Mode evolution and nanofocusing of grating-coupled surface plasmon polaritons on metallic tip

摘要

We present a detailed analysis on mode evolution of grating-coupled surface plasmonic polaritons (SPPs) on a conical metal tip based on the guided-wave theory. The eigenvalue equations for SPPsmodes are discussed, revealing that cy-lindrical metal waveguides only support TM01 and HEm1surface modes. During propagation on the metal tip, the grat-ing-coupled SPPs are converted to HE31, HE21, HE11 and TM01 successively, and these modes are sequentialy cut off except TM01. The TM01 mode further propagates with drasticaly increasing effective mode index and is converted to loca-lized surface plasmons (LSPs) at the tip apex, which is responsible for plasmonic nanofocusing. The gap-mode plas-mons can be excited with the focusing TM01 mode by approaching a metal substrate to the tip apex, resulting in further enhanced electric field and reduced size of the plasmonic focus.