Pre-ablation features formed by focusing a femtosecond laser beam with dual axicons to c-Si in vacuum

► Focusing a femtosecond laser beam with dual axicon optics to the vacuum chamber was demonstrated. ► 786 and 393 nm of the wavelength focused on the c-Si in the vacuum formed a hole with submicron-meter scale. ► Irradiation of 786 nm wavelength formed humps and swells on a scale of ∼100 nm, leading to the periodical ripple pattern. ► The 393 nm laser irradiation created the regular rippled pattern with the spatial period of 60 nm.