considering the intrapulse interference mechanism, taking into account the two-photon absorption spectrum of the samples. Our results indicate that tuning the relative position between the nonlinear absorption and the pulse spectrum can be used as a novel strategy to optimize the two-photon absorption in broadband molecular systems. [Copyright &y& Elsevier]

Abstract: This Letter studies the degenerate two-photon absorption (2PA) spectra of three diazoaromatic compounds using Z-scan with fs-pulses. The 2PA spectra exhibit resonant enhancement of the nonlinearity as the excitation approaches the linear absorption. The absence of 2PA to the ππ∗ band is related to the weaker donor/acceptor groups, and lower symmetry of diazoaromatic compounds, in agreement with our semi-empirical calculations. The higher magnitude of the 2PA cross-section, in comparison with single azoaromatics, demonstrates the positive effect of increasing conjugation. The decrease of 2PA cross-section with temperature is attributed to thermally induced torsions, which decreases the effective conjugation of diazoaromatic molecules. [Copyright &y& Elsevier]