Polarization-Dependent Scattering of Nanogratings in Femtosecond Laser Photowritten Waveguides in Fused Silica.

The properties of polarization-selective, light-guiding systems upon subwavelength nanogratings formation in the case of type II refractive index traces induced by femtosecond laser pulses in bulk fused silica were studied. Polarization-dependent scattering is analyzed both in simulation using a finite-difference, time-domain method and in experiments. We argue that the polarization-sensitive optical guiding of type II waveguides is due to polarization-dependent scattering of nanogratings. Optical designs can then be suggested where the guiding efficiency of type I traces can be combined with type II anisotropies. A low-loss waveguide polarizer is demonstrated based on the modulation of the evanescent field emerging from type I waveguides using polarization-dependent scattering of neighboring nanogratings.

Systematic study of laser ablation with GHz bursts of femtosecond pulses.

We report on crater formation, line scribing and cavity milling experiments on Silicon, Copper, Aluminum and stainless steel with GHz bursts of femtosecond pulses. The intra-burst repetition rate has been varied between 0.88 and 3.52 GHz, the number of pulses per burst between 50 and 3200, the burst fluence between 8 and 80 J/cm2. For these experiments, a 100-W femtosecond GHz-burst laser has been developed on an industrial laser basis, delivering a total burst energy up to 1 mJ at 100 kHz, with an adjustable number of pulses per burst. The results highlight the conditions to obtain high-ablation efficiency, show how to optimize the machining quality and point out the burst duration as the relevant parameter for femtosecond GHz machining.

Fused silica ablation by double femtosecond laser pulses: influence of polarization state.

Glass processing is a subject of high interest for many industrial fields such as optics manufacturing, smart electronics or medical devices. With respect to nanosecond technology, the use of femtosecond lasers allows to achieve high processing quality thanks to nonlinear absorption properties. Nevertheless, the throughput of femtosecond processing is still very low when compared to other laser technologies. Temporal and spatial pulse shaping is a smart and flexible solution to further increase the efficiency of femtosecond laser processing by driving efficiently both electron dynamics and absorption involved during laser irradiation. In the present work, the effect of temporal pulse shaping on fused silica ablation is investigated by single-wavelength (1030nm) double femtosecond pulses pump-pump experiment. Two sub-pulses are focused on the top surface of fused silica with two different polarization configurations: (i) orthogonally-crossed linear polarization or (ii) counter-rotating circular polarization. The investigated parameters are the pulse-to-pulse delay, set with a delay line, the total fluence and the polarization configuration. The results are discussed in term of optical transmission, modification and ablation thresholds, and ablated volume. A numerical model describing the electron dynamics and the absorbed energy density is also presented to support interpretation of experimental results. It is demonstrated that pulse-to-pulse delay has a major influence on ablated volume, modification and ablation threshold. Polarization state has also, to a lesser extent, a significant influence on ablated volume. Their cooperative effect on the ablation efficiency is discussed.

High-efficiency femtosecond ablation of silicon with GHz repetition rate laser source.

We report on silicon ablation with a 20 W GHz amplified femtosecond laser source. This novel laser delivers burst energies up to 400 µJ, providing flexible intra-pulse repetition rates of 0.88 or 3.52 GHz, up to 200 pulses with ∼350 fs pulse duration. High-efficiency, high-quality ablation can be achieved through optimally determining the number of pulses, intra-pulse repetition, and average pulse energy within a burst. Due to such optimization, we demonstrate a specific ablation rate of 2.5 mm3/min/W with a burst containing 200 pulses at 0.88 GHz, which is the highest one reported so far for fs laser ablation, to the best of our knowledge. GHz ablation is sensitive to the selection of laser parameters. We conceptually discuss the contributions of the pulses within a burst to heat-accumulation-based incubation and material ablation.

Dual-color control and inhibition of direct laser writing in silver-containing phosphate glasses.

We report on dual-color control of femtosecond direct laser writing (DLW) in a noncommercial silver-containing zinc phosphate glass, thanks to an additional illumination with a cw (continuous wave) UV laser, either after the femtosecond irradiation or simultaneously. By tuning the cw UV power, we demonstrate the tunable control and inhibition of the production efficiency of laser-induced fluorescent silver clusters, leading up to 100% inhibition for simultaneous co-illumination when the laser writing is performed close enough to the permanent structuring threshold. The role of the cw UV illumination is discussed in terms of inhibition of the silver cluster precursors or of dissolution of the laser-induced silver clusters. These results show the ability of laser writing inhibition in our photosensitive silver-containing phosphate glass, which is a necessary step to further develop super-resolution laser writing approaches, such as STED-like DLW, either of fluorescent silver clusters or of silver metallic nanoparticles with plasmonic properties.

Patterning linear and nonlinear optical properties of photosensitive glasses by femtosecond structured light.

We report on structured light-induced femtosecond direct laser writing (DLW) under tight focusing in non-commercial silver-containing zinc phosphate glass, which leads to original patterns of fluorescent silver clusters. These fluorescence topologies show unique features of frustrated diffusion of charged species, giving rise to distorted silver cluster spatial distributions. Fluorescence and second harmonic generation correlative microscopy demonstrate the realization of structured light-induced direct laser poling, resulting from a laser-induced permanent and stable electric field buried inside the modified glass. Thus, structured light-induced DLW remarkably enables both linear and nonlinear patterning. This work highlights the interest of optical phase engineering to obtain nontrivial beam profiles and subsequent photo-induced patterns that cannot be reached under Gaussian beam irradiation.