Polycrystalline silicon waveguides for integrated photonics

Silicon photonics is an expanding domain of research since its booming a decade ago. Leveraging decades of research and development from the electronics industry, silicon photonics is the ideal candidate to overcome the bottleneck that microelectronics is facing with regard to the interconnect bandwidth limitations. In addition to being a platform compatible for both photonics and electronics, silicon is transparent in the mid-infrared regime, has a high refractive index for tight light confinement (i.e., small footprints), and presents a high nonlinear refractive index that is of high interest for optical signal processing applications. However, the integration of a silicon photonic layer is still challenging due to either the deposition flexibility or the material quality. In this thesis, a technique is presented to integrate a silicon layer with the deposition flexibility of amorphous silicon and the material quality of crystalline silicon, whilst being low-cost and having a thermal budget of < 450 °C making it compatible with the CMOS technology. Using a laser treatment, the as-deposited amorphous silicon is locally crystallised into polycrystalline silicon, a composite material of made of crystalline silicon crystallites surrounded by an amorphous silicon matrix. Both film and wire structures are processed and the material quality is assessed through optical microscopy, Raman spectroscopy, and X-ray diffraction. The optical quality of the polycrystalline wire waveguides is also investigated in the linear and nonlinear regime. In parallel to the planar silicon photonics geometry, silicon core fibres are also investigated in this work. These novel fibres offer the capability to integrate the functionality of silicon photonics platforms directly inside fibre architectures. Amorphous core fibres can be drawn with the lowest losses but as for the planar geometry, the material lacks electronics capabilities. On the other hand, polycrystalline silicon core fibres, which are suitable for both optical and electrical applications, can be drawn but their propagation losses remain high. In this work, two silicon core fibre material improvement methods, based on laser recrystallisation and tapering, are presented. To assess the material improvement, fibres are analysed through optical microscopy, Raman spectroscopy and X-ray diffraction. Finally, the optical losses of the improved fibres are measured on an optical transmission setup