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.

Precise nanostructuration of surface and the subsequent upgrades in material properties is a strong outcome of ultra fast laser irradiations. Material characteristics can be designed on mesoscopic scales, carrying new optical properties. We demonstrate in this work, the possibility of achieving material modifications using ultra short pulses, via polarization dependent structures generation, that can generate specific color patterns. These oriented nanostructures created on the metal surface, called ripples, are typically smaller than the laser wavelength and in the range of visible spectrum. In this way, a complex colorization process of the material, involving imprinting, calibration and reading, has been performed to associate a priori defined colors. This new method based on the control of the laser-driven nanostructure orientation allows cumulating high quantity of information in a minimal surface, proposing new applications for laser marking and new types of identifying codes.

The sensitivity of grating-coupled Surface Plasmon Polaritons (SPPs) on metallic surface has been exploited to investigate the correlation between ripples formation under ultrashort laser exposure and SPPs generation conditions. Systematic examination of coupling of single ultrashort laser pulse on gratings with appropriate periods ranging from 440 nm to 800 nm has been performed. Our approach reveals that a surface plasmon is excited only for an appropriate grating period, the nickel sample exhibits fine ripples pattern, evidencing the plasmonic nature of ripples generation. We propose a systematic investigation supported by a comprehensive study on the obtained modulation of such a coupling efficiency by means of a phenomenological Drude-Lorentz model which captures possible optical properties modification under femtosecond irradiation.

Ablation of Cu and Al targets has been performed with 170 fs laser pulses in the intensity range of 10 12 -10 14 W/cm 2 . We compare the measured removal depth with 1D hydrodynamic simulations. The electron-ion temperature decoupling is taken into account using the standard two-temperature model. The influence of the early heat transfer by electronic thermal conduction on hydrodynamic material expansion and mechanical behavior is investigated. A good agreement between experimental and numerical matter ablation rates shows the importance of including solid-to-vapor evolution of the metal in the current modeling of the laser matter interaction.

International audienceThe electronic behavior of various solid metals (Al, Ni, Cu, Au, Ti, and W) under ultrashort laser irradiation is investigated by means of density functional theory. Successive stages of extreme nonequilibrium on picosecond time scale impact the excited material properties in terms of optical coupling and transport characteristics. As these are generally modelled based on the free-electron classical theory, the free-electron number is a key parameter. However, this parameter remains unclearly defined and dependencies on the electronic temperature are not considered. Here, from first-principles calculations, density of states are obtained with respect to electronic temperatures varying from 10^-2 to 10^5 K within a cold lattice. Based on the concept of localized or delocalized electronic states, temperature dependent free-electron numbers are evaluated for a series of metals covering a large range of electronic configurations. With the increase of the electronic temperature we observe strong adjustments of the electronic structures of transition metals. These are related to variations of electronic occupation in localized d bands, via change in electronic screening and electron-ion effective potential. The electronic temperature dependence of nonequilibrium density of states has consequences on electronic chemical potentials, free-electron numbers, electronic heat capacities, and electronic pressures. Thus electronic thermodynamic properties are computed and discussed, serving as a base to derive energetic and transport properties allowing the description of excitation and relaxation phenomena caused by rapid laser action

International audienceUltrafast laser irradiation can trigger anisotropically structured nanoscaled gratinglike arrangements of matter, the laser-induced periodic surface structures (LIPSSs). We demonstrate here that the formation of LIPSS is intrinsically related to the coherence of the laser field. Employing several test materials that allow large optical excursions, we observe the effect of randomizing spatial phase in generating finite domains of ripples. Using three-dimensional finite-difference time-domain methods, we evaluate energy deposition patterns below a material's rough surface and show that modulated pattern, i.e., a spatially ordered electromagnetic solution, results from the coherent superposition of waves. By separating the field scattered from a surface rough topography from the total field, the inhomogeneous energy absorption problem is reduced to a simple interference equation. We further distinguish the contribution of the scattered near field and scattered far field on various types of inhomogeneous energy absorption features. It is found that the inhomogeneous energy absorption which could trigger the low-spatial-frequency LIPSSs (LSFLs) and high-spatial-frequency LIPSSs (HSFLs) of periodicity Λ>λ/Re(n˜) are due to coherent superposition between the scattered far field (propagation) and the refracted field, while HSFLs of Λ<λ/Re(n˜) are triggered by coherent superposition between the scattered near field (evanescent) and the refracted field. This is a general scenario that involves a topography-induced scattering phenomenon and stationary evanescent fields, being applied to two model case materials that exhibit large optical excursions upon excitation (W, Si) and nonplasmonic to plasmonic transitions. We indicate the occurrence of a general light interference phenomenon that does not necessarily involve wavelike surface plasmonic excitation. Finally, we discuss the role of interference field and scattered field on the enhancement of LIPSSs by simulating interpulse feedback effects and provide the electromagnetic origin of grooves (Λ>λ) related to a feedback-driven topography evolution. Those results strongly suggest the electromagnetic interpretation of LIPSSs in interplay with an evolving surface topograph

Femtosecond laser-induced volume nanograting formation is numerically investigated. The developed model solves nonlinear Maxwell's equations coupled with multiple rate free carrier density equations in the presence of randomly distributed inhomogeneities in fused silica. As a result of the performed calculations, conduction band electron density is shown to form nanoplanes elongated perpendicular to the laser polarization. Two types of nanoplanes are identified. The structures of the first type have a characteristic period of the laser wavelength in glass and are attributed to the interference of the incident and the inhomogeneity-scattered light waves. Field components induced by coherent multiple scattering in directions perpendicular to the laser polarization are shown to be responsible for the formation of the second type of structures with a subwavelength periodicity. In this case, the influence of the inhomogeneity concentration on the period of nanoplanes is shown. The calculation results not only help to identify the physical origin of the self-organized nanogratings, but also explain their period and orientation.

The capability to organize matter in spontaneous periodic patterns under the action of light is critical in achieving laser structuring on sub-wavelength scales. Here, the phenomenon of light coupling to Marangoni convection flows is reported in an ultrashort laser-melted surface nanolayer destabilized by rarefaction wave resulting in the emergence of polarization-sensitive regular nanopatterns. Coupled electromagnetic and compressible Navier-Stokes simulations are performed in order to evidence that the transverse temperature gradients triggered by non-radiative optical response of surface topography are at the origin of Marangoni instability-driven self-organization of convection nanocells and high spatial frequency periodic structures on metal surfaces, with dimensions down to λ/15 (λ being the laser wavelength) given by Marangoni number and melt layer thickness. The instability-driven organization of matter occurs in competition with electromagnetic feedback driven by material removal in positions of the strongest radiative field enhancement. Upon this feedback, surface topography evolves into low spatial frequency periodic structures, conserving the periodicity provided by light interference.