Self-organization of surfaces on the nanoscale by topography-mediated selection of quasi-cylindrical and plasmonic waves

Abstract

Using coupled electromagnetic and hydrodynamic calculations, we elucidate theoretically the topographic transition from a random metallic surface to a periodic sub-wavelength grating by ultrashort laser ablation. The origin of this transition lies in the successive selection of hybrid surface waves scattered by random nanoholes. Contrary to the common belief that surface plasmon polaritons play the dominant role in the process and define the grating periodicity, we show that both quasi-cylindrical and surface plasmon waves are involved, whereas the diversity in the resulting spacings λ/2–λ (λ is the laser wavelength) is the manifestation of a broad frequency overlap of these waves, controlled by their relative phase shifts with respect to the plasmonic counterparts. The topography evolution imposes the dominant contribution to the surface sub-wavelength pattern by selecting the appropriate wave character from plasmonic modes to evanescent cylindrical waves. With the radiation dose, the grating periodicity exhibits a pronounced blue shift due to reinforced dipole–dipole coupling between the nanoholes and surface curvatures in the laser-processed area. This allows the creation of regular patterns with tunable periodicity.

Maxwell Meets Marangoni—A Review of Theories on Laser‐Induced Periodic Surface Structures

“…When considering a large set of such nanoholes interacting collectively, the numerical simulations revealed the transition from a statistically rough surface to well-ordered quasi-periodic LSFL-I structures through multi-pulse ablation feedback based on electromagnetic scattering. As previously reported in experiments for strong absorbing materials, [92][93][94] the spatial LIPSS period Λ LSFL-I then reduces with an increasing number of laser pulses per irradiated spot, finally saturating at values around 3 /4 in the center of the ablated spot, [47] see Figure 13.…”

“…Depending on the electrical properties (metallic, dielectric), this leads to a directional radiation characteristic (radiative and nonradiative scattered fields, propagation) and, thus, to a specific, anisotropic field distribution of the induced SEWs. [45][46][47] In the case of plasmonically active materials, it should also be noted that the excitation of SPPs requires the irradiation of a transverse magnetic (TM) wave, [31] which causes the polarization dependence of LSFL, e.g., on metals. [34] The SEW model can be extended to nonnormal incident radiation, where differences between s-and p-polarized light must be Interference of a p-polarized electromagnetic wave (wavelength ) incident under the angle with an SEW generated by scattering at a defect (scattering center).…”

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

“…Without HSFL structures on the oxide layer (d HSFL = 0 nm), the intensity distribution corresponds to a non-organized pattern, only affected by the initial roughness present at the oxide surface set to R HSFL = 20 nm here. For metals it has been demonstrated that the roughness at the surface is an important parameter that may influence the final periodicity of the LIPSS [8,17]. However, this is only valid when the structures are produced at the first irradiated interface (the surface), in our case, the oxide layer, and not at the interface between two different materials.…”

“…However, there have been recent significant advances in models based on finite-difference time-domain calculations (FDTD) that describe the formation of ablative LSFL in order to account for the periodicity of LIPSS on various materials under realistic irradiation conditions, including the formation of random defects at the surface [16]. For metals, Rudenko et al [17] implemented a numerical FDTD approach for calculating the electromagnetic fields scattered from ensembles of isolated dipole-like scatterers. In addition, FDTD simulations were used for the description of the processes that occur when different types of structures are formed in dielectrics under femtosecond laser irradiation [18].…”

High-Rate Laser Surface Texturing for Advanced Tribological Functionality

“…The pulse distance pd was set of about the focus spot diameter and with little overlap in orthogonal direction (slow axis) as the line distance ld between the raster scanned lines was half of the spot diameter. In initial studies, such kind of weak surface roughness is considered as inter-pulse feedback mechanism for the self-organizing formation of regular surface patterns as induced upon multi-pulse laser irradiations [30][31][32]. This material response is also confirmed by the SEM micrograph presented in Figure 3c, showing more prominent surface features originating from the higher accumulated irradiation dose that was due to higher pulse overlap at smaller line distance.…”