Fabrication of micro-pillar with high aspect ratio on monocrystalline diamond by galvanometer-assisted femtosecond laser milling

Diamond micro/nano-pillar is widely applied in quantum transportation and nanomechanics, but it is very difficult to be machined by conventional mechanical methods, duo to its feature of high hardness, wear-proof and extremely small scale. Thus, a novel strategy of galvanometer-assisted femtosecond laser milling (GFM) was proposed to overcome the difficulty of fabricating the micro-pillar with high aspect ratio on monocrystalline diamond. Through numeric calculation, it is found that GFM trajectory possesses the special distribution consisting of separate pulse-concentrated regions and expanded flat-distributed region, which was then experimentally proved beneficial for improving the energy consumption and highly promoting the material removal rate. Moreover, the GFM was found to be much easier to generate micro-pillar, whereas in the case of conventional non-galvanometer milling (NGM) micro-pillar underwent severe degeneration or height loss under various average laser powers. Through optimizing the machining parameters, the micro-pillar was successfully prepared with an aspect ratio of as high as 42.25 by GFM. A 5 x 5 micro-pillar array of monocrystalline diamond was also fabricated with consistent diameter, height and aspect ratio. With Raman spectroscopy, the graphitization induced by GFM was slight, though it was unavoidably introduced with relatively high average laser power. Besides, the graphitizing level on the lateral face of the machined micro-pillar was found to non-linearly change with different machining parameters, because the material removal mode underwent a transformation from graphitization dominated to sublimation dominated with the increasing of applied energy density.