Fig. 1. Schematic diagram of fabrication process of metallic nanogaps by shower beam ion etching

Fig. 2. Process monitoring of individual metallic nanogap structure.（a） The initial diabolo-shaped metallic pattern fabricated by electron beam lithography； （b-e） The morphologies of （a） after shower ion beam etching for 50， 100， 150 and 200 s， respectively； （f） An enlarged view of the nanogap in （e）

Fig. 3. Fabrication of multiple metallic nanogap structures using shower ion beam etching

Fig. 4. Height variation analysis of nanostructures

Fig. 5. Fabrication of suspended metallic nanogap

Fig. 6. Polarization dependence of SERS effect in metallic nanogap structures

Fig. 7. （a） Raman spectra of Ag nanogap structure directly attached to Si substrate （red curve） and the Ag nanogap structure on the top of triangle-shaped HSQ pillars （purple curve）； （b-c） Electric field distribution in the xz-planes of above two metallic nanogap structures at 532 nm