  We report an experimental investigation on the irradiation of copper thin
films with high repetition rate femtosecond laser pulses (1040 nm, 50 MHz), in
ambient air and liquid water. We observe a novel, striking phenomenon of
chromatic copper oxides (cupric oxide and cuprous oxide) annuli generation. The
characteristic features of the chromatic copper oxide annuli are studied by
exploiting micro-Raman Spectroscopy, Optical and Scanning Electron
Microscopies. In the case of irradiation in water, the seldom investigated
effects of the immersion time, after irradiation with a fixed number of pulses,
is analyzed, and an intriguing dependence of the color of the chromatic annuli
on the immersion time is observed. This remarkable behavior is explained by
proposing an interpretation scenario addressing the various processes involved
in the process. Our experimental findings show that cuprous oxide nanoparticles
(size of ~20 nm) and cuprous oxide nanocubes (nanocube edges of ~30, ~60 nm)
with extremely fine sizes can be effectively generated by exploiting high
repetition rate laser-assisted oxidation.
