Landau Damping Induced Limits in Nanogap Metal-Insulator-Metal Plasmonic Waveguides and Cavities

Abstract

Plasmonic structures permit the focusing of light into volumes far below the diffraction limit. In particular Metal-Insulator-Metal (MIM) gap plasmonic structures can reach nanoscale energy confinement if the gap is sufficiently miniaturized. Under classical models, gap plasmonics can achieve indefinite confinement, down to the single atom level. However, these classical models fail to consider quantum effects that occur as the confinement approaches the single nanometer level. Recently, it has been demonstrated that Landau Damping, the absorption of highly confined plasmonic energy, is the dominant effect in highly confined MIM devices until the tunneling regime is reached. However, the effects of Landau Damping on MIM gap devices are poorly understood. In this work, we analyze the effects of Landau Damping on MIM gap devices, specifically MIM waveguides and cavities. It is found that in waveguides, Landau Damping does not limit the confinement but does limit the maximum propagation length achievable. Moreover, in cavity structures, Landau Damping causes the Quality Factor to drop significantly as the gap is further miniaturized. In terms of quantum optics applications, this causes the radiative spontaneous emission enhancement to actually decrease as the gap is miniaturized sufficiently and a saturation of the coupling-loss ratio limiting the achievement of strong coupling. These effects will limit the possibilities for high performance nanogap plasmonic devices.