Femtosecond laser surface structuring and oxidation of chromium thin coatings: Black chromium

•Oxidation of the chromium thin film to chromium oxide by femtosecond laser with a fundamental wavelength of 1064 nm.•Solar absorber from chromium oxide that low percentage reflectance.•Femtosecond laser oxidation, with a de-focused laser.•Chromium oxide formation by femtosecond laser in normal ambient.

In view of their potential applications as selective solar absorbers, chromium coatings on float glass substrates were nano/micro structured by femtosecond laser in air. Raman and X-rays diffraction investigations confirmed the formation of an ultra-porous α-Cr2O3 layer at the surface; higher is the input laser power, enhanced is the crystallinity of the α-Cr2O3 layer. The α-Cr2O3 layer with the Cr underneath it in addition to the photo-induced porosity acted as a classical ceramic-metal nano-composite making the reflectance to decrease significantly within the spectral range of 190-1100 nm. The average reflectance decreased from 70 to 2%.