Freeform maskless nanolithography of silicon via subsurface laser writing

Functional nanostructuring of crystalline silicon typically relies on mask-based lithography or laser methods restricted to periodic textures. We report a subsurface-to-surface laser-writing approach that patterns silicon without masks or resists, enabling programmable freeform nanoscale features. Etch-selective seeds are inscribed tens of micrometers beneath the surface, and routine polishing followed by a brief wet etch converts them into deterministic nanoscale surface reliefs. The method affords precise, reproducible geometric control, including curved motifs, and yields linewidths down to 250 nm at 800 nm pitch. In single scans over 3-mm2 fields, we fabricate high-aspect-ratio nanogratings exhibiting ~25% relative diffraction efficiency at 532 and 633 nm. By leveraging the design freedom of volumetric laser modification, this strategy provides a route to structural color, light-management textures, and silicon metasurfaces for nanophotonics.