  Nanophotonic waveguides with sub-wavelength mode confinement and engineered
dispersion profiles are an excellent platform for application-tailored
nonlinear optical interactions at low pulse energies. Here, we present fully
air clad suspended-silicon waveguides for infrared frequency comb generation
with optical bandwidth limited only by the silicon transparency. The achieved
spectra are lithographically tailored to span 2.1 octaves in the mid-infrared
(2.0-8.5 um or 1170--5000 cm-1) when pumped at 3.10 um with 100 pJ pulses.
Novel fork-shaped couplers provide efficient input coupling with only 1.5 dB
loss. The coherence, brightness, and the stability of the generated light are
highlighted in a dual frequency comb setup in which individual comb-lines are
resolved with 30 dB extinction ratio and 100 MHz spacing in the wavelength
range of 4.8-8.5 um (2100-1170 cm-1). These sources are used for broadband gas-
and liquid-phase dual-comb spectroscopy with 100 MHz comb-line resolution. We
achieve a peak spectral signal-to-noise ratio of 10 Hz^0.5 across a
simultaneous bandwidth containing 112,200 comb-lines. These results provide a
pathway to further integration with the developing high repetition rate
frequency comb lasers for compact sensors with applications in chip-based
chemical analysis and spectroscopy.
