Competition between subwavelength and deep-subwavelength structures ablated by ultrashort laser pulses

Femtosecond laser-induced periodic subwavelength and deep-subwavelength structures (SWS; DSWS) have attracted attention due to their subdiffraction resolution of surface and inner volume patterning. Understanding of the richness of laser–matter interaction during formation of SWS and DSWS is another quest which can help to find control for nanoscale fabrication. Lack of control over SWS and DSWS formation has impacted their wider use and calls for a deeper insight into the relationship between them. Herein we present a systematic study defining a criterion for imprinting either SWS or DSWS, which is based on a competition and their mutual incompatibility discriminated by the laser fluence and pulse accumulation. Structure evolution of SWS and DSWS is highly dependent on the localized effective laser fluence, which determines the instantaneous optical permittivity by the laser-excited electrons creating an active plasma layer. The proposed universal SWS and DSWS competition mechanism involving the laser-induced plasma wave at the plasma–substrate interface ties together many previous observations and unifies the discussed mechanisms of surface nanoripple formation.