Generally, the light transmission through the hole in the metal thin film gradually decreases as the size of the hole becomes smaller. However, when holes with subwavelength size are periodically arranged, the extraordinary transmission occurs at some visible frequencies due to the interaction of the surface plasmon in the metal with the incident light. Furthermore, although the incident light is in the long wavelength region such as terahertz waves and microwaves, where it is hard to excite the surface plasmon, the extraordinary transmission also occurs in the nanogap consisting of two metal films separated by a nanometer-scale distance. In this case, when the electromagnetic waves are normally incident on the nanogap, electric charges are accumulated at the gap area via light-induced current in the metal films, creating a huge electric field inside the gap. This allows the nanogap to act as an antenna, enabling the extraordinary transmission in the long wavelength regime. Therefore, the study of the interaction of electromagnetic waves with nanogap, called “Gap-based plasmonics”, is one of our major research projects.
The key factor of the gap-based plasmonics is the field enhancement which is the amplitude ratio of the electric field formed inside the nanogap to the incident electric field. In addition, it is highly dependent on the nanogap structure, the metal thickness, and the gap width and material because of the charge accumulation at the gap region. According to this, we demonstrated various examples such as terahertz modulation, molecular sensing, light-matter interactions in a very narrow space, and quantum plasmonics using the field enhancement of gap-based plasmonics. Recently, we have developed a nanogap active device with a tunable gap width from zero to tens of microns to simultaneously cover from visible to microwaves.
THz transmission via the effective nanogap thickness
Opt. Express 29(14), 21262-21268 (2021)
Quantum plasmonics in a nanogap
Nat. Commun., 9, 4914(2018)
Flexible nanogap active device
Nano Lett., 21, 4202-4208 (2021)