Controllable Fabrication Of Surface Micro/Nano Structure By Femtosecond Laser Pulse Train For Surface Enhanced Raman Scattering

Femtosecond ultrashort laser micromachining has been extensively researched for the last decade, due to its unique advantages. The development of femtosecond laser pulse train offered an effective tool for micro/nano manufacturing, in which high quality, high efficiency and high accuracy are obtained. Based on the theory of electron dynamic control, we design the femtosecond laser pulse train and apply it to the process of materials. By controlling the excitation and ionization of electrons, we can control the subsequent phase change process and photoreduction. Based on the previous research, we fabricated several effective surface enhanced Raman scattering substrates making use of femtosecond laser pulse train. The main contents of this issue can be summarized as the following three:(1) We fabricated an effective SERS substrate by linealy polarized femtosecond laser pulse train in two steps. By adjusting the direction of linearly polarized laser, nanopillar structures were induced. When the pulse delay Δt=1000 fs, the highest enhancement factor was obtained, which is 1.1×109. We demonstrated the advantages of nanopillar structure over nanoripple structure in both experimental and theoretical aspects.(2) Applying circularly polarized femtosecond laser pulse train, we fabricated another effective SERS substrate in silver nitrate solution in one step. A composite surface structure consist of nanoparticles and micro bulges were induced on the silicon sample, on which silver nanoparticles were reduced and deposited. After the optimization of femtosecond laser parameters, which include laser fluence, scanning speed and pulse delay, the highest enhancement factor was obtained, which is 1.12×107.(3) Femtosecond laser single pulse were used to fabricate silicon sample in silver nitrate solution. It was demonstrated that besides the silver nanoparticles deposited on the silicon surface, the majority of them were out there floating in the solution. After process, the silicon sample were submerged in the solution for several days. Then the silver nanoparticles deposited on the silicon surface act as crystal nucleus, and gradually grow to be silver nanosheets, which were 70-80 nm in thickness. The silver nanosheets were effective SERS substrates, and its enhancement factor was estimated to be 2.21×107. One advantage of this kind substrate is that it would improve processing efficiency on a large scale.