Infrared frequency comb generation and spectroscopy with suspended silicon nanophotonic waveguides

Abstract

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