Excitation of erbium ions in SiO2 with Si nanocrystals via virtual Auger process

Here we propose a possible mechanism of fast Er3+ ions excitation in SiO2 matrix with Si nanocrystals. We show that the presence of Si nanocrystals allows for non-resonant optical pumping of erbium ions by virtual Auger transition, which is the second order process via an intermediate virtual state: the first step is the optical transition inside the Si NC, and the second one is Auger excitation of Er3+ ion accompanied by intraband transition of the confined carrier. This mechanism of excitation can take place when the energy of photon absorbed is larger than the sum of the confined electron-hole ground state energy and the excitation energy of Er3+ ion. We have calculated the excitation cross-section as a function of the excitation energy for erbium ions situated both inside and outside the NC. We show that virtually all Er3+ ions inside NCs can be excited directly into the first excited state I-4(13/2) (responsible for the 1.5 mu m emission) by the laser pulse of duration 5-10 ns. The results obtained for ions located outside NCs demonstrate the efficiency of the virtual excitation Auger process for transition of erbium ions into the higher excited states. 1.5 mu m PL appears in this case as a result of nonradiative relaxation of excited ions down to the I-4(13/2) state. Correspondingly, the rise time of the 1.5 mu m PL should be about several microseconds. The cross-sections calculated demonstrate the efficiency of such Auger process.