Infrared frequency comb generation and spectroscopy with suspended silicon nanophotonic waveguides

Nanophotonic waveguides with sub-wavelength mode confinement and engineered dispersion are an excellent platform for application-tailored nonlinear optical interactions at low pulse energies. We present fully air-clad suspended silicon waveguides for infrared frequency comb generation with optical bandwidth limited only by the silicon transparency. Precise lithographic control over the waveguide dispersion enables tailored infrared frequency comb generation across a bandwidth of 2.0-8.8 mu m (1130-5000 cm(-1)), with the broadest simultaneous bandwidth covering 2.0-7.7 mu m. Novel fork-shaped couplers provide efficient input coupling with only 1.5 dB loss. The coherence, brightness, and 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 4.9-8.8 mu m (1130-2050 cm(-1)) using three different waveguide widths. 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 root Hz across a simultaneous bandwidth of 6.3-8.2 mu m (1220-1590 cm(-1)) 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.