Since many important molecules have strong “fingerprints” in the mid-infrared (mid-IR, between 3μm and 15μm), this wavelength spectrum is currently gaining significant attention for applications ranging from pollution detection, quality control in the food industry, early cancer diagnosis, security and safety [1, 2]. Molecular sensing devices in the mid-IR are currently being developed and are in the process of commercialization. An appealing approach is to create molecular sensing devices in the mid-IR based on low cost integrated mid-IR chips. A key building block is a high brightness integrated broadband light source that would allow the detection of several molecules characterized by distinct absorption lines in parallel. Such an integrated broadband source, referred to as a supercontinuum source, has been already demonstrated in the mid-IR on a chalcogenide chip . However, demonstrating mid-IR supercontinuum on group IV materials, in order to exploit the advantages of reliable CMOS fabrication technology, remains a challenge. So far, numerous CMOS-compatible supercontinuum sources have been demonstrated in silicon nitride-on-insulator [4, 5], silicon-on-insulator [6, 7], silicon germanium-on-insulator  and silicon-on-sapphire platforms . However, these sources are limited up to 3.5µm and 6µm due to the absorption in the silica and sapphire substrate, respectively. More recently, the silicon germanium-on-silicon platform [10, 11], emerged as an attractive platform for mid-IR photonics, with transparency potentially extending up to 15μm depending on the Ge content .
Here we report experimentally the first octave spanning supercontinuum generation from a SiGe waveguide in the actual mid-IR with 5mW on-chip power exceeding that produced so far in any other Si-based platform (0.15mW in SiGe/SiO2  and ~1mW in silicon-on-sapphire ). Our 4.25µm x 2.70µm cross-section air-clad SiGe-on-Si waveguide has been designed and manufactured to achieve single mode operation at 4µm, low anomalous dispersion and strong fundamental TE mode confinement in the core nonlinear material (~96% at 4μm). Losses as low as 0.4dB/cm were measured between 3.8 and 5µm. The achieved supercontinuum covered more than an octave between 2.95 and 6.0µm was generated by pumping a 7cm long waveguide with ~ 200fs pulses at 4.15µm and 63MHz repetition rate, as delivered by a Miropa-fs optical parameter amplifier. These results were supported by simulations and the related generation of a high brightness supercontinuum establishes silicon germanium-on-silicon as a promising platform for integrated nonlinear photonics in the mid-IR, with the potential to extend the operating range to beyond 8μm.
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