NanoSlim
Fiber lasers and amplifiers have grown considerably through their use in industry (materials processing), civil engineering (sensors) and medical (LASIK), to name a few examples. Such fibers rely on the doping of the glass by luminescent ions, generally rare-earth ions. As the luminescent properties of the latter depend on their atomic environment, the choice of material is paramount. Silica is the preferred material for optical fibers because it has many mechanical and economic advantages. However, the use of this glass limits certain luminescent properties. The development of new applications therefore requires new glass compositions. In this context, the encapsulation of luminescent ions in nanoparticles is proposed since it makes it possible, in principle, to engineer the luminescence properties through the environmental control of rare-earth ions. Such fibers would combine the advantages of silica and offer spectroscopic properties which would not exist in this glass. The development of these fibers is confronted with the problem of transparency management. Nanoparticles with a size smaller than around 50 nm are required to obtain sufficiently low light scattering induced losses. The current approach to control their size consists in preparing a preform (rod drawn in fiber) already containing small nanoparticles. However, it is difficult to carry out such a preform, as well as to maintain the integrity of the small nanoparticles during the drawing which takes place at 2000°C. This project aims to propose a radically different approach. Indeed, we propose to take advantage of the drawing step to obtain small nanoparticles in the fiber starting from "big" nanoparticles in the preform.
This process is based on the elongation of the nanoparticles during the drawing and their break-up induced by Rayleigh-Plateau instabilities. Such effects have already been evidenced to take place during the fiber drawing of optical fibers, as characteristic arrays of particles (produced by break up of a particle present in the preform) can be seen on Fig. 1, SEM image of the core of a particle-rich optical fiber and Fig. 2, X-ray nanotomography. Observed break-up depends on various parameters such as the size of the nanoparticles, the viscosity of the matrix and of the nanoparticles and the surface tension between the two media. Yet, due to the high temperatures of the material and the small scale of these objects, some of these parameters are still unknown. Thus, to develop in a controlled manner such a process, the NanoSlim project focuses on carrying out a systematic study of these parameters.
The study will focus on Europium-doped silica fiber, codoped with magnesium. During this project, 3 tasks will be covered, to study the thermodynamic and hydrodynamic effects occurring during fiber drawing:
- Production of model samples: monoliths of silica with nanoparticles of controlled size and composition ( mainly MgO and xMgO-(1-x)SiO2).
- Experimental analysis: very specific characterization techniques are involved in the Nanoslim project for studying size, morphology, structure and composition at nanometric scales (atom probe tomography, high resolution electron microscopy, etc.) and luminescence properties.
- Numerical simulations: analysis of the process will also be conducted through numerical simulations. Two complementary approaches will be implemented: finite elements and molecular dynamics.