Paulo Henrique D. Ferreira

Femtosecond laser induced synthesis of Au nanoparticles mediated by chitosan.

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.

Metallic nanoparticles grown in the core of femtosecond laser micromachined waveguides

3D-waveguides containing silver nanoparticles have been fabricated in tungsten lead–pyrophosphate glass by femtosecond laser micromachining. Nucleation and growth of nanoparticles occur in a single step process when high repetition rate laser (MHz) is employed, while an additional annealing is required for the irradiation using kHz laser system. The presence of nanoparticles locally changes the refractive index, and, therefore, the elliptical structures produced by direct laser writing were able to guide light. By increasing the pulse energy applied during the micromachining, the waveguide size increased from 2 to 30 μm, while their propagation loss decrease from 1.4 to 0.5 dB/mm at 632.8 nm.

Femtosecond Laser Patterning of the Biopolymer Chitosan for Biofilm Formation

Controlling microbial growth is crucial for many biomedical, pharmaceutical and food industry applications. In this paper, we used a femtosecond laser to microstructure the surface of chitosan, a biocompatible polymer that has been explored for applications ranging from antimicrobial action to drug delivery. The influence of energy density on the features produced on chitosan was investigated by optical and atomic force microscopies. An increase in the hydrophilic character of the chitosan surface was attained upon laser micromachining. Patterned chitosan films were used to observe Staphylococcus aureus (ATCC 25923) biofilm formation, revealing an increase in the biofilm formation in the structured regions. Our results indicate that fs-laser micromachining is an attractive option to pattern biocompatible surfaces, and to investigate basic aspects of the relationship between surface topography and bacterial adhesion.

Enhancing multi-photon induced excitonic emission of ZnO single crystals by shaping fs laser pulses

We report on the control of multi-photon excited emission of the ZnO exciton band via spectral phase modulation of the femtosecond excitation pulses. It was observed that the optimum spectral phase that enhances the exciton emission results in a pulse-train temporal profile, whose separation corresponds to an energy of 82 meV, which is close to the energy of LO phonon sidebands in ZnO emission. Such a result suggests that exciton–LO phonon coupling can be explored to coherently enhance the exciton emission in ZnO single crystals with respect to the defect luminescence band.

Optical microdevices fabricated using femtosecond laser processing (Conference Presentation)

Femtosecond laser processing techniques have been widely employed to produce micro or nanodevices with special features. These devices can be selectively doped with organic dyes, biological agents, nanoparticles or carbon nanotubes, increasing the range of applications. Acrylate polymers can be easily doped with various compounds, and therefore, they are interesting materials for laser fabrication techniques. In this work, we use multiphoton absorption polymerization (MAP) and laser ablation to fabricate polymeric microdevices for optical applications. The polymeric sample used in this work is composed in equal proportions of two three-acrylate monomers; while tris(2-hydroxyethyl)isocyanurate triacrylate gives hardness to the structure, the ethoxylated(6) trimethyl-lolpropane triacrylate reduces the shrinkage tensions upon polymerization. These monomers are mixed with a photoinitiator, the 2,4,6-trimetilbenzoiletoxifenil phosphine oxide, enabling the sample polymerization after laser irradiation. Using MAP, we fabricate three-dimensional structures doped with fluorescent dyes. These structures can be used in several optical applications, such as, RGB fluorescent microdevices or microresonators. Using azo compounds like dopant in the host resin, we can apply these structures in optical data storage devices. Using laser ablation technique, we can fabricate periodic microstructures inside polymeric bulks doped with xanthene dyes and single-walled carbon nanotubes, aiming applications in random laser experiments. In structured bulks we observed multi-narrow emission peaks over the xanthene fluorescence emission. Furthermore, in comparison with non-structured bulks, we observed that the periodic structure decreased the degree of randomness, reducing the number of peaks, but defining their position.

Enhancement of Au nanoparticle formation by shaping fs-pulses

The influence of fs-pulse train period on the Au nanoparticles production was studied. Using a pulse shaping technique, the period was tunned in order to match Raman resonances that enhance the Au nanoparticle formation process.

A Simple Calibration Method for Liquid Crystal Pulse Shaper

We present a simple method to calibrate spatial light modulator (SLM) employed for pulse shaper. Such method is based on light scattering induced by abrupt index refraction change instead of traditional interferometric methods.

Study of Two-Photon Excited Fluorescence in MEH-PPV

The two-photon excited fluorescence of MEH-PPV was measured using femtosecond laser pulses. We carried out intensity and fluorescence spectrum measurements as a function of the ultrashort pulse shape.