Fig. 1. Schematic of the experimental setup. BS, beam splitter; DM, dichroic mirror; L1, achromatic doublet; L2, plano-convex lens; P, polarizer; WS, white-light source.

Fig. 2. (a)–(c) SEM images of LIPSS on the surface of nickel at different scanning speeds. Scale bars, 2 μm; (d) and (f) high-magnification AFM 3D images at ν=300 and 100 μm/s; (e) and (g) cross sectional profiles for (d) and (f). Laser fluence, fixed at 0.15  J/cm2. AFM scanning area, 3  μm×3  μm. Scanning direction and polarization orientation are both horizontal.

Fig. 3. Transition boundary of LIPSS morphology on the surface of nickel with respect to the scanning speed and the laser fluence. Parameters for the SEM images, inset, of HSFL and LSFL are 0.1  J/cm2 and 150 μm/s, and 0.12  J/cm2 and 400 μm/s, respectively. Scale bars, 2 μm.

Fig. 4. Schematic diagram of the HSFL area and the LIPSS area. Scanning direction and the polarization orientation are both horizontal. Processing parameters, 0.12  J/cm2, 100 μm/s, and 300 fs pulse delay, respectively. Scale bar, 5 μm.

Fig. 5. (a)–(c) Partially magnified SEM images of the LIPSS at different θ on the surface of nickel. Scale bar, 2 μm; (d) schematic diagram of the direct scanning process; (e) K as a function of θ at different laser fluences. Scanning speed, fixed at 100 μm/s (N=450).

Fig. 6. K as a function of pulse delay on the surface of nickel. Total laser fluence of the dual-pulse train and the scanning speed, fixed at 0.15  J/cm2 and 100 μm/s (N=450).

Fig. 7. (a) K as a function of θ at different pulse delays; (b) K as a function of pulse delay at different θ. Total laser fluence of the dual-pulse train and the scanning speed, fixed at 0.15  J/cm2 and 100 μm/s (N=450).