Taking the shine off copper

Amin Abdolvand and his collaborators at the University of Dundee in the UK have discovered that intense nanosecond pulses of laser light can transform copper's characteristically lustrous surface to a deep, absorbent black. Although the Dundee team conducted its experiments in air, oxidation or some other chemical reaction is not the transformation's principal cause. Rather, the pulses modified the surface's vertical structure through laser-induced melting and ablation. Achieving the effect depends on how the pulses are delivered. Abdolvand and his colleagues used a neodymium–doped yttrium orthovanadate laser, which emitted 12-ns pulses at a repetition rate of 30 kHz. When focused on the Cu surface, each pulse delivered 2.6 J/cm² to a spot about 60 μm wide. Stepping the beam over the surface ensured that the modification took the form of discrete patches arranged in a closely spaced grid. As the accompanying figure shows, the modified surface resembled an upturned egg carton whose pyramidal cups have been pushed in. Of the various firing patterns that Abdolvand and his colleagues tested, a square grid with a spatial period of 70 μm yielded the biggest change: Reflectivity from the UV through to the red was reduced from 90% to 3% and in the near-IR from 80% to 30%. Several mechanisms can account for the absorption boost, including light-trapping in the microcavities between the pyramids. Adding strong thermal absorbance to Cu's already strong thermal conductance could find uses in devices that carry away radiant heat. (G. Tang, A. C. Hourd, A. Abdolvand, Appl. Phys. Lett. 101, 231902, 2012.)—Charles Day