Femtosecond Laser Textured Surfaces for Radiative Cooling: Black Metals

There is a growing need for cost-effective methods for modifying the surfaces of a wide range of materials over large areas. Here, low reflectance R < 2% surfaces have been created over the near-to-mid IR spectral window of 2−20μm by ablating W, Al, and Cu with high average intensity 20-120 TW/cm^2 200 fs laser pulses at 1030 nm wavelength. The resulting chemical modification of the surfaces after laser ablation at ambient room conditions was assessed with X-ray photoelectron spectroscopy (XPS). The results show a consistent decrease in the metallic component together with an increase in metal oxides. Energy dispersive spectroscopy (EDS) showed a similar increase in oxygen content over a micrometer depth scale. The reduced refractive index of the metal oxides compared to the corresponding metals contributes to the reduction in infrared (IR) reflectance, together with the 3D hierarchically textured surface structure. IR-black metals show great potential for radiative cooling at elevated temperatures relevant to industrial and space applications.