Aperiodic nature of nanograting inscribed by femtosecond pulses.

A systematic study was performed, both experimentally and theoretically, to investigate the structural periodicity of nanogratings inscribed by focused femtosecond pulses on the surface of dielectrics. The results surprisingly show that although nanogratings generally appear periodic they are in fact intrinsically aperiodic. In the perpendicular writing scheme, the groove spacing gradually decreases from the middle part towards both sides. In the parallel writing scheme, the groove spacing varies quasiperiodically and the variation differs with respect to pulse-to-pulse spacing. Constant groove spacing was obtained only for a particular pulse-to-pulse spacing. These Gaussian-apodized and quasiperiodic variations are found to be intrinsic. They arise from the fact that the grooves are created, depending on the writing scheme, either by a series of local-lobes with different amplitudes and material feedbacks or by a repeated generated leading side-lobe with similar but not identical amplitudes and material feedbacks. The production of each single grooves result from a nonlinear and localized process from which arises its aperiodic nature. All these intrinsically Gaussian-apodized and quasiperiodic variations can be well interpreted based on the incubation-based nanoplasmonic model.