Orientation of periodic grating structures controlled by double-pulse irradiation

The formation of laser-induced periodic surface structures (LIPSS) is demonstrated on a titanium surface irradiated by a double-pulse beam cross-polarized at time delays from Δτ = 0–1 ps. The first-pulse fluence F1 and the delayed pulse fluence F2 are kept below the formation threshold FTH = 65 mJ/cm2 of the periodic grating structure on Ti. We find that periodic grating structures with LIPSS orientation of 45° relative to both polarizations are produced at delay times of 0 and 120 fs. To control LIPSS orientation, a double-pulse beam with a time delay of 0 fs is demonstrated in which a beam composed of a first pulse maintaining constant fluence of F1 = 70 mJ/cm2 and a delayed pulse varying from F2 = 0–70 mJ/cm2. The LIPSS orientations are in a range of 0°–45° and decrease as the normalized fluence F2/F1 decreases. We find that the orientation of LIPSS produced by double-pulse irradiations is in relatively good agreement with the direction obtained by the vector sum of laser fields $$E_{1}^{4}$$E14 and $$E_{2}^{4}$$E24. This tendency suggests that a multi-photon process at the metal surface might characterize the LIPSS orientation.