Convection roll-driven generation of supra-wavelength periodic surface structures on dielectrics upon irradiation with femtosecond pulsed lasers

Abstract

The significance of the magnitude of Prandtl number of a fluid in the propagation direction of induced convection rolls is elucidated. Specifically, we report on the physical mechanism to account for the formation and orientation of previously unexplored supra-wavelength periodic surface structures in dielectrics, following melting and subsequent capillary effects induced upon irradiation with ultrashort laser pulses. Counterintuitively, it is found that such structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength. It turns out that the extent to which the hydrothermal waves relax depends upon the laser beam energy, produced electron densities upon excitation with femtosecond pulsed lasers, magnitude of the induced initial local roll disturbances and the magnitude of the Prandtl number with direct consequences on the orientation and size of the induced structures. It is envisaged that this elucidation may be useful for the interpretation of similar, albeit large-scale periodic or quasi-periodic structures formed in other natural systems due to thermal gradients, while it can also be of great importance for potential applications in biomimetics. PACS: 78.20.Bh 64.70.D-42.65.Re 77.90.+k The predominant role of convective flow in nonequilibrium spatial pattern formation has been demonstrated in various phenomena in nature such as on Earth's land and sea, as well as on planet's surface when large thermal or wind speed gradients are developed [1-4]. Similar fluid instabilities are also encountered in many industrial applications: heat exchangers [5] , evaporative cooling devices, and chemical vapor process [6], film flow in inclined porous substrates used in oil pipes [7], manufacturing of high purity semiconductor crystals [8].Similar patterns and more specifically, periodical structure formation are also induced on the surface or volume of many solids upon irradiation with laser beams [9][10][11]. This modification usually requires a solid to liquid phase transition followed by fluid movement and capillary effects. Whether classical Navier-Stokes equations and how a Newtonian fluid mechanics could determine quantitatively the characteristics of the molten material dynamics, still remain an open question.The formation of surface and bulk periodic structures gives rise to unique material properties and it has received considerable attention over the past decades due to a wide scope of applications regarding micro/nano-structuring of materials [12]. In this context, one timely area of exploration has been the ultrashort pulsed laser induced periodic structuring of dielectrics, due to its applicability in telecommunications, biomedicine and biomimetics [13][14][15][16][17]. The predominant physical mechanism to explain the formation of laser induced periodic surface structures (LIPSS) suggests that excited electron densities modify the dielectric constant and the refractive index n [18] leading to low spatial frequency LIPSS (LSFL). Alternative mechanisms to...

Surface Plasmon Polariton Triggered Generation of 1D-Low Spatial Frequency LIPSS on Fused Silica

“…However, these so-called one-Dimensional LSFL (1D-LSFL) reveal periodicities larger than the wavelength of the applied laser and an orientation parallel to the polarization of the laser under normal irradiation, and neither conform with the established SPP theory. This supra-wavelength (Λ LSFL > λ) type of LIPSS has been reported in several publications for punctual laser irradiation of dielectrics [34][35][36] with their generation being attributed to hydrodynamic instabilities within a convection roll-driven formation [34]. Beside these supra-wavelength structures, the common way LSFL are shown on fused silica for punctual irradiation is Λ LSFL < λ and still being parallel to the polarization of the laser [12,15,28,36].…”

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

“…Figure 15 schematically summarizes the physical processes considered in the TTM approach. Finally, it should be noted that the TTM was already implemented as part of more complex LIPSS simulations based on FDTD (see Section 3.1.3) for dielectrics [46,107] and metals, [47] and in combination with hydrodynamic simulations (see Section 3.2.1) for metals, [92,108] semiconductors, [109,110] and dielectrics [111] and in molecular dynamics (MD) simulations (see Section 3.2.3) of metals. [112]…”

“…Including multi-pulse feedback, some authors discussed a convection-role mechanism based on the Marangoni effect that may account for supra-wavelength periodic structures, such as supra-wavelength LSFL or grooves. [92,108,111,118] However, when hydrodynamic theories based on the compressible NSEs are used in combination with a 3D-FDTD model, including as input the absorbed energy densities deposited by ultrashort laser pulses via electromagnetic scattering, many experimental findings can be reproduced. [91] Figure 16 assembles these phenomena as plots of color-encoded cross-sectional surface profiles ordered via the applied number of laser pulses for three characteristic fluence ranges in the regimes below (left column), close to (middle column), and above the ablation threshold fluence (right column).…”

Evolution of microstructures on stainless steel induced by ultra-short pulsed laser ablation

“…The LSFL structure can be minimized by the use of circular polarized light, which additionally has a strong impact on the precursor ripple formation and following CLP coverage of the surface [50]. However, the larger gray precursor ripple structure evolves in both polarization states with periodicity larger the incident wavelength with about 2 μm spacing and convection at the surface molten layer might play a crucial role [51].…”