Optical properties of plasmonic metasurface with sub-nm gaps - Refractive index variation originating from electron transports -

Plasmonic metasurfaces consisting of two-dimensionally arrayed metallic nano-objects on a plane have drawn attention in terms of its exotic optical characteristics. Although investigations of metasurfaces conducted to date have focused on structures with sub-wavelength spatial scale, recent experimental studies have demonstrated those with much smaller size in which the gap distances between the nano-objects reach to sub-nm length where quantum mechanical effects becomes important.
In our presentation, we theoretically and numerically investigate the plasmonic metasurface with sub-nm gaps in terms of the refractive index variation due to quantum mechanical effects. To take into account quantum mechanical effects in the analysis, we employ time-dependent density functional theory(TDDFT) treating the constituent nano-particles by a jellium model. Furthermore, to distinguish the importance of the quantum mechanical effects, another simulation based on conventional classical electromagnetism is also employed and compared with the TDDFT. SALMON(https://salmon-tddft.jp/) developed by our group has been used for those numerical calculations. We will show that drastic change of the refractive index starts around gap distances 0.2 nm where electron transports take an important place.