Noble metal nanoparticles (NPs) are able to localize the electromagnetic (EM) energy of the incident light into subwavelength volumes. In addition, they can mediate the radiation from coupled emitters into the far-field. This allows to consider metal NPs as optical nanoantennas (NAs), analogous to common radio antennas.
Good optical NAs meet some requirements for their practical applications, namely, they possess a large scattering cross-section to collect maximal EM energy and display a large electric field enhancement to optimally transfer freely propagating EM energy into localized modes. To achieve this high enhancement, not only the photophysics of metal NP-emitter coupling in terms of all of the decay channels should be fully understood and controlled but also the geometry of the system should be very well defined and reproducible. Self-assembly is widely used for this purpose, as it allows to produce a large number of identical structures with defined positioning of all elements. In this chapter, we first discuss the general rules governing the decay rates of emitters in the vicinity of metal NPs. This will be followed with the discussion of general self-assembly strategies for NAs and applications of NAs for biosensing and single molecule studies. An alternative way to design NAs by using light-harvesting NPs will be briefly introduced. Finally, the challenges in the fabrication of NAs and their applications will be outlined.