Regularities of nanofocusing of a surface plasmon wave near the nanoapex of a metallic microtip

Nanofocusing of the electromagnetic energy of the optical frequency range into a nanosized spatial area in the vicinity of the nanoapex of a metallic microtip with a radius of curvature of the apex on the order of a few nanometers, which occurs when the surface plasmon wave converges to the tip, is studied. The metal boundary near the apex is considered approximately as the surface of the paraboloid of revolution. It is shown that, in the approximation of the absence of losses in the metal, the electric field distribution near the nanoapex is determined only by its radius of curvature and the frequency of the plasmon wave normalized to the plasma frequency of the metal. The impact of absorption in the metal on this distribution is considered. For microtips made of highly conductive materials, the absorption in the metal in the optical frequency range is shown to have almost no influence on the size of the focal nanoregion.