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Metal/Insulator/Metal nanocavities (MIMs) are highly versatile systems fornanometric light confinement and waveguiding, and their optical properties aremostly interpreted in terms of surface plasmon polaritons. Although classicelectromagnetic theory accurately describes their behavior, it often lacksphysical insight, letting some fundamental aspects of light interaction withthese structures unexplored. In this work, we elaborate a quantum mechanicaldescription of the MIM cavity as a double barrier quantum well. We identify thesquare of the imaginary part of the refractive index of the metal as theoptical potential, and find that MIM cavity resonances are suppressed if theratio n/\k{appa} exceeds a certain limit, which shows that low n and high\k{appa} are desired for strong and sharp cavity resonances. Interestingly, thespectral regions of cavity mode suppression correspond to the interbandtransitions of the metals, where the optical processes are intrinsicallynon-Hermitian. The quantum treatment allows to describe the tunnel effect forphotons, and reveals that the MIM cavity resonances can be excited by resonanttunneling via illumination through the metal, without the need of momentummatching techniques such as prisms or grating couplers. By combining thisanalysis with spectroscopic ellipsometry on experimental MIM structures, and bydeveloping a simple harmonic oscillator model of the MIM for the calculation ofits effective permittivity, we show that the cavity eigenmodes coincide withlow-loss zeros of the effective permittivity.

A Semi-Classical View on Epsilon-Near-Zero Resonant Tunneling Modes in Metal/Insulator/Metal Nanocavities