Nanogap optical antennas enable enhancement of light-matter interactions owing to their great field enhancement. Interesting properties have been studied over recent years, such as high-order harmonics generation, electrically driven absorption, optical rectification and nonlinear tunnelling effect. However, as the gap size is shrunk down to the nanometer scale, losses dramatically increase and coupling efficiency of antenna with free-space decreases. This work reports conditions of perfect coupling, also called critical coupling, between a periodic array of nanogap metal-insulator-metal (MIM) antennas and free-space. We demonstrate that critical coupling is still achievable, even for the thinnest gaps.

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Ludivine Emeric, Claire Deeb, Fabrice Pardo, and Jean-Luc Pelouard, "Optical critical coupling in nanogap antennas," Proc. SPIE 11082, Plasmonics: Design, Materials, Fabrication, Characterization, and Applications XVII, 110821G (Presented at SPIE Nanoscience + Engineering: August 15, 2019; Published: 9 September 2019); https://doi.org/10.1117/12.2529531.6083788869001.