Excitation and superfocusing of surface plasmon polaritons on a silver-coated optical fiber tip

We theoretically study the transfer of energy from a fiber mode to surface plasmon polaritons at the silver/air interface in the taper region of a silver-coated fiber tip. We describe conditions, under which a transfer efficiency from the fiber mode to the surface plasmon polariton of up to 46% can be achieved. Propagation of the surface plasmon polaritons towards the tip of the tapered structure leads to a strong subwavelength localization of the excitation enabled by a substantial increase of the surface plasmon polariton wave vector approaching the tip. The resulting strong field enhancement and confinement has a tremendous potential for applications in ultrahigh resolution tip-enhanced near-field optical microscopy.