Nanofocusing of a novel plasmonic fiber tip coupling with nanograting resonance

Abstract

The nanofocusing of plasmonic tips which can realize localized electric field enhancement beyond the diffraction limit is highly desired for light–matter interaction and super-resolution imaging research. A novel plasmonic fiber tip made of a metal-coated fiber tip coupling with nanograting resonance is theoretically and numerically presented using the finite difference time domain method. The nanofocusing of metal-coated fiber tip is investigated under the internal excitation of the radial vector beam and the evolution of internal optical radial vector mode to surface plasmon polaritons (SPPs) mode is calculated under the excitation wavelength of 632.8 nm. The circular nanogratings are designed on the metallic sidewalls of the plasmonic fiber tip, and the resonance enhancement mechanism is studied with the interference of different propagation modes. It is found that the positions of the circular slits are determined by the interference of SPP modes and the interference of internal optical mode and external SPP mode. The nanogratings are nonperiodic, and with the decrease of the effective refractive index of the optical mode, the pitch length between two slits gets bigger. The results show that the novel plasmonic fiber tip coupling with nanograting resonance can achieve one order of magnitude higher in light intensity compared with the existing plasmonic tips without compromise on spatial resolution and signal-to-noise ratio.