LAUSR

Nanogap antennas are compelling structures for squeezing light into ultrasmall volumes. However, when gaps are shrunk to the nanometer scale, the mode losses dramatically increase. In this Letter, we report the conditions of critical coupling between the arrays of nanogap resonant metal-insulator-metal (MIM) antennas and free space. Adapting the antenna density, critical coupling is achievable for any thickness of insulator, from 100 down to 0.1 nm. The fundamental optical mode can be described as continuous transitions through three types of modes: a perfect MIM mode, coupling between the MIM mode and surface plasmon polariton, and a gap plasmon mode. We found that the space between adjacent antennas is an essential parameter to perform critical coupling for thinner gaps. These results pave the way towards understanding extreme confinement in nanogap antenna structures such as MIM or nanoparticle arrays.