Research progress of plasmonic nanofocusing

Surface plasmons (SPs) are the surface waves of collective oscillations of free electrons at metal-dielectric interface, which have the ability to overcome the diffraction limit and to enhance the giant near-field. Tapered metallic nanostructures that support surface plasmons' propagation are highly attractive to nanophotonic applications because of their waveguiding and field-focusing properties. This distinct morphologic feature enables the functionality known as nanofocusing. As a result, the plasmons can be guided in these nanostructures and finally focused on the sharp apex to greatly enhance the local field. This attractive effect can be widely used for effective remote-excitation detection/sensing. In this paper, we review various types of plasmonic nanofocusing structures operating in the visible and infrared region. We focus on their fundamentals, fabrications, and applications. Firstly, we discuss the mechanisms of the plasmonic nanofocusing. Then, the characteristics of various tapered metallic nanostructures of SPs are reviewed, including on-chip waveguides, metal tips and bottom-up fabricated nanowires. For applications, some prototypes of plasmonic nanofocusing for bio/chemo sensing are demonstrated. Finally, a summary and outlook of plasmonic waveguides are given.