Functional Surface Fabrication Of Micro/Nanostructures By Femtosecond Laser

A lot of interesting phenomenon in the evolution of millions of years, such as gecko leap over rooftops, colorful butter fly wings, and self-cleaning lotus are a source of inspiration for scientists and researchers. They found that the ordered uniform micro/nanostructures on the surface can improve material properties for mechanics, optical absorption and hydrophobicity, etc. Therefore, the development of efficient and economic techniques for producing functional surface micro/nanostructures has the great significance.Recently, with development of laser technique, especially femtosecond laser has promoted rapidly development in the field of micro/nanofabrication. Due to the characteristics of high fabricating precision, minimum micro-cracks and trivial heat-affected-zone, femtosecond laser micromachining technique makes it possible to fabricate the ordered uniform micro/nanostructures on the surface. Because of the pulse duration of femtosecond laser is considerably shorter than electron-lattice relaxation time. During the absorption of femtosecond pulse, the lattice temperature remains unchanged. Femtosecond laser-material interaction is highly nonequilibrium. Hence, the processing of femtosecond laser fabrication depends on the interaction between photons and electrons.Based on previous studies, this study focuses to reveal the interaction mechanism between the fs laser and materials theoretically and experimentally. We obtained the optimized parameters through lots of experiments, which affects the surface topography. Meanwhile, we propose a method with high-pressure nitrogen to improve processing technology and fabricate a large area and uniform substrate. We first propose a two-step approach that effectively combines fs laser fabrication and thermal dewetting process to obtain a nanoparticle array distributed throughout the micro/nano structure. The proposed technique provides an opportunity to equip microchips with the surface-enhanced Raman scattering(SERS) capabilities of high sensitivity, chemical stability, and homogeneous signals.The research work of this paper can be summarized as the following two:(1)We fabricate large and uniform periodic micro/nano structures on a silicon wafer using a fs laser pulse. Through the experiment, it is found that morphological characters are associated with laser polarization, pulse number, and machining environments. This research found that the scan direction is perpendicular to the fs laser polarization and the interval is 2μm with high-pressure nitrogen will raise processed quality.(2)We report a simple, cost-effective and repeatable method for fabricating a large-area and uniform substrate for SERS. The silicon, micro-machined by femtosecond laser, is coated with gold film, and then treated through dewetting process. Abundant and uniform Au NPs adhered on machined silicon structure will provide a SERS-active substrate with high sensitivity, chemical stability and spatial homogeneous signals. This research found that when the temperature rise rate is set at 30 ℃/min and kept at 1000℃ for 1h, the enhancement factor of the SERS substrate is 9.2×107 with a 5 nm-thick film coated. Moreover the greatest Raman intensity deviation is estimated to be in the range of 6% for the substrate exposed in the air for a month.