Oxidation ruled transition from normal to anomalous periodic structures with femtosecond laser irradiation on Cr/Si films.

Elucidation of the underlying physics for laser-induced periodic surface structures (LIPSSs) is of great importance for their controllable fabrication. We here demonstrate a periodic structure transition from normal to anomalous morphology, upon femtosecond laser irradiation on 50-nm thick Cr/Si films in an air pressure-tunable chamber. As the air pressure gradually decreases, the amount of surface oxide induced by preceding laser pulses is found to reduce, and eventually triggering the structure evolution from the anomalously oriented subwavelength to normally oriented deep-subwavelength LIPSSs. The intriguing structure transition is explained in terms of the competitive excitation between the transverse-electric scattered surface wave and transverse-magnetic hybrid plasmon wave, which is ruled by the thickness of the preformed oxide layer indeed.