Adiabatic nanofocusing of plasmons by sharp metallic grooves: Geometrical optics approach

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Abstract
In this paper, we demonstrate the possibility of efficient adiabatic nanofocusing of gap plasmons by sharp metallic V grooves or dielectric wedges covered with metal. The geometrical optics approach and the approximation of continuous electrodynamics are used for the analysis. In particular, it is demonstrated that both the phase and group velocities of an incident symmetric (with respect to the magnetic field) plasmon tend to zero at the tip of the groove, and the plasmon adiabatically slows down, eventually dissipating in the metal. The amplitude of the plasmon strongly increases near the tip of the groove. However, unlike nanofocusing by a sharp metal conical tip, even in the absence of dissipation, the amplitude of the plasmon near the tip of a V groove remains finite. The dependence of the maximal local-field enhancement on structural parameters, dissipation in the metal, angle of incidence, etc., is analyzed. It is also shown that a symmetric gap plasmon can effectively be guided by the groove, forming a channel plasmon-polariton—a special plasmon mode propagating along the tip of a metallic V groove. An existence condition for these strongly localized plasmon-polaritons is derived and discussed.