Integrated broadband dual-polarization Ge-rich SiGe mid-infrared Fourier-transform spectrometer.

Miniaturized on-chip spectrometers covering a wide band of the mid-infrared spectrum have an immense potential for multi-target detection in high-impact applications, such as chemical sensing or environmental monitoring. Specifically, multi-aperture spatial heterodyne Fourier-transform spectrometers (SHFTS) provide high throughput and improved tolerance against fabrication errors, compared to conventional counterparts. Still, state-of-the-art implementations have only shown single-polarization operation in narrow bandwidths within the near and short infrared. Here, we demonstrate the first, to the best of our knowledge, dual-polarization ultra-wideband SHFTS working beyond 5 μm wavelength. We exploit the unique flexibility in material engineering of the graded-index germanium-rich silicon-germanium (Ge-rich SiGe) photonic platform to implement a SHFTS that can be operated in an unprecedented range of 800  cm, showing experimental resolution better than 15  cm for both orthogonal polarizations and free spectral range of 132  cm, in the wavelength range between 5 and 8.5 μm.