N.M. Barbosa Neto

Investigation of Ground- and Excited-State Photophysical Properties of 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphyrin with Ruthenium Outlying Complexes

The present work employs a set of complementary techniques to investigate the influence of outlying Ru(II) groups on the ground- and excited-state photophysical properties of free-base tetrapyridyl porphyrin (H(2)TPyP). Single pulse and pulse train Z-scan techniques used in association with laser flash photolysis, absorbance and fluorescence spectroscopy, and fluorescence decay measurements, allowed us to conclude that the presence of outlying Ru(II) groups causes significant changes on both electronic structure and vibrational properties of porphyrin. Such modifications take place mainly due to the activation of nonradiative decay channels responsible for the emission quenching, as well as by favoring some vibrational modes in the light absorption process. It is also observed that, differently from what happens when the Ru(II) is placed at the center of the macrocycle, the peripheral groups cause an increase of the intersystem crossing processes, probably due to the structural distortion of the ring that implies a worse spin-orbit coupling, responsible for the intersystem crossing mechanism.

Singlet excited state absorption of porphyrin molecules for pico- and femtosecond optical limiting application

This work employs the Z-scan technique with 120fs 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 460to800nm. 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.

High-efficiency multipass optical limiter

We report experimental results on a highly efficient optical limiter that uses a three-mirror, multipass ring configuration. The optical limiters enhanced performance is based on the accumulative effect that is achieved when the laser beam passes through the sample several consecutive times. Its feasibility was demonstrated for media with different limiting action processes, namely, reverse saturable absorption, nonlinear scattering, and thermal lensing. We employed a frequency-doubled, Q-switched, 10-ns Nd:YAG laser operating at a 5-Hz repetition rate.

INFLUENCE OF PHOTODEGRADATION ON THE OPTICAL LIMITING PROCESS OF CHLOROPHYLL A

The influence of photodegradation on the optical limiting process of chloroform solutions of chlorophyll A is investigated. Optical limitation experiments were carried out with a frequency-doubled, 10 ns Nd:YAG laser at a 5 Hz repetition rate, as a function of sample degradation. Optical limitation becomes less effective in photodegraded samples, which may be explained by the decrease in the concentration of chlorophyll A molecules. It is concluded that the optical limiting process is mainly related to the reverse saturated absorption mechanism, and contributions from light scattering due to photodegradation and the cavity bubbles appear irrelevant.

The influence of pH in nonresonant third-order nonlinearities of amino acid solutions

Abstract We report on the use of the Z -scan technique with femtosecond pulses at 775 nm to investigate the electronic charge delocalization in l -arginine, l -proline and l -serine amino acids. We have observed a variation in the nonresonant nonlinear transmittance with the pH, which reflects the variation in the relative concentration of the cationic and anionic forms and, consequently, the electronic charge delocalization. For l -serine, for instance, at a pH around 6.0 the n 2 rises to its maximum value corresponding to the one expected for the maximum concentration of the zwitterionic form. A good correspondence between the measured nonlinear transmittance variation and the titration curve was observed for the three amino acid samples, in the pH range studied. The n 2 values measured range from 10 −17 to 10 −16 cm 2 /W for pH varying from about 2 to 9.2.

Dynamic optical nonlinearities in di-furfuryl ether solution

Dynamic nonlinear refraction and absorption of di-furfuryl ether dissolved in dichloro-methane are investigated with a frequency-doubled Q-switched and mode-locked Nd:YAG laser. The nonlinear absorption presents a strong reverse saturation that seems promising for use in optical limiting devices. Three contributions are observed for the nonlinear refraction: one fast process related to the singlet population, and two slow accumulative contributions arising from the triplet population and thermal lensing. The time evolution of the optical nonlinearities, modeled by means of a five-energy-level diagram, allows the determination of excited state cross-sections as well as the intersystem crossing lifetime.