  Waveguide integrated optical modulators in the mid-infrared (mid-IR)
wavelength range are of significant interest for molecular spectroscopy in one
hand, as on-chip synchronous detection can improve the performance of detection
systems, and for free-space communications on the other hand, where optical
modulators working in the atmospheric transparency windows are crucially
missing. Here we report for the first time the demonstration of optical
modulation in mid-IR photonic circuit reaching wavelengths larger than 8 um.
Moreover, optical modulation in an unprecedented wavelength range, from 5.5 to
11 um wavelength is shown, relying on a broadband Ge-rich graded-SiGe platform.
This first demonstration is used as a proof of concept, to experimentally
confirm the free-carrier absorption effect modeling. These results pave the way
towards efficient high-performance electrically-driven integrated optical
modulators in the mid-IR wavelength range.
