Surface plasmon at a metal-dielectric interface with an epsilon-near-zero transition layer

A metal-dielectric interface is usually modeled by a zero-width layer, where dielectric permittivity suffers a discontinuity jump. In reality, there is a narrow transition region, which includes an epsilon-near-zero (ENZ) layer. We show that due to a continuous dielectric function the dispersion of the surface plasmon is modified. The energy of the surface plasmon radiates through the ENZ layer, giving rise to additional radiative losses. Direct evidence of the effects related to the transition layer is given by the observation of plasmonic resonance in the presence of a strong electric field normal to the metal film. The electric field affects the electron density within the transition layer that leads to a measurable shift of the plasmonic resonance.