Mid-Infrared Supercontinuum Generation in Germanium Waveguides

On-chip mid-infrared (mid-IR, from 3 to 15m wavelength) supercontinuum (SC) generation is of great interest for integrated sensing devices, since it enables parallel detection of multiples species [1]. The generation of mid-IR SC in group IV platforms is particularly significant, since it would allow low-cost, large-scale production and electro-optic integration. Mid-IR SC generation has been demonstrated in several group-IV platforms [1] - [4]. Recently, the wide transparency window up to 15m and theoretical predictions of attractive nonlinear properties [5] led researchers to explore germanium as a promising material for mid-infrared photonics. However, experimental SC generation in pure germanium waveguides remained elusive so far. Our group reported SC generation up to 8.5m in silicon-germanium on silicon waveguides [3]. We have also demonstrated the possibility, with this platform, of precisely controlling the SC coherence properties [6] - [8]. The great potential of germanium-based platforms for on-chip mid-IR SC generation was recently confirmed, with the demonstration of a SC extending from 3 up to 13m in a germanium-rich graded index silicon-germanium on silicon waveguide [9]. To achieve such a broad spectrum, however, the waveguide was pumped at 7.5m, whereas short wavelength pumps are preferable in fully integrated broadband sources.