Multi-shot exposure of silicon surface in air by mid-infrared (MIR, 2.5–5 microns) femtosecond laser pulses results in an initial (Np = 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 (Np = 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.

中文翻译：

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硅表面多发中红外飞秒激光曝光期间从深亚波长波纹到近波长波纹的拓扑转变

通过中红外（MIR，2.5-5 微米）飞秒激光脉冲对空气中的硅表面进行多次曝光，导致初始（Np = 2-5 次）出现一个亮点，具有异常取向的双极浅深-亚波长波纹（周期 ~ (0.2–0.4)λ，平均沟槽烧蚀率 ~ 10–20 nm/shot，沟槽深度 < 100 nm），通过扫描共聚焦激光轮廓测量法可视化。在更长的曝光时间（Np = 10-20 次照射），照射点变得明显黑色，表现出正常取向的、几乎单极的近波长波纹和超深沟槽（平均烧蚀率≤60 nm/次，沟槽深度 ~ 400- 600 纳米）。通过熔体位移形成的异常双极深亚波长波纹观察到的明显拓扑转变，