Fabrication of nanogap structures through spatially shaped femtosecond laser modification with the assistance of wet chemical etching

Fabricating nanostructures with an extremely small feature size through a near-infrared femtosecond laser is a considerable challenge. In this Letter, we report a flexible, facile, and mask-free method that enables the formation of nanogap structures with a controllable size on silicon. This method involves spatially shaped femtosecond laser single-pulse modification assisted with chemical etching. Nanogaps obtained after etching can be divided into two categories, namely a ring dimer with a nanogap (type I) and Crack-nanogap (type II). The nanogap between the ring dimer could be reduced to 68 nm with a gradual increase in the laser fluence. For the Crack-nanogap obtained through crack propagation induced by stress release during a wet etching process, the smallest gap size is approximately 9 nm.