Low-loss silicon core fibre platform for broadband nonlinear photonics in the mid-infrared.

Broadband mid-infrared light sources are highly desired for wide-ranging applications that span free-space communications to spectroscopy. In recent years, silicon has attracted great interest as a platform for nonlinear optical wavelength conversion in this region, owing to its low losses (linear and nonlinear) and high stability. However, most research in this area has made use of small core waveguides fabricated from the silicon-on-insulator platform, which suffer from high absorption losses of the silica cladding, limiting their ability to generate light beyond 3 ${\mu}$m. Here we design and demonstrate a compact silicon core, silica clad waveguide platform that has low losses across the entire silicon transparency window. The waveguides are fabricated from a silicon core fibre that has been tapered to engineer the mode properties to ensure efficient nonlinear propagation in the core with minimal interaction of the mid-infrared light with the cladding. These waveguides exhibit many benefits of fibre platforms, such as high coupling efficiency and power handling capabilities, allowing for the generation of mid-infrared supercontinuum spectra with high brightness and coherence spanning almost two octaves (1.6-5.3 ${\mu}$m).