Controllable formation of nanogaps in thin metallic film by rear side irradiation with ultrashort pulsed laser

Metallic nanogap electrode shows great potential in many fields. However, its fabrication is still a highly challenging task because it goes beyond the capability of many nanofabrication technologies. We demonstrate a nanogap formation method in metallic film surface using femtosecond laser rear side irradiation. In this method, laser beam passes through the transparent substrate, heating the film material adjacent to film/substrate interface; ultrafast heating induced thermal shock and stress confinement in an enclosed space contribute to the formation of nanogap structures. Experimental results indicate that laser fluence is a crucial process parameter influencing the formation of structures in rear side irradiation. When it is higher than the ablation threshold in a proper range, nanogap can be formed in irradiation region definitely. Length and direction controlled nanogaps with width of sub-50 nm are realized in our experiments for the first time, which shows nonlinear dependence characteristic of direct tunneling. Compared with other nanogap fabrication techniques, this room temperature nanogap fabrication method has the advantages of simple process, easy operation and cost-efficiency.