Efficient on-chip molecule and bio-agent detection can be achieved by accessing strong molecular absorption lines in the mid-infrared, but it requires high output power broadband mid-IR sources. Here, we report supercontinuum generation in an air-clad Si 0.6 Ge 0.4 ∕Si waveguide that emits a broad spectrum spanning from 3.0 μm to 8.5 μm. These waveguides have anomalous dispersion and low propagation loss (<0.4 dB∕cm) in the mid-IR, which leads to a supercontinuum output with a high average power of more than 10 mW on-chip. The realization of broadband mid-IR sources with high spectral brightness makes the SiGe-on-Si platform promising for a wide range of applications.

We demonstrate coherent supercontinuum generation spanning over an octave from a silicon germanium-on-silicon waveguide using ∼ 200 f s pulses at a wavelength of 4 µm. The waveguide is engineered to provide low all-normal dispersion in the TM polarization. We validate the coherence of the generated supercontinuum via simulations, with a high degree of coherence across the entire spectrum. Such a generated supercontinuum could lend itself to pulse compression down to 22 fs.

We report a simple post-process technique that harnesses a hybrid chalcogenide/silicon-germanium system for the control of waveguide dispersion. By adding a chalcogenide top cladding to a SiGe/Si waveguide, we can substantially change the dispersive properties, which underpin the generation of a supercontinuum. In our particular example, we experimentally show that a shift from anomalous to normal dispersion takes place. We numerically study the dispersion dependence on the chalcogenide thickness and show how to use this additional degree of freedom to control the position of the zero dispersion wavelengths and hence the spectral span of the supercontinuum. Finally, we compare our approach with more traditional techniques that use geometry for dispersion tailoring.

Abstract:We characterize the nonlinear optical response of low loss Si 0.6 Ge 0.4 / Si waveguides in the mid-infrared between 3.3 μm and 4 μm using femtosecond optical pulses. We estimate the three and four-photon absorption coefficients as well as the Kerr nonlinear refractive index from the experimental measurements. The effect of multiphoton absorption on the optical nonlinear Kerr response is evaluated and the nonlinear figure of merit estimated providing some guidelines for designing nonlinear optical devices in the mid-IR. Finally, we compare the impact of free-carrier absorption at mid-infrared wavelengths versus near-infrared wavelengths for these ultra-short pulses. References and links1. R. Soref, "Mid-infrared photonics in silicon and germanium," Nat. Photonics 4(8), 495-497 (2010). 2. B. Jalali, "Nonlinear optics in the mid-infrared," Nat. Photonics 4(8), 506-508 (2010). Brun, S. Ortiz, P. Labeye, S. Nicoletti, and C. Grillet, "Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared," Opt. Express 23(7), 8261-8271 (2015).

Absorption spectroscopy based on supercontinuum generation in the mid-infrared is a powerful technique to analyze the chemical composition of samples. Furthermore, phase-coherent supercontinuum sources can enable fast data acquisition with coherent, stable pulses that allow single-shot measurements. We report here a numerical study of the coherence of an octavespanning mid-infrared supercontinuum source that was experimentally obtained in an air-clad SiGe/Si waveguide. We show that engineering two closely spaced zero-dispersion wavelengths that enclose an anomalous dispersion band centered around a fixed pump wavelength can produce supercontinuum pulses with high spectral density and full coherence at the extreme ends of the spectrum. This work is important for absorption spectroscopy, on-chip optical frequency metrology, and f-to-2f interferometry applications.