Transformation from nano-ripples to nano-triangle arrays and their orientation control on titanium surfaces by using orthogonally polarized femtosecond laser double-pulse sequences

One-dimensional and two-dimensional laser-induced periodic structures (nano-ripples and nano-triangle arrays) are fabricated on titanium surfaces by direct scanning with orthogonally polarized and equal-energy femtosecond laser double-pulse sequences (OP pulses). Clear and regular nano-ripples and nano-triangle arrays are formed when the laser fluence is slightly higher than the ablation threshold of titanium and the time delay is approximately 2 ps or shorter. By changing only the scanning speed, we achieve the transformation from nano-ripples to nano-triangle arrays. The orientation of nano-ripples and that of one of the three sides of nano-triangle arrays are always perpendicular to the scanning direction rather than the laser polarization. The mechanism underlying this phenomenon is as follows. First, the nano-ripples are formed due to periodic energy deposition along the direction of surface waves (surface plasmon polaritons; SPPs), which is along the scanning direction during the scan process. Subsequently, the preformed nano-ripples irradiated with subsequent laser pulses undergo hexagonal convection flow and are transformed into nano-triangle arrays. The preformed nano-ripples perpendicular to the scanning direction have a positioning effect on the formation of nano-triangle arrays, as verified in two-step fast scan experiments. Large-area patterning of nano-triangle arrays producing three-directional structural color is demonstrated in this paper.