Optical properties of plasmonic metasurface with sub-nm gaps - Extremely large third-order nonlinear optical effects caused by electron transports -

Plasmonic metasurfaces composed of periodically arrayed metallic nano-objects on 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 sub-nm length where quantum mechanical effects become important.
We computationally investigate the metasurface with sub-nm gaps in terms of 3rd-order nonlinear optical responses originating from quantum mechanical effects. To take into account quantum mechanical effects, we employ time-dependent density functional theory(TDDFT) treating the constituent nano-particles by a jellium model. SALMON(https://salmon-tddft.jp/) developed by our group has been used.
We will show that, at gap distances of ≥ 0.6 nm, the 3rd-order nonlinearities increase as the gap distance decreased, owing to enhancement of the induced charge densities at the gaps. At smaller gap distances down to 0.1 nm, larger 3rd-order nonlinearity will appear. This extremely large 3rd-order nonlinearities were found to originate from electron transport by quantum tunneling currents through the sub-nm gaps.