Excitable Er fraction and quenching phenomena in Er-dopedSiO2layers containing Si nanoclusters

Abstract

This paper investigates the interaction between Si nanoclusters ͑Si-nc͒ and Er in SiO 2 , reports on the optical characterization and modeling of this system, and attempts to clarify its effectiveness as a gain material for optical waveguide amplifiers at 1.54 m. Silicon-rich silicon oxide layers with an Er content of 4 -6 ϫ 10 20 at./ cm 3 were deposited by reactive magnetron sputtering. The films with Si excess of 6 -7 at. %, and postannealed at 900°C showed the best Er 3+ photoluminescence ͑PL͒ intensity and lifetime, and were used for the study. The annealing duration was varied up to 60 min to engineer the size and density of Si-nc and optimize Si-nc and Er coupling. PL investigations under resonant ͑488 nm͒ and nonresonant ͑476 nm͒ pumping show that an Er effective excitation cross section is similar to that of Si-nc ͑ϳ10 −17 -10 −16 cm 2 ͒ at low pumping flux ͑ϳ1016 -10 17 cm −2 s −1 ͒, while it drops at high flux ͑Ͼ10 18 cm −2 s −1 ͒. We found a maximum fraction of excited Er of about 2% of the total Er content. This is far from the 50% needed for optical transparency and achievement of population inversion and gain. Detrimental phenomena that cause depletion of Er inversion, such as cooperative up conversion, excited-stated absorption, and Auger deexcitations are modeled, and their impact in lowering the amount of excitable Er is found to be relatively small. Instead, Auger-type short-range energy transfer from Si-nc to Er is found, with a characteristic interaction length of 0.4 nm. Based on such results, numerical and analytical ͑Er as a quasi-two-level system͒ coupled rate equations have been developed to determine the optimum conditions for Er inversion. The modeling predicts that interaction is quenched for high photon flux and that only a small fraction of Er ͑0.2-2 %͒ is excitable through Si-nc. Hence, the low density of sensitizers ͑Si-nc͒ and the short range of the interaction are the explanation of the low fraction of Er coupled. Efficient ways to improve Er-doped Si-nc thin films for the realization of practical optical amplifiers are also discussed.

Energy transfer mechanism and Auger effect in Er3+ coupled silicon nanoparticle samples

“…As reported in different works, 11,12,18,43 the coupling constant scales with distance following an exponential decay law: this is a clear signature of an electronic exchange mechanism between donor and acceptor generating from wave function overlap ͑Dexter transfer͒. 16 The other possible transfer mechanisms, dipole-dipole interaction ͑Förster͒ and indirect radiative excitation do not likely occur in our system: the former being too slow with respect to the fast transfer time we measured ͑tens of nanoseconds͒, while the latter does not explain the enhancement of Er ions effective excitation cross section, being less efficient than direct excitation itself.…”

“…7,8 A widely spread believe is that this kind of mechanism is the main cause for the low fraction ͑ϳ3-5%͒ of optically active Er ions population effectively coupled to Si-np reported so far in the literature. [9][10][11][12] In this study, we report spectroscopic experiments performed in samples where the coupled Er content is about 52% of the optically active ions ͑i.e., 13.5% of total Er 3+ concentration͒ ͑Ref. 13͒ and the measured internal optical gain in active waveguides is as large as 1 dB/cm.…”

“…In this perspective, the main and, possibly, only detrimental issue in achieving a complete inversion of Er 3+ population for amplification purposes is given by the effective interaction distance between sensitizers and ions, as recently reported in literature. 11,12,17,18 …”

Bipolar pulsed excitation of erbium-doped nanosilicon light emitting diodes

“…The saturation of the 1.54 lm emission may be attributed to both a limited amount of optically active Er ions and to the onset of nonradiative recombination processes. 11,16,23 The presence of the last is evidenced by the decrease of the luminescence decay time shown in Fig. 4, which we will discuss later.…”

Achieving optical gain in waveguide-confined nanocluster-sensitized erbium by pulsed excitation

“…This coupling is an active area of experimental research. 9,10 However, net gain in the system is only possible if gain from inverted Er 3+ is greater than loss due to carrier absorption.…”