Coloration of a Ti target under its ablation with picosecond laser pulses in liquids is ascribed to the formation of self-organized nanostructures. The density of nanostructures increases in an avalanche-like way with the number of laser pulses. The Raman peaks observed in the laser-exposed areas may be due to the auto-SERS effect.
@article{arxiv.1003.2796,
title = {Laser-assisted coloration of Ti: oxides or nanostructures?},
author = {E. V. Barmina and E. Stratakis and C. Fotakis and G. A. Shafeev},
journal= {arXiv preprint arXiv:1003.2796},
year = {2010}
}