In-situ high-resolution visualization of laser-induced periodic nanostructures driven by optical feedback

Abstract

Optical feedback is often evoked in laser-induced periodic nanostructures. Visualizing the coupling between surfaces and light requires highly-resolved imaging methods. We propose in-situ structured-illumination-microscopy to observe ultrafast-laser-induced nanostructures during fabrication on metallic glass surfaces. This resolves the pulse-to-pulse development of periodic structures on a single irradiation site and indicates the optical feedback on surface topographies. Firstly, the quasi-constancy of the ripples pattern and the reinforcement of the surface relief with the same spatial positioning indicates a phase-locking mechanism that stabilizes and amplifies the ordered corrugation. Secondly, on sites with uncorrelated initial corrugation, we observe ripple patterns spatially in-phase. These feedback aspects rely on the electromagnetic interplay between the laser pulse and the surface relief, stabilizing the pattern in period and position. They are critically dependent on the space-time coherence of the exciting pulse. This suggests a modulation of energy according to the topography of the surface with a pattern phase imposed by the driving pulse. A scattering and interference model for ripple formation on surfaces supports the experimental observations. This relies on self-phase-stabilized far-field interaction between surface scattered wavelets and the incoming pulse front.

Laser-induced periodic surface structures: Arbitrary angles of incidence and polarization states

“…In addition to the ability to model arbitrary polarization states and incidence angles, further advantages of the FDTD approach over the efficacy theory include the ability to model topography-driven interpulse feedback effects [11,39] and to study surface topography in the spatial domain. The interpulse feedback mechanism has been shown as the main process driving the final rippled topography [37,[69][70][71]. The interpulse feedback effect here is taken into account by the socalled holographic ablation model (HAM) first introduced by Skolski et al [11]: After the simulation of the first pulse, the topography of the sample is updated according to the calculated energy absorption, implying a change of material permittivity to 1 (material removal) when the local energy density exceeds the ablation criterion.…”

“…To overcome theses limitations, a numerical approach [10][11][12][13][33][34][35][36][37][38] adopting the finite-difference time-domain (FDTD) method was developed within the last decade to simulate the formation of LIPSSs. The main advantages of the FDTD approach over the pioneering efficacy theory are the flexibility to model light-matter interactions for any geometries directly from Maxwell's equations, a natural and straightforward incorporation of both topography-driven interpulse feedback effect [11,12,35], and intra-pulse feedback of permittivity dynamics [13,33,34].…”

“…Very recently, multiphysical simulations combining Maxwell's equations and Navier-Stokes equations were proposed to unravel the origin of LIPSSs beyond the laser absorption stage, in which the first stage is to model the absorption stage using the FDTD method [39,40]. Despite these fruitful outcomes, to the authors' knowledge, all the published FDTD approaches on LIPSSs are restricted to normal incidence and linearly polarized light [10][11][12][13][33][34][35][36][37][38][39] with only very few exceptions on nonlinearly polarized beams at normal incidence [41]. Yet, it is widely known that the incident angle is an important parameter which determines the period [42][43][44][45][46][47][48] and even the orientation of LSFLs [18].…”

The Role of the Laser-Induced Oxide Layer in the Formation of Laser-Induced Periodic Surface Structures

“…Their periodicity (Λ LIPSS ) usually ranges from hundreds of nanometers up to some micrometers and it is used to classify them into the general categories as low-spatial frequency LIPSS (LSFL), when Λ LSFL ∼ λ, and high-spatial frequency LIPSS (HSFL) for Λ HSFL λ, where λ is the laser wavelength [2]. Suitable manufacturing strategies have been identified, including the optimization of laser processing parameters (laser fluence, wavelength, repetition rate, angle of incidence, number of pulses per spot area) [3][4][5][6][7][8], material properties (optical, thermal and mechanical properties) [9][10][11], and the ambient medium in which they are generated (air, vacuum, reactive atmospheres) [12][13][14][15] for applications in optics, medicine, fluid transport and tribology among others [1].…”

Chemical effects during the formation of various types of femtosecond laser-generated surface structures on titanium alloy

“…Depending on the specific final application, other factors may be relevant such as the chemistry of the irradiated areas, the resulting crystallinity, and its final optical response [5]. In recent years, such periodic surface structures have been fabricated systematically for applications in optics, medicine, fluid transport, wetting, and tribology [5][6][7][8][9][10], exploring different laser processing parameters (fluence, wavelength, repetition rate, polarization, angle of incidence, number of pulses per spot) [5,[11][12][13][14][15][16][17][18][19], in different atmospheres (air, vacuum, reactive gasses) [20][21][22][23]. The simplest case of LIPSS is represented by parallel lines that are formed at the sample surface usually perpendicular to the laser polarization with periods that can be 100 times smaller than the laser beam diameter.…”