Renato J. Martins

Laser induced periodic surface structuring on Si by temporal shaped femtosecond pulses.

We investigated the effect of temporal shaped femtosecond pulses on silicon laser micromachining. By using sinusoidal spectral phases, pulse trains composed of sub-pulses with distinct temporal separations were generated and applied to the silicon surface to produce Laser Induced Periodic Surface Structures (LIPSS). The LIPSS obtained with different sub-pulse separation were analyzed by comparing the intensity of the two-dimensional fast Fourier Transform (2D-FFT) of the AFM images of the ripples (LIPSS). It was observed that LIPSS amplitude is more emphasized for the pulse train with sub-pulses separation of 128 fs, even when compared with the Fourier transform limited pulse. By estimating the carrier density achieved at the end of each pulse train, we have been able to interpret our results with the Sipe-Drude model, that predicts that LIPSS efficacy is higher for a specific induced carrier density. Hence, our results indicate that temporal shaping of the excitation pulse, performed by spectral phase modulation, can be explored in fs-laser microstructuring.

Effect of SLM pixelation on two-photon fluorescence by applying an off-centered quadratic spectral phase mask

Spatial light modulators (SLMs) have become a valuable tool to shape ultrashort laser pulses, which have found innumerous applications in different areas of ultrafast science. Despite the fact that this family of devices can be used to produce almost any arbitrary pulse shape, as long as enough bandwidth is available, there are limitations related to their discrete nature that may lead to distortions in the expected pulse shape. Here we investigate such collateral temporal distortions by quantifying the effect imposed by the discrete nature (pixelation) of liquid-crystal-based SLMs on the two-photon excited fluorescence signal of a conjugated polymer, when the central position of a quadratic spectral phase mask is scanned across the laser bandwidth. We conclude that the observed temporal distortions are related to replica pulses generated by an interplay between the additional linear phase resultant of an off-centered quadratic phase mask and pixelation of the applied phase mask.

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.

Nonlinear characterization of fs-laser written Gorilla Glass waveguides

In this work, we performed the nonlinear characterization of Gorilla Glass waveguides produced by fs-laser microfabrication with different writing parameters. Their nonlinear refractive index n2 was determined by the dispersive-scan technique. We found that the waveguides n2 values are lower than the one for the pristine material and they depend on the writing parameters. Raman spectroscopy measurements showed important structural modifications related to non-bridging oxygens (NBOs) on the fs-laser irradiated glass, which may lead to the reduction of the third-order polarizability.

Fabrication of waveguides in Gorilla Glass with fs-pulses and its nonlinear features (Conference Presentation)

Ultrafast waveguide fabrication has been an active research area since its demonstration, leading to numerous applications. Recently reported high quality waveguide in Gorilla Glass has promoted it as a good candidate for optical devices. In this study, 120-fs laser pulses centered at 520, 650 and 775 nm at a repetition rate of 1 kHz were applied to investigate the influence of the wavelength on micromachining. Grooves ablated onto Gorilla Glass surface with different pulse energies and scanning speeds presented similar features and threshold pulse energy, regardless the excitation wavelength. Fifteen millimeter long waveguides were produced 100 μm below sample surface with pulse energy varying from 250 nJ up to 5 μJ (scanning speed of 200 μm/s). Waveguides longitudinal and transversal profiles were analyzed via optical microscopy and its guiding properties characterized in an objective-lens based coupling system at 633 and 775 nm. Guide modes intensity distribution show that for waveguides fabricated with higher pulse energy light is guided further from the core, while for lower fabrication energy light is guided closer to the center in a more fundamental mode. Considering that light traveling through 15 mm of material in confined mode, we coupled 775 nm fs-pulses into fabricated waveguides. By monitoring the spectrum of the guided light as input pulse energy increased, spectral broadening assigned to self-phase modulation effects was observed followed by white-light generation starting at 450 nm. In conclusion, we found that micromachining on Gorilla Glass is wavelength independent and inscribed waveguides present desirable nonlinear features.

Coherent Control of the Emission Bands of ZnO using Genetic Algorithms

We investigate the coherent control of the emission bands in ZnO crystal by pulse shaping. The coherent control was made through genetic algorithm based software. We have observed a significant increase in the ZnO.