Realizing a point-like cavity and its efficient coupling

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.