Nanoantennas : new prospects in light-matter-interactions
The possibilities of nanoantennas for an enhanced interaction of light with quantum systems is theoretically investigated. A versatile framework is introduced to describe this interaction. A classical electrodynamics description of nanoantennas enables to freely design and tune them for specific means. But this approach leads to a semiclassical description of the entire system. We introduce a quantization scheme for nanoantennas based on their quasinormal modes, which enables a fully quantum treatment of the hybrid system. We show how nanoantennas can be designed to enhance the rates of so-called dipole-forbidden transitions of quantum systems, since the electromagnetic fields in the vicinity of a nanoantenna may vary much stronger than for plane waves. Specifically, an electric quadrupole excitation rate can be enhanced in the gap of a dimer nanoantenna. In regard to the specifics of an assumed quantum system, this excitation can cause a luminescence enhancement.We suggest using a hybrid quantum system consisting of a nanoantenna and a quantum system as an ultra-bright single-photon source. Due to its interaction with a nanoantenna, a quantum system can spontaneously emit radiation at higher rates when compared to free space. But this enhancement is not the only important figure of merit for single-photon sources. A trade-off between emission rate and nonclassicality of the emitted light is found as well. We investigate how quantum systems may be strongly coupled to nanoantennas. Fundamental trade-offs may set ultimate limits for achievable coupling-to-loss-ratios. To realize the strong coupling regime, nanoantennas have to be sufficiently small, which causes comparably low efficiencies. If a strongly coupled hybrid system is pumped, its dynamics and spectral response depends considerably on the excitation strength. The effects of strong coupling are most pronounced for weak excitations.