Femtosecond laser processing has been extensively used in micromachining, especially for the precision processing of hard and brittle materials. However, the precision of the materials ablated by femtosecond laser is not easy to control. This paper reports an experimental and theoretical study on the ablation characteristics of fused silica using high repetition rate femtosecond laser. An experimental study of microchannels milling on the fused silica was carried out. The influence of pulse energy, repetition rate, scanning velocity, scanning times on the size and morphology of the microchannels was obtained. Simultaneously, the experimental data on the depth and width of microchannels under different parameter combinations were acquired through the orthogonal experiment. The prediction model of aspect ratio was obtained by BP neural network algorithm. Finally, the verification test was established and showed that the experimental results were consistent with the theoretical results. It would provide a theoretical basis for further study on the microchannels fabrication of femtosecond laser.
Reducing the reflection of silicon surface is an effective way to enhance its optical absorption performance in optical and optoelectronic devices. In this paper, the influence mechanism of heat accumulation effect existing in the material substrate on the multi-scale porosity properties of surface structure during femtosecond laser irradiation is investigated. Micro-nano structures will lose their multi-scale porous properties at high-repetition-rate laser irradiation due to excessive agglomeration, nucleation or melting. By rapidly cooling the material substrate, the porosity of surface micro-nano structure are optimized, and the antireflection performance of the material surface is improved obviously. Our study opens a novel and convenient route for preparation of broadband antireflective black silicon surfaces for various applications.
Micro-structures offer superior functions such as superhydrophobicity, selfcleaning, anti-wear and drag reduction. In this paper, various microstructures were fabricated by rear-side picosecond laser irradiation of two-layer materials. The material of underlying layer was commonly commercial available ink; the material of surface layer was PMMA. The high light absorptivity of underlying material result in significantly reduced absorption depth. The laser source could therefore be regarded as plane heat source, leading to better surface morphologies after the mater-laser interaction. The results showed that convex structures were obtained at a lower laser fluence; with increase of laser fluence, a doughnut-like structures were obtained; with further rise of laser fluence, bowl-like structures would be obtained. Moreover, the size of microstructures could be tuned by adjusting laser processing parameters such as laser power, frequency and laser-mater interaction time. This method provides an insight for fabrication of functional surface.
This study was performed using picosecond pulses to obtain the three-dimensional micro-nano-hierarchical porous structures on the surface of titanium via the combination of the ablation and the deposition of ablated particles. For the repetition rate of 100 kHz and the scanning speed of 10 mm/s, there were secondary nano-tree-like micro-nano structures via the deposition of the ablated material formed on the primary microstructures. However, when the scan speed decreased, the primary microstructures were invisible owing to too much material deposition. When the repetition rate increased to 500 kHz, the ablated particles were irradiated by the posterior pulse before deposition onto the surface of material, and agglomerated into spidernet-like nanostructures. Then, the upper layer of the secondary micro-nano structure was molten and came into micro-nano porous fractal structures. Both the secondary micro-nano porous structures showed the hierarchy in the vertical and horizontal direction of the surface of titanium. Contact angle measurement after 3 months indicated that the nano-tree-like micro-nano structures showed super-hydrophilcity, and the spidernet-like nano and micro-nano porous fractal structures showed super-hydrophobicity.
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