Non-resonant excitation of rare-earth ions via virtual Auger process

The luminescence of rare-earth ions (REI) is often intensified by defects associated with REIs or excitons bound to these defects. In this paper we show that the presence of such a state opens the possibility of non-resonance optical pumping via the process involving virtual Auger transition. It is the second order perturbation process when an electron arrives in an virtual intermediate state due to the optical transition (the first step) and the Auger transition is the second one. We have calculated the cross-section of such an excitation process when the optical transition is accompanied by creation of the exciton bound to the defect associated with REI and obtained a simple analytical expression for the cross-section. The excess energy of the excitation quanta is taken away by multiphonon emission. The electron–phonon interaction with local phonon vibrations of the bound exciton is assumed to determine the multiphonon process. It is shown that the probability of the process under study exceeds considerably the probability of direct optical 4f–4f4f–4f absorption even in the case when the energy distance between the excitation quantum energy and the exciton energy is about 0.1 of the exciton energy. The excitation mechanism considered leads to the appearance of a broad unsymmetrical band in the excitation spectrum with the red side much wider and flatter than the blue one.

Research highlights
► We consider the virtual Auger excitation of the rare-earth luminescence.
► A bound exciton or an excited defect play role of a virtual state.
► The efficiency of this second-order perturbation process is demonstrated.