Large Third-Order Nonlinear Optical Effect Induced by Plasmonic Metasurface with Sub-nm Gaps

Plasmonic metasurfaces consisting of metallic nano-objects periodically arrayed in two-dimension are an attractive platform to tactfully manipulate light-matter interaction. Although conventional researches for plasmonic metasurfaces have focused on periodic structure of wavelength or sub-wavelength spatial scale, in recent years, there have been reported experimental studies that fabricate sub-nm gap distances between nano-objects by using a self-assembly approach [1] , [2] . Owing to the extremely short gap distance, these metasurfaces have been shown to realize exotic optical properties. Particularly in reference [2] , it has been demonstrated that a plasmonic metasurface with a gap distance of 0.6 nm subjected to an off-resonant incident pulse exhibits large third-order nonlinear optical effect with negligibly small heat accumulation, being suitable for all-optical switch. However, such sub-nm scale is close to those of atoms and molecules, and quantum mechanical effects should be inherently important. In fact, through the last decade, it has been theoretically and experimentally revealed that an isolated nano-dimer consisting of two metallic nano-objects with a sub-nm gap shows drastic changes in its optical property by quantum tunneling current flowing at the gap [3] , [4] . Nevertheless, there has been almost no discussion so far on quantum mechanical effects in plasmonic metasurfaces with sub-nm gaps.