Effect of transmission characteristics of femtosecond laser focusing on micro-/nanostructure morphology of titanium alloy

Various types of micro-/nanostructures can be fabricated by femtosecond laser irradiation on metal surfaces. It has received extensive attention and research because of its potential application value. In this paper, the effect of laser focusing position on the micro-/nanostructure formation on the surface of a titanium alloy (TC4) was systematically researched. The experimental results show that, in the process of femtosecond laser focusing, a spatial distribution of laser energy with a narrower beam center and a ring around the beam is formed in front of the geometric focus and the distribution of laser energy changes along the laser transmission direction. This property can affect the morphology of the micro-/nanostructure on the surface of irradiated materials. It is believed that the new spatial distribution of laser fluence caused by focusing is due to the nonlinear process of plasma defocus and laser self-focusing caused by multi-photon or tunnel ionization in front of the target by focusing the femtosecond laser. The discovery of the nonlinear characteristics of the focused femtosecond laser and the application of the unique energy spatial distribution to the surface morphology of the fabricated surface put forward a new idea for the direct fabrication of micro-/nanostructures via femtosecond laser, which has certain significance for the development of micro-/nanofabrication technology of femtosecond lasers.