Large area, high speed inscription of laser-induced periodic surface structures (LIPSS) in Cr using a high repetition rate fs-laser

Summary form only given. The formation of Laser-Induced Periodic Surface Structures (LIPSS) is a universal phenomenon [1] that can be observed in a wide variety of materials under certain irradiation conditions. Although the study of sub-wavelength LIPSS in semiconductors and dielectrics irradiated below bandgap has been particularly intense [2], the formation of LIPSS in metals have also been given attention due to its specific features [3,4,5] that have been related to the characteristics of electron-phonon coupling, plasma density and electron diffusion upon fs-laser irradiation at low repetition rates. In this work we report on the unique characteristics of low spatial frequency LIPSS patterns fabricated in Cr upon high repetition rate fs-laser irradiation employing beam scanning. Highly regular patterns with sub-wavelength period can be produced for a wide range of repetition rates (100´s kHz range), over large areas (~cm²) and high scan speeds (~m/s).The experiments have been performed with a fs-fibre-laser amplifier (Tangerine, Amplitude Systemes) delivering ≈ 400 fs at 1030 nm and a repetition rate from 50 kHz up to 2 MHz. A galvo-scanner system enables to raster-scan the beam over the sample surface by focusing it with a f=100 mm, f-theta lens to a spot diameter of ≈ 30 μm. Different irradiation conditions (laser repetition rate, beam polarization, pulse energy, scan speed and scan line separation...) have been explored. Figure 1 shows AFM images of LIPSS patters formed by scanning the surface of a sample at v=1500 mm/s, and a repetition rate of 500 kHz for three different pulse energies. In all cases the ripples are formed perpendicular to the polarization axis with period (typically -0.85 μm) smaller than the laser wavelength. The best results in terms of modulation are observed for the scanning direction perpendicular to the polarization axis. High energies (Fig. 1a) produce well-defined and aligned ripples bu- the uniformity is worse than for lower ones (Fig. 1b) featuring a ripple substructure. If the pulse energy is further reduced, (Fig. 1c) the ripple structure becomes discontinuous. Extremely homogenous large-area grating structures with a modulation depth of several hundreds of nanometers have been produced for different repetition rates and scan speeds.In the work we analyse the role of the different irradiation parameters on the characteristics of the generated structures and discuss their origin in the frame of the models presently used for LIPSS formation in metals [3,4,5] with emphasis on the possible influence of thermal accumulation and geometrical factors on the homogeneous propagation of the sub-wavelength structure over macroscopic regions.