ИСТИНА

We present a theoretical investigation of a single atom (in two-level or three-level scheme) fluorescence affected by a nanoantenna illuminated be a laser field. The nanoantenna is a metal nanoparticle or a cluster of two nanoparticles with size much smaller than wavelength of the incident laser field. Such plasmon antennas can effectively concentrate the energy of the incident optical radiation in the nanoscale region converting the far field into the near field [1]. Also such devices can considerably enhance radiative and absorptive properties of a single atom (or a molecule). Both this aspects allow handling successfully the fluorescence intensity and the shape of fluorescence spectrum changing geometrical properties of the nanoantenna. Capability to control fluorescence spectrum and its intensity opens also novel perspectives in creating elements for quantum information processing, generating entangled states, creating novel high-precision biosensors, etc. [2, 3]. Two key factors affect in this case the fluorescence spectrum of an atom. First, it is the local field, which can be drastically enhanced by illuminating the nanoantenna with the laser field. This local field enhancement leads to the increase of the atom's absorption efficacy and to splitting the lines in the resonance fluorescence spectrum due to the dynamical Stark effect. The latter reveals as three lines in the fluorescence spectrum in the case of a two-level atom and in 5 to 7 lines (instead of a single line) for the three-level atom. Second factor is that the nanoantenna varies the fluorescence rate of the atom. Moreover the radiative and non-radiative decay rates can be independently changed. As a result, the fluorescence intensity can be greatly increased or almost quenched. Additionally, one can manipulate the widths of the resonant lines in the spectrum. Three-level atoms show here much more degrees of freedom to play with than their two-level counterparts.