Photoactivated processes in optical fibers: generation and conversion mechanisms of twofold coordinated Si and Ge atoms.

In this work we present an extensive investigation of nanoscale physical phenomena related to oxygen-deficient centers (ODCs) in silica and Ge-doped silica by means of first-principles calculations, including nudged-elastic band, electron paramagnetic resonance parameters calculations, and many-body perturbation theory (GW and Bethe-Salpeter equation) techniques. We show that by neutralizing positively charged oxygen monovacancies we can obtain model structures of twofold Si and Ge defects of which the calculated absorption spectra and singlet-to-triplet transitions are in excellent agreement with the experimental optical absorption and photo-luminescence data. In particular we provide an exhaustive analysis of the main exciton peaks related to the presence of twofold defects including long-range correlation effects. By calculating the reaction pathways and energy barriers necessary for the interconversion, we advance a double precursory origin of the [Formula: see text] and Ge(2) centers as due to the ionization of neutral oxygen monovacancies (Si-Si and Ge-Si dimers) and as due to the ionization of twofold Si and Ge defects. Furthermore two distinct structural conversion mechanisms are found to occur between the neutral oxygen monovacancy and the twofold Si (and Ge) atom configurations. Such conversion mechanisms allow to explain the radiation induced generation of the ODC(II) centers, their photobleaching, and also their generation during the drawing of optical fibers.

Deep Levels and Electron Paramagnetic Resonance Parameters of Substitutional Nitrogen in Silicon from First Principles.

Nitrogen is commonly implanted in silicon to suppress the diffusion of self-interstitials and the formation of voids through the creation of nitrogen-vacancy complexes and nitrogen-nitrogen pairs. Yet, identifying a specific N-related defect via spectroscopic means has proven to be non-trivial. Activation energies obtained from deep-level transient spectroscopy are often assigned to a subset of possible defects that include non-equivalent atomic structures, such as the substitutional nitrogen and the nitrogen-vacancy complex. Paramagnetic N-related defects were the object of several electron paramagnetic spectroscopy investigations which assigned the so-called SL5 signal to the presence of substitutional nitrogen (NSi). Nevertheless, its behaviour at finite temperatures has been imprecisely linked to the metastability of the NSi center. In this work, we build upon the robust identification of the SL5 signature and we establish a theoretical picture of the substitutional nitrogen. Through an understanding of its symmetry-breaking mechanism, we provide a model of its fundamental physical properties (e.g., its energy landscape) based on ab initio calculations. Moreover by including more refined density functional theory-based approaches, we calculate EPR parameters (↔g and ↔A tensors), elucidating the debate on the metastability of NSi. Finally, by computing thermodynamic charge transition levels within the GW method, we present reference values for the donor and acceptor levels of NSi.

v-P2O5 micro-clustering in P-doped silica studied by a first-principles Raman investigation.

Synthetic vitreous silica is currently the preferred material for the production of optical fibres because of the several excellent properties of this glass, e.g. high transmission in the visible and IR domains, high mechanical strength, chemical durability, and ease of doping with various materials. For instance, fiber lasers and amplifiers exploit the light amplification properties provided by rare-earth ions employed as dopants in the core of silica-based optical fibers. The structure and composition of the nearest neighbor shell surrounding rare-earth ions in silica-based optical fibers and amplifiers have been intensively debated in the last decade. To reduce aggregation effects between rare-earth ions, co-dopants such as phosphorus and aluminium are added as structural modifiers; phosphorus-doping, in particular, has proved to be very efficient in dissolving rare-earth ions. In this work, we provide further insights concerning the embedding of P atoms into the silica network, which may be relevant for explaining the ease of formation of a phosphorus pentoxide nearest-neighbor shell around a rare-earth dopant. In particular, by means of first-principles calculations, we discuss alternative models for an irradiation (UV, x-, γ-rays) induced paramagnetic center, i.e. the so called room-temperature phosphorus-oxygen-hole center, and its precursors. We report that the most likely precursor of a room-temperature phosphorus-oxygen-hole center comprises of a micro-cluster of a few (at least two) neighboring phosphate tetrahedra, and correspondingly that the occurrence of isolated [(O-)2P(=O)2]- units is unlikely even at low P-doping concentrations. In fact, this work predicts that the symmetric stretching of P=O bonds in isolated [(O-)2P(=O)2]- units appears as a Raman band at a frequency of ~1110 cm-1, and only by including at least another corner-sharing phosphate tetrahedron, it is shown to shift to higher frequencies (up to ~40 cm-1) due to the shortening of P=O bonds, thereby leading to an improved agreement with the observed Raman band located at ~1145 cm-1.

Vibrational properties of vitreous GeSe2 with the Becke-Lee-Yang-Parr density functional.

The vibrational properties of vitreous GeSe(2) are studied within the Becke-Lee-Yang-Parr (BLYP) generalized gradient approximation to density functional theory. For this purpose, we consider two models. The first one is derived from a chemically ordered network, previously generated through classical molecular dynamics. The second one is generated through first-principles molecular dynamics simulations based on the BLYP functional and shows a larger variety of structural motifs. We describe the structural and electronic properties of our models, including bond lengths, first-neighbor coordinations, and the electronic density of states. The vibrational properties are addressed through the vibrational density of states, the infrared spectrum, and the Raman spectrum. For the chemically ordered model, the vibrational properties obtained with the BLYP functional are of comparable quality with respect to previous results obtained with the Perdew-Wang (PW) functional. However, a global consideration of our results indicates that the BLYP functional yields an overall improved description for the vibrational properties of vitreous GeSe(2). The BLYP dielectric constants are closer to experimental values. Furthermore, we found that the Raman spectrum of the model generated through first-principles molecular dynamics within the BLYP scheme is largely improved with respect to an analogous model generated within the PW scheme.

Vibrational spectra of vitreous SiO2 and vitreous GeO2 from first principles.

Using a density-functional approach, we calculate the principal vibrational spectra of vitreous SiO2 and vitreous GeO2 and discuss their analogies and differences. For both glasses, we generate model structures consisting of a random network of corner-sharing tetrahedra and differing only by their packing density. The comparison between calculated and measured neutron structure factors supports the validity of our model structures. Our investigation then extends to the vibrational properties, including the inelastic-neutron, infrared, and Raman spectra. For these spectra, good agreement with experiment is also found. Our results support the picture that silica and germania are constituted by a continuous random network of corner-sharing tetrahedra. In particular, the good agreement with experiment for the Raman spectra supports the average intertetrahedral angles of 148° and 135° found in our models of vitreous SiO2 and vitreous GeO2, respectively. The concentration of small ring structures in these glasses is also discussed.

Medium-range structural properties of vitreous germania obtained through first-principles analysis of vibrational spectra.

We analyze the principal vibrational spectra of vitreous GeO(2) and derive therefrom structural properties referring to length scales beyond the basic tetrahedral unit. We generate a model structure that yields a neutron structure factor in accord with experiment. The inelastic-neutron, the infrared, and the Raman spectra, calculated within a density-functional approach, also agree with respective experimental spectra. The accord for the Raman spectrum supports a Ge-O-Ge angle distribution centered at 135 degrees. The Raman feature X(2) is found to result from vibrations in three-membered rings, and therefore constitutes a distinctive characteristic of the medium-range structure.