Study of the energy transfer mechanism in different glasses co-doped with Si nanoaggregates an Er3+ ions

The risetime of the 1.54 mu m luminescence emission in Er: Si-nc co-doped glasses provides information on the mechanisms leading to population of the luminescent level, i.e. on the energy transfer time. In this paper we present a detailed study of this risetime in silica glasses after excitation in the wavelength region 360-860nm by a continuously tunable pulsed Ti:Al2O3 laser. The emission risetime turns out to be dependent on the sample fabrication method used, i.e. implantation or co-sputtering and on the annealing temperature used to precipitate the silicon aggregates. The fastest energy transfer time observed in our samples is tau(tr) <= 2 mu s. The risetime was found to be weakly dependent on the excitation wavelength up to around 700mn where a sudden increase is observed. This increase is more pronounced in samples where the annealing temperature was such to produce amorphous aggregates rather than crystalline ones. The difference in the nature of electronic states excited by near-infrared pumping in the two types of samples containing crystalline or amorphous aggregates is also evidenced by CW photoluminescence excitation (PLE) measurements which reflect the absorption coefficient of the sensitizing centers. In samples containing amorphous aggregates we observe an exponential dependence of the PL emission intensity versus excitation energy, which is indicative of an Urbach tail due to the disordered structure of aggregates. In the sample containing crystalline aggregates we observe an indirect interband transition behavior, with an energy gap of 1.56eV.