Mid-infrared transmitting Ge-Sb-Se chalcogenide glass fibres: for potential use in medical diagnostics

This Project is aimed at developing and fabricating mid-infrared (MIR) transmitting germanium-antimony-selenium (Ge-Sb-Se) chalcogenide glass fibres for passive transmission in MIR sensing and as new high-purity low-loss optical fibres for active MIR supercontinuum generation (SCG). The work reported in this Thesis can be divided into three main categories: (i) a study of four Ge-Sb-Se glass compositions (atomic %): (Ge19Sb15Se66), (Ge20Sb10Se70), (Ge24Sb4Se72) and (Ge22Sb8Se70), in terms of their thermal properties, optical properties and cytotoxicity to match potential glass pairs for small-core step-index MIR SCG fibres; (ii) to develop and optimise methods of achieving high-purity chalcogenide glasses and (iii) a preliminary study of the structure of Ge-Sb-Se glasses along the stoichiometric tie-line x(GeSe2)-(1x)( Sb2Se3), to guide future selection of candidate glass pairs to draw to step-index fibre. The MIR spectral region covers the wavelengths 3-50 μm and characteristic vibrational absorption spectra unique to each molecular type. Vibrational spectroscopy can detect subtle changes in the specific spectral response within this region. Molecular vibrations are indicative of changes within biological cells relative to normal biological cells, signifying the presence or absence of a disease. This Project contributes to the collaborative MINERVA Project which is developing a remote skin cancer detection system using MIR absorption spectroscopy aiming to carry out disease diagnosis in vivo. Providing broadband photons at MIR wavelengths has previously presented difficulties. Conventional MIR blackbody light sources are weak and optical fibres for transmitting MIR light to/from tissue in vivo can be limited by strong material absorption such as silica glass > 2.4 μm and tellurite, and heavy metal fluoride, > 4.75μm. In contrast, chalcogenide glasses have been shown to transmit MIR light out to 25 μm and MIR SCG from ~ 2 – 15.1 µm has recently been demonstrated in chalcogenide glass fibre. This Thesis reports on the characterisation of four Ge-Sb-Se glass compositions to match potential glass pairs for fabrication of step-index fibres based on particular thermal properties and desired fibre numerical aperture (NA). Three glass pairs are drawn to fibre: Pair I (Ge22Sb8Se70) and (Ge24Sb4Se72), Pair II (Ge19Sb15Se66) and (Ge20Sb10Se70), and Pair III (Ge20Sb10Se70) and (Ge24Sb4Se72). Difficulties emerged and are examined in the extrusion and fibre drawing processes arising from a mismatch in the glass pair’s physical properties. Thus, a hierarchy of the order of selection of physical properties is suggested, with matching the glass transition temperature (Tg) deemed to be the top priority. The optical properties of the fabricated fibres are characterised in terms of predicted NA, near-field imaging and optical loss measurements. The minimum loss achieved (2.42 dB/m at 6.66 µm wavelength) is for Pair I (Core: Ge22Sb8Se70 and cladding: Ge24Sb4Se72). The effect of heat-treating to purify the precursor elements Sb and Se on subsequent fibre loss is observed and it is established that glass purity was a critical factor affecting the intensity of hydride and oxide impurity absorption bands punctuating the 2.5 – 10 µm wavelength transmission region. Therefore, distillation methods are explored as a means of generating high-purity chalcogenide glasses and a new distillation rig is developed. Preliminary cytotoxicity tests on a fibre fabricated from Pair I are conducted to provide the foundations of a procedure for future chalcogenide glass fibre cytotoxicity testing. The initial data demonstrated the potential of etching the Ge-Sb-Se chalcogenide glass fibres in propylamine to reduce any cytotoxic response caused by the Ge-Sb-Se fibres. Neutron diffraction experiments are combined with Tg and density measurements along the stoichiometric tie-line x(GeSe2)-(1-x)( Sb2Se3), to aid in a greater understanding of the structure-property relationship of Ge-Sb-Se glasses for the future selection of candidate glass step-index fibre pairs. The preliminary work established that the stoichiometric glasses are predominantly made up of [GeSe4] and [SbSe3] units. From the neutron diffraction data, it is suggested that the average bond length of a Sb-Se bond was 2.62 ± 0.001 Å and the average bond length of a Ge-Se bond was 2.37 ± 0.001 Å. Extracting the coordination of the Ge and Sb elements is found to be difficult on account of an overlap of the Ge-Se and Sb-Se peaks. Therefore further analysis, using nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) is suggested. It is shown that, as the vitreous structure changes from higher levels of [SbSe3] units to incorporate more [GeSe4] (thus a reduction of Sb), the Tg of the glass increases and the density decreases. A close match in Tg (< 23 °C) is recommended as critical for the successful fabrication of a Ge-Sb-Se chalcogenide glass fibre. Thus, knowledge of the trend in Tg, dependent on the ratio of [SbSe3] units to [GeSe4] units, is an initial step in selecting theoretical Ge-Sb-Se compositions with a closer match of thermal properties as candidate glass step-index fibre pairs. Having a more accurate guide to select theoretically Ge-Sb-Se glass compositions to match thermal properties is suggested to save time synthesising and characterising obsolete compositions. It is concluded that Ge-Sb-Se chalcogenide glass fibres, developed through this Project, are strong candidates towards achieving MIR SCG small-core fibres, with the potential application for the transmission of MIR to and from potentially cancerous skin tissue samples. Therefore, enabling in vivo mapping for an immediate diagnostic response