Local Electric Field Enhancement During Nanofocusing of Plasmons By a Tapered Gap

We demonstrate that during plasmon nano-focusing in a tapered gap (V-groove), local electric field experiences much stronger enhancement than the magnetic field. Two distinct asymptotic regimes are found near the tip of the groove: the electric field approaches either zero or infinity when dissipation is above or below a critical level (at a fixed taper angle), or taper angle is smaller or larger than a critical angle (at a fixed level of dissipation). Tapered gaps are shown to be the best option for achieving maximal field enhancement, compared to nano-wedges and tapered rods. An optimal taper angle is determined.