Ge-rich graded SiGe waveguides and interferometers from 5 to 11 µm wavelength range

The mid-infrared (mid-IR) wavelength range hosts unique vibrational and rotational resonances of a broad variety of substances that can be used to unambiguously detect the molecular composition in a non-intrusive way. Mid-IR photonic-integrated circuits (PICs) are thus expected to have a major impact in many applications. Still, new challenges are posed by the large spectral width required to simultaneously identify many substances using the same photonic circuit. Ge-rich graded SiGe waveguides have been proposed as a broadband platform approach for mid-IR PICs. In this work, ultra-broadband waveguides are experimentally demonstrated within unprecedented wavelength range, efficiently guiding light from 5 to 11 µm. Interestingly, losses from 0.5 to 1.2 dB/cm are obtained between 5.1 and 8 µm wavelength, and values below 3 dB/cm are measured from 9.5 to 11.2 µm wavelength. An increase of propagation losses is seen between 8 and 9.5 µm; however, values stay below 4.6 dB/cm in the entire wavelength range. A detailed analysis of propagation losses is reported, supported by secondary ion mass spectrometry measurement, and different contributions are analyzed: silicon substrate absorption, oxygen impurities, free carrier absorption by residual doping, sidewall roughness and multiphonon absorption. Finally, Mach-Zehnder interferometers are characterized, and wideband operation is experimentally obtained from 5.5 to 10.5 µm wavelength.