Direct Laser Interference Patterning (DLIP) and Laser Induced Period Surface Structures (LIPSS) [1] are two distinct technologies for periodic surface texturing. Based on different physical phenomena, they enable structures often showing dissimilar morphology and pitch. It has been reported, that by superposing LIPSS over DLIP, a final multi-scale, hierarchical morphology can be generated, where the two structures coexist [2]. Certain applications, such as antireflectivity, demand a 2D nanostructured surface with a periodicity of a few hundred of nm [3]. The DLIP method can reach this resolution by using UV and an important incidence angle of the beams, which is incompatible with the scanner-based approach. Here, we report a novel approach in DLIP structuring based on the use of a galvo scanner with large aperture (30 mm) and an F-theta lens combining large entrance pupil (> 20 mm) and relatively small focal length (30 mm). We show that by using a 10 ps laser source emitting at λ=1064 nm, this set-up makes possible a DLIP pitch value ΛDLIP as low as 1.4 µm which becomes comparable with the LIPSS period (~800 - 900 nm). Interestingly, we identified a process window (fluence, number of passes, polarization) where LIPSS formation on stainless-steel surface is strongly affected by the presence of DLIP. By orienting the linear polarization orthogonally to the interference pattern, we show that LIPSS can be generated in a highly regular way with a much-reduced periodicity (~470 nm, see figure 1) than conventional LIPSS produced in similar conditions without DLIP (~830 nm). This phenomenon allows to divide by 3 the DLIP period. Moreover, it is possible to produce a similar nanostructured surface with a period of 700 nm (ΛDLIP /2). In addition, we show that 2D nanostructuring is possible as well.

In order to further enhance the nanostructuring resolution, the setup has been converted to the visible wavelength and thus, similar experiments have been performed with an irradiation at 532 nm, allowing a DLIP period of 700 nm. We show that the same nanostructuring phenomenon occurs giving rise to the formation of regular LIPSS with a periodicity of ~240 nm.
All the generated structures have been characterised by SEM and analysed by using 2D Fast Fourier Transform. We believe that our results represent a promising approach for the high throughput generation over large surface of highly regular structures in the range of few hundreds of nm.

[1] J. Bonse, Quo Vadis LIPSS, Quo Vadis LIPSS? -Recent and Future Trends on Laser-Induced Periodic Surface Structures, Nanomaterials., 10, 1950 (2020)
[2] S. Alamri, F. Fraggelakis, T. Kunze, B. Krupop, G. Mincuzzi, R. Kling and F. Lasagni, On the interplay of DLIP and LIPSS upon ultra-short laser pulse duration, Materials, 12, 1018 (2019)
[3] Y. M. Song, H. J. Choi, J. S. Yu and Y. T. Lee, Design of highly transparent glasses with broadband antireflective subwavelength structures, Optics Express, 18, 12, pp 13063-13071 (2010)