Abstract
The guided surface plasmons mode inside the slit of noble metals and its short-pitch resonance are predicted with waveguide theory and verified by finite difference time domain (FDTD). Additionally, surface plasmons modes inside the slit of noble metals, propagation modes inside normal planar waveguide and evanescent-like modes inside the planar waveguides formed with strong absorbing materials are proved unifiable. The short-pitch resonances inside the metal slit is proved to be the result of high effective refractive index of the propagating surface plasmon mode inside the slit when the slit width reduces to several dozens of nanometers. The roles and contributions of the each kind of mode to the near-field enhancement at the immediate exit of the slit are analyzed theoretically and with FDTD through specially designed structures. The near-field enhancement is proved originating from the guided surface plasmon mode in the metal slit other than the surface plasmon modes along the incident and exit surfaces.