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NanoCrystals in Fibre Lasers

Periodic Reporting for period 2 - NCLas (NanoCrystals in Fibre Lasers)

Reporting period: 2020-01-01 to 2021-06-30

In the NCLas project, a disruptive technology for the synthesis of glasses containing functional nanocrystals (NCs) will be introduced and developed. It is based on a novel hybrid nanosintering process, which allows for the incorporation of a large variety of NCs and functionalization of glasses in different formats for various applications. Previous attempts to either grow NCs inside glass by glass heat treatment or incorporate NCs during glass formation showed unconvincing results. In this nanosintering process, key enabling steps include reducing the sintering temperature, developing specialized NC core-shell structures and adjusting the glass composition, thus achieving a chemically inert environment and matching the refractive index of NCs and glass. This technology will be exploited to produce low-loss, NC-functionalised glass fibres. Fibre lasers are energy efficient, compact and offer maintenance-free operation, ultra-short pulses, high power, and low noise. Today’s commercial fibre lasers are fabricated from robust, durable silica glass. The operation of oxide fibre lasers can be extended to an enormous spectral range (~400 – 3000 nm) by doping oxide glasses with laser-active nanocrystals optimized for particular laser wavelengths, thus enabling a huge variety of new applications. We will demonstrate two highly relevant fibre lasers: (i) a Ti3+:sapphire-NC (Ti:Sa) fibre laser tuneable around 800 nm for biophotonic applications; (ii) a Pr3+:yttria-NC 1300-nm fibre laser enabling a much-awaited wavelength extension in telecommunications and also fitting into one of the biophotonic windows. An interdisciplinary team mitigates the high risk of NCLas with demonstrated experience in their fields and highly complementary backgrounds. The consortium challenges all steps from material synthesis to device demonstration. NCLas makes a significant contribution to Key Enabling Technologies such as Nanotechnology, Photonics and Advanced Materials.
According to the plan, the first year of this 4-year-project concentrated mainly on the investigation, characterisation and provisioning of the matrix glass nanoparticles as well as the laser-active nanocrystals Ti3+:sapphire and Pr3+:yttria. For the latter, five different routes of synthesis have been investigated and the resulting NCs thoroughly characterised in order to select the most suitable technology for the project. For the chosen multi-component matrix glass BaO-Ga2O3-GeO2 (BGG), several compositions have been synthesised without crystallisation and aloss coefficient < 0.003 cm-1. The necessary rate equation simulation tools for the project developed as well as capabilities for writing fibre Bragg gratings have been confirmed on a first project fibre.
In early experiments, the laser-active NCs have been sintered in a mixture with BGG powder and other alternative glass powders by subjecting them to an appropriate furnace cycle with 30 minutes holding time. Some combinations indicate the survival of the NCs at least up to the softening temperature of the respective glass. First fibre drawing experiments were directed towards investigation of the NC survival when they are subjected to the heating cycle on the drawing tower, rather than producing fibres of good optical quality. In some cases, the survival of the fluorescence for both active ions, Ti3+ and Pr3+ could be confirmed, but requires further investigations.
While BGG glass as a core material can be drawn into fibre of good optical quality using the rod-in-tube method, the drawing experiments with BGG using the powder-in-tube method exhibited a rather strong crystallization. In matrix glass study, a number of synthesized and commercial glass compositions have been investigated with respect to their crystallization behavior. In various sinter experiments without and with the active NCs, several glass candidates were identified that also allow refractive index matching to one of the NCs. In powder-in-tube experiments using glass NPs mixed with active NCs, optical fibres without matrix glass crystallizations were successfully drawn. So far, the active NCs dissolved frequently during drawing, but in a few instances active crystalline nanoparticles were observed. Building on these results, further research will focus on speeding up the thermal process as well as stabilizing the active NCS e.g. by core-shell structures.
Despite the early stage of the project, NCLas is already progressing beyond the state of the art. To the best of our knowledge, this is the first comparative study on synthesis, characterisation and luminescence properties of Pr3+:Y2O3: nanocrystals. Because the FET Open format focuses on future technologies in an early stage of research, the main impact of NCLas during the duration of the project will be generated on the scientific and technological side. The radically new nanosintering process will offer unprecedented parameter control over a wide range of material combinations and will establish a new class of functional hybrid materials. The incorporation of a wide variety of new functional NCs into hybrid materials, especially in fibres and waveguide, is expected to trigger new research directions towards the exploitation of new functionalities. Such a process can be exploited not only in the field of lasers, but also for active coatings or new metamaterials with properties that combine in a synergistic manner the properties of glass and the NCs. However, the nanosintering process itself is also expected to be a valuable research tool in materials science. To foster exchange within the scientific community and beyond, the project partners have developed a communication and dissemination strategy, that includes the organisation of a Summer School, an official NCLas website, a project video, scientific and other publications.
The socio-economic impact of NCLas will be mainly realised towards or after the end of the project. The two planned demonstrators are highly relevant for industry and society: The Ti:sa fibre laser will replace their complex, bulky, and expensive solid-state equivalents and provide platform to transfer new bio-photonic research from the labs into the real world all the way to point-of-care solutions. The Pr-doped fibre amplifier will provide a long-awaited solution for the important telecom window near 1.3 µm wavelength, where commercial gain devices are missing, thereby increasing the transmission bandwidth for digital society (e.g. Industry 4.0 IoT, etc.)
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