Ultraconfined Plasmons in Ultrathin Gold-Based Hybrid Plasmonic Structures.

Plasmons in few-nanometer-thick metals have attracted great research interest due to their strong optical confinement and electro-optic tunability. However, further optical confinement by reducing the metal thickness to subnanometer scale is difficult, and the dramatic momentum mismatch makes the excitation extremely challenging. Here, we overcome these challenges by integrating ultrathin gold with a silicon nanoribbon array (separated by a few-nanometer-thick silica) to form a hybrid plasmonic structure. The coupling between the ultrathin gold and high-refractive-index silicon not only increases the in-plane wavelength compression ratio up to ∼8 around 1410 nm wavelength but also squeezes the optical fields strongly into the silica spacer in the out-of-plane direction. Using a plasmon conversion approach, we efficiently excite ultraconfined plasmons in a 3.5 nm-thick gold-based hybrid structure with ∼45% absorption. By replacing the intrinsic silicon with highly doped silicon, we further create a compact plasmonic tunnel junction array, enabling direct electrical excitation of ultraconfined plasmons.