Dynamically Tunable Plasmonic Lens between the Near and Far Fields Based on Composite Nanorings Illuminated with Radially Polarized Light

We present a plasmonic lens with a dynamically tunable focal length in the mid-infrared region that is composed of gold film with annular slits and embedded vanadium dioxide (VO2) nanorings. When illuminated with radially polarized light, a perfect circularly symmetric focusing spot can be dynamically tuned between the near and far fields by decreasing temperature, and it can always be focused beyond the diffraction limit. A physical model of the tunable plasmonic lens is also proposed to explain the mechanisms by which the focal length is dynamically tuned. The field enhancement and the focal length can be further tuned by adding concentric annular slits and VO2 nanorings, respectively, while keeping the focusing spot size small. This provides a further step in the development of a perfect tunable plasmonic lens.