Reflectivity and microrelief transitions in femtosecond laser-induced Grey and Black AISI 321 stainless steel surfaces

Laser-induced surface microstructuring has emerged as a powerful strategy for tailoring optical properties of metals for applications in energy systems, sensing, and stealth technologies. The present study provides a systematic investigation of femtosecond laser-induced Grey and Black surface states on AISI 321 stainless steel and its covering layers, revealing a previously unresolved transition between oxidation-dominated and topography-enhanced light absorption regimes, and quantitatively decoupling their respective contributions. The applied femtosecond laser irradiation produces two distinct classes of low-reflectivity surface microstructures with markedly different morphologies and optical responses. At lower laser intensities, a Grey surface forms via a Two-Scale Laser-Induced Periodic Surface Structures (TS-LIPSS), where high-frequency classical Laser-Induced Periodic Surface Structures (LIPSS) coexist with incipient microcraters. At higher intensities, a Black surface develops, characterized by a Quasi-Periodic Laser-Induced Deep Surface Structures (QP-LIDSS) composed of densely distributed deep craters. Correlated structural, chemical, and optical analyses reveal different dominant mechanisms of reflectivity reduction. The Grey surface is primarily governed by thin oxide layers (∼20 nm) superimposed on nanoscale texturing, whereas the Black surface combines thicker oxides (∼50 nm) with pronounced crater-assisted light trapping. UV–Vis–NIR spectroscopy shows a stepwise reflectivity decrease from 47% (untreated) to 34% (Grey) and 14–16% (Black), supported by grazing-incidence scattering and COMSOL-based modelling. Quantitative analysis of roughness parameters demonstrates that no single descriptor is sufficient to capture the optical behavior. Instead, reflectivity correlates with combined parameter groups - core, amplitude, spatial development, and with the interfacial area ratio emerging as the most sensitive indicator when coupled with spectral and scale-dependent analysis. These results establish a transition from oxidation-driven light suppression to coupled oxidation–topography-controlled effect, offering a scalable route to engineer stainless-steel surfaces via femtosecond laser treatment. The ability to tune the optical response via laser processing offers a scalable route to creating functional surfaces with enhanced optical and electromagnetic performance. Unlike typical studies focusing solely on the final high-roughness dual-scale morphology, this work demonstrates how the transition from oxidation-driven light suppression on sub-wavelength ripples (TS-LIPSS) to geometric trapping on dual-scale structures (QP-LIDSS) can be utilized to precisely tune the optical state of industrial alloys without complex chemical treatment.