Superblackening Stainless Steel by Ultrafast Laser Direct Writing of Micro/Nano-Composite Structures

While achieving ultimate light trapping on metal surfaces is critical for advancing optoelectronic applications, the fabrication of superblack metal surfaces remains challenging. In the present work, we realize the broadband superblack functionality on 316L stainless steel by introducing hierarchical micropillars composited with nanotextures fabricated through picosecond ultrafast laser direct writing. Specifically, a Gaussian-weighted pulse accumulation model is established to reveal the edge-enhanced “M-shaped” energy distribution characteristics of the hypotrochoidal trajectory strategy, which are identified as the intrinsic mechanisms governing the precision formation of high-aspect-ratio micropillars. Through systematic optimization of laser processing parameters, high-quality micropillar arrays with a remarkable mid-diameter aspect ratio of 7.66, a minimal top diameter of 10.2 μm and a low sidewall taper angle of 5.7° are successfully fabricated. Subsequently, optimal nanotextures with high fidelity are nondestructively superimposed onto the micropillar skeleton via an in situ laser scanning process. The resulting hierarchical micro/nano-composite structures ultimately achieve an ultralow solar-weighted reflectance of 1.39% in the UV–vis–NIR range, which is attributed to the synergistic mechanisms of macroscopic geometric trapping and microscopic antireflection. This work provides a scalable all-laser route for manufacturing high-performance metallic optical functional surfaces.