2) Exotic doping

It is well-known that noble metal ou semiconductor nanoparticles (NPs) exhibit high nonlinear optical coefficients, especially in the neighbourhood of their resonance (surface plasmon resonance for metals and exciton for semiconductors). NPs may also be studied in the scope of energy transfer towards active ions in order to enhance amplifier’s or laser’s efficiency. However, these objects, widely studied in thin films or in bulk materials, had never been introduced in the core of an optical fiber in a clearly demonstrated manner. This quasi-absence of realisation is partly ascribable to the difficulty of preserving nanoparticles during the different steps of glass fabrication and fibre drawing . In effect, while melting temperature of metallic gold is close to 1064°C, the glass-working temperature is around 2000°C. In order to attempt to stabilise gold nanoparticles in the core of a fibre, a first stategy is to take advantage of the sol-gel synthesis of silica glass, which limits the impact of high temperature treatments. This approach, developped in Lille since 2002 in the case of monoliths, consists of a chemical way yielding nanoporous glasses that may be impregnated by solutions contening the desired precursors. The use of a gold precursor or of a solution contening gold nanoparticles actually leads to drawable doped monoliths. We have shown that the so-formed nanoparticles were able to resist to several heat-treatments of the glass at high temperature (about 2000°C) and that they could survive until the final fibre geometriy is obtained. The linear and non-linear optical properties of such fibres fabricated using these monoliths are clearly modified by the presence of nanoparticles.

Another option consists in doping the porous gel or the glass with NPs precursors, then to prompt their crystal growth in a localised manner under a laser beam. In this domain, our team has acquired international acknowledgement. In particular in porous sol-gel silica, 2D or 3D gratings of metal nanocristals (Au, Ag) or semiconductors nanocrystals (CdS, PbS) have been generated in different irradiation conditions(pulsed or CW lasers at various wavelengths). Photo-thermal or photo-chemical mechanisms have been identified depending on the employed laser and precursors. This work, falling within the framework of a project supported by ANR POMESCO, has led us to define the best experimental conditions allowing efficient synthesis of stable NPs in silica xerogels. Extrapolation of these techniques to dense glasses, then to optical fibres, is going on, the localised aspect of the NPs growth being particuliarly interesting for the use of optical nonlinearities in pulsed lasers.

Left: Photograph and absorption spectra showing SPR resonance of gold NPs produced in silica xerogel under femtoseconde laser irradiation.

Right: Photograph and HRTEM image of CdS NPs produced in the vicinity of a gel surface by visible CW laser irradiation.

The fields of health nuclear industry are nowadays enquirer of fibred technologies. In medicine for example, Beside the biomedical laser or fibrescopy, which are among the most widespread applications of optical fibre, these fibred technologies can lead to the achievement of reliable, precise and innovative measurement systems in order to offer to practitioners real-time useful tools, more and more efficient during medical interventions on patients. In the nuclear domain, dose or temperature metrology increasingly makes use remote sensors, among which optical fibre has a role to play in accordance with its immunity regarding parasitic electromagnetic waves. Silica fibres are particularly interesting because they are able to resist to harsh environments, for example to radiation doses of several kilograys (kGy). In this context, our team considers developing new doped silica rods, as well as new fibres for ionizing radiation metrology (X-ray, g, protons…). The operating modes of such devices are based on radio-induced luminescence phenomena, either spontaneous (Radioluminescence RL or scintillation) or optically stimulated (OSL). To generate this luminescence, pure silica preforms prepared by sol-gel process or by MCVD, then the corresponding fibres, have been doped with one or two doping species (Cu⁺, Ce³⁺, Tb³⁺, Gd³⁺ ions, germanium oxide …).

Fibred dosimeters should play a major role in the future in the area of radiotherapy and of high energy physics, being given their intrinsic advantages (radiation resistance, multi-core fibres, low intrusiveness, etc.). As an example, the conception of fibred dosimeters could allow localised and remote measurements of radiations, which would represent a real progress in cancer treatment by radiotherapy, but also in the survey of nuclear plants or for radioactive waste storage.

We have already demonstrated the potentiality of doped fibres for UV, X, g dosimetry and also under proton beam. The SURFIN project (new materials for the survey of nuclear plants through optical fibres), funded by ANDRA and French PIA over the period 2017-2022, is coordinated by PhLAM, performed in collaboration with Laboratoire Hubert Curien (LabHC) at University of Saint-Etienne, the Institut de Chimie de Clermont-Ferrand (ICCF) at University of Clermont-Auvergne and the Institut de Physique de Nice (InPhyNi) at University of Sophia Antipolis. Another project, this one having medical purpose, aims at exploiting the small dimensions of such optical fibres, as well as developing their sensitivity and their response dynamics, for a real-time and precise definition of dose or dose-rate profiles in radiotherapy X or in pulsed protontherapy. This is the reason of FIDELIO project (Fiber-based In-vivo Realtime Dosimetry for Pulsed Radiotherapy), funded by French ANR over the period 2021-2025 and to which we participate with InPhyNi, LabHC and the iXBlue Photonics Division firm in Lannion. Other research operations concerning proton beam dosimetry through optics fibres have been conducted in collaboration with the Canadian particle accelerator centre TRIUMF in Vancouver.