Fig. 3: Formation dynamics of nanogratings.

a–c Nonlinear modulations of SPPs by excitation of e–h pairs in Si-on-Ag films under intense ultrafast laser illumination. a Sketch of band structure of amorphous Si, and generation of e–h pairs by two-photon absorption as well as linear absorption due to dangling bonds with light energy lower than band gap. b Transient evolution of spatially averaged e–h pair density, N_e-h, and the permittivity (real part) of amorphous Si, ε_si, considering a Si-on-Ag film illuminated by a femtosecond laser pulse (shaded area). c Modal profiles of SPPs supporting by the Si-on-Ag film in the initial linear regime (upper panel) and with ε_si corrected by the e–h pairs (bottom panel). The thickness of the Si film is 50 nm, and the laser pulse has 130-fs duration, 0.08 J cm⁻² fluence, and 1030 nm wavelength. d–g Sketch of forming oxidized-ridge (d) and ablative-groove (e) nanogratings by controlling laser scanning speeds, and their exemplified numerical simulations (f, g). Both experimentally and numerically, we observe the formation of oxidized-ridge gratings with a scanning speed lower than 0.5 mm s⁻¹, and of ablative-groove gratings with a scanning speed higher than 2 mm s⁻¹