Supra-wavelength surface structures on silicon by femtosecond laser defocus structuring for antireflection

• We explore the feasibility of manufacturing SWSs by femtosecond laser defocus structuring. • The near field enhancement effect and Marangoni convection are proposed to explain the formation mechanism of SWSs. • Antireflective SWSs sample with reflectance < 4.5% in the wavelength of 400–800 nm is successfully fabricated. • The photo-thermal test indicates the efficient photo-thermal conversion ability of SWSs silicon. Laser-induced periodic surface structures (LIPSS) offer a flexible surface nano-patterning antireflective strategy. However, due to the subwavelength size (Λ < λ) of LIPSS, the antireflection performance is limited. Supra-wavelength surface structures (SWSs) with feature size (Λ > λ) are good alternative, benefiting in light trapping in antireflection. However, the formation mechanism of SWSs is not clear and the antireflection studies of SWSs are few. In this work, we explore the feasibility of manufacturing antireflective SWSs on silicon by femtosecond laser defocus structuring. As the defocus distance increases, the spot size will increase and the cross-sectional energy intensity will decrease, providing proper energy threshold for the generation of SWSs. The near field enhancement effect and hydrodynamic flow and Marangoni convection are proposed to explain the possible formation mechanism of SWSs. By optimizing the processing parameters (defocus distance, scan speed, laser power and scan interval), antireflective SWSs sample (reflectance < 4.5% in the wavelength of 400–800 nm, omnidirectional reflectance R < 2.8% at different incident angles) has been successfully fabricated. Moreover, the photo-thermal test indicates the efficient photo-thermal conversion ability of SWSs silicon. This novel manufacturing method is simple and flexible, which opens new doors for the potential applications of antireflective silicon in photo-thermal and photovoltaic.