Confining photons in the smallest possible volume has long been the theme of the nanophotonics community. In this study, we suggest two types of point-like cavities: 1) 3D nano-diabolo metallic cavities and 2) plasmonic crystal point-like cavities, which both extremely squeeze photons in a 3D fashion with mode volumes of < 10−6 λ3 and intensity enhancements of >105. The former confines photons as forming the three-dimensionally tapered sub-5-nm air-gaps at the center of complementary nano-diabolo structures and the latter as tapering the metal-insulator-metal plasmonic crystal cavities along all three dimensions.
Especially, the plasmonic crystal point-like cavities could couple to the integrated waveguides with over 90% efficiency by optimally selecting the number of air-holes in the plasmonic crystals. Based on the theoretical study, we fabricated the 3D nano-diabolo metallic cavity with a minimum gap size of 4 nm using the proximal milling technique, and the plasmonic crystal point-like cavity with the integrated waveguide using the electron-beam ion-deposition and focused ion-beam milling techniques. We strongly believe that our proposed point-like cavities could open a new research theme based on 3D nano-metallic structures, providing both extreme photon density and efficient coupling.