Efficient integration of sub-5-nm-gap plasmonic crystal cavities with plasmonic waveguides

We propose seamless integration of a tightly focused sub-5-nm-gap plasmonic crystal cavity with a plasmonic waveguide. A three-dimensionally tapered 2-nm-gap plasmonic crystal cavity exhibiting a mode volume of 6 × 10^-6 (λ/n)³ can efficiently couple to an integrated waveguide with over 90% efficiency by optimally selecting the number of air-holes in the plasmonic crystal. Along with the theoretical study, we fabricated the three-dimensionally tapered plasmonic crystal cavity integrated with a waveguide by using e-beam-induced deposition and focused ion-beam milling techniques.