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This paper reports the synthesis of Au nanoparticles by 30-fs pulses irradiation of a sample containing HAuCl4 and chitosan, a biopolymer used as reducing agent and stabilizer. We observed that it is a multi-photon induced process, with a threshold irradiance of 3.8 × 10(11) W/cm2 at 790 nm. By transmission electron microscopy we observed nanoparticles from 8 to 50 nm with distinct shapes. Infrared spectroscopy indicated that the reduction of gold and consequent production of nanoparticles is related to the fs-pulse induced oxidation of hydroxyl to carbonyl groups in chitosan.

We report on a method to study the dynamics of triplet formation based on the fluorescence signal produced by a pulse train. Basically, the pulse train acts as sequential pump-probe pulses that precisely map the excited-state dynamics in the long time scale. This allows characterizing those processes that affect the population evolution of the first excited singlet state, whose decay gives rise to the fluorescence. The technique was proven to be valuable to measure parameters of triplet formation in organic molecules. Additionally, this single beam technique has the advantages of simplicity, low noise and background-free signal detection.

Objective: We investigated the main characteristics of human dental tissue under Nd:YAG picosecond laser ablation., Summary Background Data: The use of ultrashort laser pulses for teeth ablation prevents overheating and is an alternative for mechanical material removal; it also minimizes the volume of damaged material., Methods: Laser pulses of picosecond at 15 Hz repetition rates from a Q-switched and mode-locked Nd:YAG laser were focused on sound human molars for 30 seconds. Variation of light intensity in the pulse train allowed us to obtain drilled holes with different characteristics. Enamel and dentin surfaces were examined by optical and scanning electron microscope (SEM). The samples consisted of three sound human molars. The ablation rate was determined after taking an average of all samples., Results: Images from the SEM showed an interesting contrast between the morphology of the ablated enamel and dentin regions. In enamel, the ablated region appears to be more superficial than in dentin. The dentin fragility normally causes cracks that originate in the ablated region. The ablation rates in both enamel and dentin demonstrate a saturation behavior as the laser intensity increases. Furthermore, the ablation rate in dentin is about eight times greater than in enamel for the same laser fluence., Conclusion: Our results show an important correlation between the surface morphology and the pulsed laser fluence, which is compatible to the ablation mechanisms presented when ultrashort laser pulses are used.

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 reports on the two-photon absorption (2PA) cross-section for bis (n-butylimido) perylene (BuPTCD), bis (benzimidazo) perylene (AzoPTCD), bis (benzimidazo) thioperylene (Monothio BZP), bis (phenethylimido) perylene (PhPTCD), bis (benzylimido) perylene (BePTCD) and n-pentylimido-benzimidazo perylene (PAzoPTCD) solutions measured using the Z-scan technique with femtosecond laser pulses at 775 nm. All perylene derivatives studied present large 2PA cross-sections, only comparable to the best ones reported in the literature. The experimental 2PA cross-sections obtained for each PTCD compound are in good agreement with a simplified sum-over-state calculation. The results of the present work indicates perylene derivatives as promising materials for two-photon applications. [Copyright &y& Elsevier]

Abstract: We present a method that uses the two-photon induced thermal lens effect as feedback signal in an ultrafast pulse shaping technique for femtosecond pulse optimization in a closed-loop evolutionary algorithm. This approach presents robustness equivalent to those that employ other nonlinear processes but has the advantage that the sample does not need to be fluorescent neither harmonic generator. The method was demonstrated in the optimization of pulses from a Kerr-lens modelocked Ti:sapphire laser using fluorescent and non-fluorescent samples. [Copyright &y& Elsevier]