Topological transition from deeply sub- to near-wavelength ripples during multi-shot mid-IR femtosecond laser exposure of a silicon surface

Multi-shot exposure of silicon surface in air by mid-infrared (MIR, 2.5–5 microns) femtosecond laser pulses results in an initial (N_p = 2–5 shots) appearance of a bright spot with abnormally-oriented, bipolar shallow deeply-subwavelength ripples (period ∼ (0.2–0.4)λ, average trench ablation rate ∼ 10–20 nm/shot, trench depth < 100 nm), visualized by scanning confocal laser profilometry. At longer exposures (N_p = 10–20 shots), the irradiated spot becomes visibly black, exhibiting normally-oriented, almost unipolar near-wavelength ripples with ultra-deep trenches (average ablation rate ≤60 nm/shot, trench depth ∼ 400–600 nm). The observed distinct topological transition from the abnormal bipolar deeply-subwavelength ripples, formed via melt displacements, to the normal unipolar ablative near-wavelength ripples was considered to be a competitive result of the related, much stronger resonant laser coupling to the second darker, rougher near-wavelength relief, accompanied by the change in mass transfer mechanisms and strong enhancement in ablation rate per shot.