Femtosecond Laser-Induced Periodic Surface Structures on Fused Silica: The Impact of the Initial Substrate Temperature.

Stephan Gräf,Frank Müller,Sebastian Engel,Clemens Kunz,Thibault Derrien

doi:10.3390/ma11081340

Abstract

The formation and properties of laser-induced periodic surface structures (LIPSS) were investigated upon fs-laser irradiation of fused silica at different initial substrate temperatures, TS. For substrate heating between room temperature, TRT, and TS = 1200 °C, a continuous wave CO2 laser was used as the radiation source. The surface structures generated in the air environment at normal incidence with five successive fs-laser pulses (pulse duration, τ = 300 fs, laser wavelength, λ = 1025 nm, repetition frequency, frep = 1 kHz) were characterized by using optical microscopy, scanning electron microscopy, and 2D-Fourier transform analysis. The threshold fluence of fused silica was systematically investigated as a function of TS. It was shown that the threshold fluence for the formation of low-spatial frequency LIPSS (LSFL) decreases with increasing TS. The results reveal that the initial spatial period observed at TRT is notably increased by increasing TS, finally leading to the formation of supra-wavelength LIPSS. The findings are discussed in the framework of the electromagnetic interference theory, supplemented with an analysis based on thermo-convective instability occurring in the laser-induced molten layer. Our findings provide qualitative insights into the formation mechanisms of LIPSS, which allow improvements of the control of nanostructure formation to be made for corresponding applications of dielectric materials in the future.

Highlights

Since their first description by Birnbaum in 1965 [1], laser-induced periodic surface structures (LIPSS) have rapidly gained increasing attention [2]
For the fused silica used in the present study, the corresponding optical constants at λ = 10.6 μm and room temperature are n = 1.98, and k =
Considering the optical penetration depth, lα = 14 μm, this means that the CO2 laser be reduced to R = 0 by using p-polarized radiation and the Brewster’s angle, θ B = 63.2◦, as the angle radiation is completely absorbed by the material

Summary

Introduction

Since their first description by Birnbaum in 1965 [1], laser-induced periodic surface structures (LIPSS) have rapidly gained increasing attention [2]. The formation of LIPSS has been demonstrated as a universal phenomenon, occurring on almost all types of materials when irradiated close to their ablation threshold [3] Due to their spatial periodicity, LIPSS are adequate for a large variety of applications in fields, such as sub-wavelength optics, tribology, and biomaterials engineering [2,4]. Based on their spatial characteristics, LIPSS are classified into low-spatial frequency LIPSS (LSFL, with a period, Λ, close to the laser wavelength, λ) [3,5,6,7] and high-spatial frequency LIPSS In the case of band gap materials, the absorption of the high intensive fs-laser radiation can lead to a highly excited electronic state, possibly enabling

Methods

Results

Conclusion