Fig. 2: Characterization of femtosecond laser-written nanogratings.

Microscopy images of nanogratings that are fabricated with a scanning speed of v_s = 0.2 mm s⁻¹ (a) and v_s = 20 mm s⁻¹ (e). Their 2D Fourier transforms and high-resolution images are displayed in b, d and f, g, respectively. δ denotes standard deviation of the periodicity. At low scanning speed, the nanogratings are self-organized via oxidation, while at high speed, they are produced through ablation. Comparison of manufacturing speeds and energy efficiency of our works with other high-speed manufacturing of nanogratings in terms of non-ablation processes (c) including oxidation in refs. ^20,23,24, reduction in ref. ²¹, and phase change in ref. ²⁵ or ablation (h). Please note that ref. ²⁹ employed the scanning beam interference lithography. i Periodicity of ablative nanogratings along y-direction. Here y = 0 denotes the center of laser spot. The ablative trace along y-direction is 6 mm, shorter than the oxidized trace of 10 mm. j photograph of a 2-inch single-crystalline Si wafer coated with 100 nm of Ag and 50 nm of amorphous Si. The vivid rainbow color of the wafer indicates the existence of highly regular nanogratings. The wafer-scale nanogratings were produced within 20 s