Introduction

At the heart of light–matter interaction lies the absorption or emission of a photon by an electronic transition, e.g. in an atom, molecule, QD or color center. Because these are generally much smaller than the wavelength of light, they interact weakly and omni-directionally with light, limiting both their absorption and emission rate. At RF similar issues were encountered and addressed long ago. Electrical circuits radiate little because they are much smaller than the corresponding wavelength. To enable wireless communication, they are connected to antennas that have dimensions in the order of the wavelength. These antennas are designed to effectively convert electrical signals into radiation and vice versa. Exactly the same concept can be applied in optics.

Hence the central idea of this chapter is that the interaction of a quantum emitter with light can be improved by near-field coupling it to the LSPR modes of a metal NP. The key idea is that the LSPRs of a metal NP create a strong local field at the NP. If an emitter is placed in this field, its absorption and emission of radiation are enhanced. The function of the NP is then analogous to an optical antenna. In this way, excitation and emission rates can be increased, and the angular, polarization and spectral dependence controlled.

This chapter first outlines these optical antenna concepts and next provides several concrete examples of how such antennas can be used to control and improve the interaction of single quantum emitters with light.