Coherence effects in LIPSS formation on silicon wafers upon picosecond laser pulse irradiations

Using different laser irradiation patterns to modify of silicon surface, it has been demonstrated that, at rather small overlapping between irradiation spots, highly regular laser-induced periodic surface structures (LIPSS) can be produced already starting from the second laser pulse, provided that polarization direction coincides with the scanning direction. If the laser irradiation spot is shifted from the previous one perpendicular to light polarization, LIPSS are not formed even after many pulses. This coherence effect is explained by a three-wave interference, - surface electromagnetic waves (SEWs) generated within the irradiated spot, SEWs scattered from the crater edge formed by the previous laser pulse, and the incoming laser pulse, - providing conditions for amplification of the periodic light-absorption pattern. To study possible consequences of SEW scattering from the laser-modified regions, where the refractive index can change due to material melting, amorphization, and the residual stress formed by previous laser pulses, hydrodynamic modelling and simulations have been performed within the melting regime. The simulations show that stress and vertical displacement could be amplified upon laser scanning. Both mechanisms, three-wave interference and stress accumulation, could enable an additional degree of controlling surface structuring.