Adiabatic nanofocusing of the fundamental modes in plasmonic parabolic potentials

We investigate the adiabatic nanofocusing of the fundamental modes in plasmonic parabolic potentials. The potentials are obtained in the metal-dielectric-metal structure, of which the dielectric layer width is modulated quadratically in the horizontal direction and linearly in the longitudinal direction. In such a structure, light is compressed in both transverse directions due to increasingly stronger parabolic potentials and decreasing dielectric layer width. We show by both closed form analytical descriptions and numerical simulations that there is a critical tapering angle, above which the field could be enhanced. In contrast to previously reported tapered structures in the adiabatic regime without transverse potentials, the structure proposed in this paper shows stronger focusing capability and allows monotonic increasing field enhancement over longer propagation distances.