A study of the non-linear optical properties of Si-nc embedded in SiO2 has been performed by using the z-scan method in the nanosecond and femtosecond ranges. Substoichiometric SiOx films were grown by plasma-enhanced chemical-vapor deposition (PECVD) on silica substrates for Si excesses up to 24 at. %. An annealing at 1250 °C for 1 hour was performed in order to precipitate Si-nc, as shown by EFTEM images. Z-scan results have shown that, by using 5-ns pulses, the non-linear process is ruled by thermal effects and only a negative contribution can be observed in the non-linear refractive index, with typical values around -10-10 cm2/W. On the other hand, femtosecond excitation has revealed a pure electronic contribution to the nonlinear refractive index, obtaining values in the order of 10-12 cm2/W. Simulations of heat propagation have shown that the onset of the temperature rise is delayed more than half pulse-width respect to the starting edge of the excitation. A maximum temperature increase of ΔT = 123.1 °C has been found after 3.5 ns of the laser pulse maximum. In order to minimize the thermal contribution to the z-scan transmittance and extract the electronic part, the sample response has been analyzed during the first few nanoseconds. By this method we found a reduction of 20 % in the thermal effects. So that, shorter pulses have to be used to obtain just pure electronic non-linearities.
Erbium-doped silicon-rich dielectrics are expected to lead to compact and scalable cost-effective optical amplifiers due to the high sensing of Er via nano-silicon. Different silicates glasses, namely: Aluminum-silicates, soda-lime glasses and fused silica codoped with Si and Er were used in order to explore the mechanism of energy transfer from Si nanoclusters (Si nc) to Er. Si excess of 5 and 15 at.% and different Er doses, so that the resulting Er peak concentration could vary from 2x1019 up to 6x1020 cm-3, were introduced in the wafers by ion implantation technique. Thermal treatments in a rapid thermal process were carried out before and after Er implantation in order to precipitate Si nc, and to find the accurate temperature to obtain the best Er emission around 1540 nm. Very intense emission, comparing to structures only doped with Er, has been detected in all co-implanted glasses. By time resolved photoluminescence experiments we measured lifetimes of the exited state of Er3+ ions ranging from 2.5 to 12 ms and an effective excitation cross-section about 1x10-17 - 6 x10-17 cm2 (depending on the Er dose and Si excess). This is orders of magnitude higher than the Er direct absorption cross-section (about 10-21 cm2). By quantifying Er emission we found only 10% of the total Er concentration was effectively excited through Si nc.
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