Microstructured Ultra-black Thin Stainless-Steel Sheets Fabricated by Sagnac Interferometric-Based Ultrafast Laser Patterning

Abstract

Antireflective surfaces that are efficient over a broad spectral range with extremely low total reflectivity have abundant potential applications in defense, space, and industries. We successfully fabricated three-dimensional micro-to-nano hierarchical structures on a thin sheet of stainless steel (SS), 100 μm in thickness, by interference-based ultrafast laser patterning for near-perfect absorption. The Sagnac-based interferometer reduces the footprint of the laser on the substrate to smaller than the diffraction limit. In this work, we achieved a groove cut of 13 μm size using a 1030 nm ultrafast laser. We achieved an average total reflectivity of 1.2% over the spectral range of 400–2000 nm and an average specular reflectivity of 0.02% over a broad range of wavelengths of UV–visible–NIR (400–2000 nm) on thin low-weight SS sheets. Further, the average total reflectivity in the spectral region of 400 to 650 nm was 1.1% only. The average groove depth from the top of the surface was 8.5 μm, and most of the material was intact in this process. As these structures are modifications of the same material, they have high mechanical strength and thermal stability and also exhibit self-cleaning ability. The patterned surface is insensitive to incident polarization and has a wide range of acceptance to the incidence cone angle of ±60°. These are the best anti-reflectivity values on an SS surface fabricated via laser surface patterning.