  Dual-comb spectroscopy is a powerful technique to measure optical spectra in
a wide spectral range with high-frequency resolution. The development of
compact systems operating in the long-wave infrared wavelength range is of high
interest for spectroscopic and sensing applications. Amongst the different
techniques to obtain optical frequency-combs, electro-optic frequency-comb
generation presents major advantages thanks to the tunable repetition rate only
limited by the bandwidth of the used electro-optical modulator. However, the
development of integrated and efficient electro-optical modulators operating in
a wide long-wave infrared spectral band is still at its infancy, and
electro-optical frequency-comb has not been demonstrated so far beyond the
telecom band. In this work, a Schottky-based modulator embedded in a Ge-rich
graded SiGe waveguide is used for electro-optic frequency-comb generation.
Considering the limited efficiency of the modulator, harmonically-rich RF
signals are used to enhance the generation of comb lines around the optical
carrier. Interestingly, this allows us to demonstrate the generation of
electro-optical combs spanning over 2.4 GHz around 8 {\mu}m wavelength. This
paves the way towards fully integrated and tunable mid-infrared electro-optic
frequency-comb generation systems.
