Nanoantennas

Focusing light to a highly confined spot with subwavelength size and high intensity has important applications in nanolithography, optical and magnetic data storage, spectroscopy, biosensing, molecular trapping, etc. However, light diffraction sets a fundamental limit on the far-field optical resolution on the order of wavelength, and it poses a critical challenge to the downscaling of nanoscale optical focusing. At the optical near-field, the light refraction also restricts the energy transmission through the subwavelength aperture with an extremely low efficiency that is proportional to (d/λ)4 [1], where d is the diameter of the aperture and λ is the wavelength of illumination light. As discussed in previous chapters, efficient light focusing can also be realized by taking advantage of its large wavevector of surface plasmon polaritons (SPPs). SPPs can be excited by incident light on some metallic interface and propagate with long decay lengths, and various plasmonic structures have been proposed to focus light [2–6]. Focusing light with orders of magnitude higher efficiency can be realized by focusing SPPs, and various nanofocusing structures have been proposed and demonstrated. In this chapter, we will discuss the use of SPPs for efficient optical energy focusing.