Quantum theory of near-field optical imaging with rare-earth atomic clusters

Scanning near-field optical microscopy (SNOM) using local active probes provides general images of the electric part of the photonic local density of states. However, certain atomic clusters can supply more information by simultaneously revealing both the magnetic and the electric local density of states in the optical range. For example, nanoparticles doped with rare-earth elements like europium or terbium provide both electric dipolar (ED) and magnetic dipolar (MD) transitions. In this theoretical paper, we develop a quantum description of active systems (rare-earth ions) coupled to a photonic nanostructure by solving the optical Bloch equations together with Maxwell’s equations. This approach allows us to access the population of the emitting energy levels for all atoms excited by the incident light, degenerated at the extremity of the tip of a near-field optical microscope. We show that it is possible to describe the collected light intensity due to ED and MD transitions in a scanning configuration. By carrying out simulations on different experimentally interesting systems, we demonstrate that our formalism can be of great value for the interpretation of experimental configurations, including various external parameters such as the laser intensity, the polarization and wavelength, the SNOM probe size, and the nature of the sample.