Spontaneous periodic ordering on the surface and in the bulk of dielectrics irradiated by ultrafast laser: a shared electromagnetic origin

Abstract

Periodic self-organization of matter beyond the diffraction limit is a puzzling phenomenon, typical both for surface and bulk ultrashort laser processing. Here we compare the mechanisms of periodic nanostructure formation on the surface and in the bulk of fused silica. We show that volume nanogratings and surface nanoripples having subwavelength periodicity and oriented perpendicular to the laser polarization share the same electromagnetic origin. The nanostructure orientation is defined by the near-field local enhancement in the vicinity of the inhomogeneous scattering centers. The periodicity is attributed to the coherent superposition of the waves scattered at inhomogeneities. Numerical calculations also support the multipulse accumulation nature of nanogratings formation on the surface and inside fused silica. Laser surface processing by multiple laser pulses promotes the transition from the high spatial frequency perpendicularly oriented nanoripples to the low spatial frequency ripples, parallel or perpendicular to the laser polarization. The latter structures also share the electromagnetic origin, but are related to the incident field interference with the scattered far-field of rough non-metallic or transiently metallic surfaces. The characteristic ripple appearances are predicted by combined electromagnetic and thermo-mechanical approaches and supported by SEM images of the final surface morphology and by time-resolved pump-probe diffraction measurements.

Femtosecond laser-induced herringbone patterns

“…Instead of having a uniform symmetry, the herringbone LIPSSs we produce have axial symmetry along the channel they form along and are orientated at an acute angle to the polarization, as opposed to standard LIPSSs, which tend to form either perpendicular or parallel to the laser's polarization [26]. Spatially modulated LIPSSs have been observed by others before, by producing periodic structures with differing orientation in different spatial locations [21,27,28]. However, unlike other spatially modulating LIPSS structures, herringbone LIPSSs are produced using only a single beam and form with axially symmetry.…”

Tribological performance of titanium samples oxidized by fs-laser radiation, thermal heating, or electrochemical anodization

“…A significant reduction of friction and wear was reported if a 100Cr6 ball of steel in engine oil as lubricant (regime of mixed friction) is used for reciprocating sliding experiments on fslaser processed titanium (Ti) or titanium alloy (Ti6Al4V). The fs-laser processing was performed at conditions where sub-micrometer sized nanostructures (so-called low spatial frequency LIPSS, LSFL [5][6][7]) were formed, covering the treated surface homogeneously. The positive tribological effect was attributed to the presence of additives contained in the engine oil covering the laser structured surfaces and preventing a direct metal to metal contact [2,3].…”

High-resolution material structuring using ultrafast laser non-diffractive beams

“…In parallel to direct focusing approaches, a different concept emerged for surface and bulk nanostructuring; the self-organization of periodic patterns (see for example Ref [19,20]. and the references therein) on excited materials.…”