Nanooptics and photonics

Phonon bottleneck in emission of rare earth ions embedded into nanocrystal structures 

R. Francinia, F. De Matteisa, Z.Ya. Zhuchenkob, A. Zolotovskyb , Yu.Bacherikovb,  and G. Tarasovb

aDipartimento di Ingegneria Industriale, Università di Roma “Tor Vergata”, Via del Politecnico, 00133 Roma, Italy

bInstitute of Semiconductor Physics, National Academy of Sciences of Ukraine, Prospect Nauki 45, 03028 Kiev, Ukraine 

E-mail: zhuchenko@isp.kiev.ua

Recently, rare earth doped nanocrystals and structures  are widely used for numerous photonic applications. In this report we consider different nanostructures doped with Er3+ ions. These systems   have been structurally studied by the TEM, EDX, and XRD techniques, and optically investigated by means of photoluminescence and up-conversion technique. In such structures, the certain structural stabilization is expected. Indeed, we have found from TEM, EDX, and XRD studies that the structures with a fully determined crystalline (mono-phase) structure can be realized. The low-temperature spectra ascribed the 4I13/2  4I15/2 transitions in Er3+ reveal some peculiarities which cannot be understand without knowledge of the crystal- field splitting of 4I13/2  and 4I15/2 manifolds. These manifolds are especially important while they are the working levels for Er-based lasers. The simulation has been performed using multi-electron molecular orbitals cluster model developed for the case of nanocrystal structures.  Both energy levels and the probabilities of transitions between the split levels have been calculated, thus allowing to reproduce the PL line shape for the 4I13/2  4I15/2  transitions. It has been found the existence of peculiar energy gap in level distribution of split 4I13/2 states. Taking into account possible absence of low-energy phonons in the phonon spectrum of spatially confined structures, the energy restricted relaxation (so called phonon bottleneck) is expected for  4I13/2  manifold. This mechanism is an origin of appearance of the “hot PL bands” in the low-temperature PL spectra in the range of the 4I13/2  4I15/2  erbium transitions. The consequences of this restricted relaxation can be also revealed by means of time-resolved luminescence.