Circular hybrid plasmonic waveguide with ultra-long propagation distance.

We propose a novel plasmonic waveguide structure, which is referred to as a circular hybrid plasmonic waveguide (HPW) and consists of a metal wire covered with low- and high-index dielectric layers. The circular HPW exhibits two distinctly different modes, namely, the strongly localized mode and the extremely low-loss mode. Our numerical calculation demonstrates that the strongly localized mode exhibits 10 order scale in normalized mode area and can be performed even in tens of nanometer sizes of waveguide geometry. In the extremely low-loss mode, the HPW exhibits ultra-long propagation distance of more than 10μm that can be achieved by forming the dipole-like hybrid mode and properly adjusting the radius of the metal wire. It is also shown that, even with this long-range propagation, the mode area of the dipole-like hybrid mode can be maintained at subwavelength scale. The simultaneous achievement of a small mode area and ultra-long propagation distance contributes to the ultra-high propagation distance to mode size ratio of the waveguide. The HPW results are very helpful for plasmonic device applications in the fields of low-threshold nanolasers, ultrafast modulators, and optical switches.