Structural and oxide-based colours on laser textured copper

Abstract

Laser texturing to create colours has recently attracted significant interest due to the rapid and non-contact nature of the technique. The complex optical response of femtosecond laser-textured copper surfaces, producing a palette of perceived colours with varying amounts of angle dependence, is presented. The colours depend on the line spacing used in the laser raster-scanning process. Grazing angle Fourier transform Infrared spectroscopy (FTIR) and Raman microscopy show increased oxide content with larger total accumulated fluence and increased pulse overlap. The increased oxide content coincides with a greater angle independence of the colours compared to colours obtained at larger line spacings. All samples have a similar sinusoidal periodic microstructure, with amplitudes ranging from 400 nm for the smallest line spacings (∼1 µm) to 100 nm for the largest line spacings (∼20 µm), and are decorated with nanoparticles. The angle dependence of the colours is quantified using reflectance measurements, which show a shift in reflectance peak with observation angle. The overall complex colour of the surfaces is attributed to a combination of copper oxides and structural colours driven by underlying nanoparticles, be they metal or metal-oxide, and sinusoidal gratings with a pitch of approximately 1 µm.