Semiconductor waveguides for mid-infrared photonics

Mid-infrared semiconductor photonics is an emerging field with wide ranging applications. One stream of research is focused on extending the well-developed silicon-based waveguide platforms into longer wavelength regimes because of the inherent transparency window of silicon in the mid-infrared regime as well as its favourable nonlinear properties. Alternative approach is to investigate the optical properties of new materials (i.e. germanium) that offer favourable properties such as broader transparency windows and large nonlinearities, etc.In this thesis, two types of novel mid-infrared waveguide platforms were investigated. The first was the semiconductor optical fibres, an innovative platform that incorporates the functional semiconductors within the robust fibre geometry. A range of different core materials were characterised from the telecommunications band into the mid-infrared regime including polycrystalline silicon, hydrogenated amorphous silicon and hydrogenated amorphous germanium. Particularly, the large nonlinearity of the hydrogenated amorphous silicon core fibres was measured systematically cross this wavelength regime previously unknown for these fibres. With the knowledge of the key nonlinear parameters including nonlinear absorption and refraction, supercontinuum generation was demonstrated in the mid-infrared where the two-photon absorption was negligible. The measurements in the mid-infrared represent the first characterisation of the material beyond 1.55 µm.The second platform was the germanium on silicon waveguides, which can be fabricated using similar techniques to the silicon integrated waveguides and are thus compatible with the widely used complementary metal-oxide-semiconductor platform. The results presented in this thesis represent the first comprehensive linear and nonlinear transmission loss characterisations of this new class of waveguide for selected mid-infrared wavelengths. By exploiting the free carriers and two-photon absorption mechanisms, high speed all-optical modulation was demonstrated across selected mid-infrared wavelengths