Round-tower plasmonic optical microfiber tip for nanofocusing with a high field enhancement

A round-tower (RT) plasmonic tip based on an optical microfiber for nanofocusing is proposed. Its operation principle is systematically analyzed by theoretical methods. Four operation regions of the theoretical model of the plasmonic tip are identified for the first time in this study. In addition, the near-field performances of the plasmonic optical microfiber tip are investigated by a finite-element simulation. The results reveal that the field enhancement at the tip apex can be improved by adjusting the slope angle of its surface plasmon polariton region. Remarkably, the proposed RT plasmonic tip can achieve a significantly enhanced field at the tip apex and simultaneously focus the optical spot to a few nanometers. The characteristics of superfocused optical spot and ultrahigh field enhancement are promising for applications in scanning near-field optical microscopy, optical recording, nanolithography, and biosensing.