Dual-comb spectroscopy with tailored spectral broadening in Si${}_3$N${}_4$ nanophotonics

Si${}_3$N${}_4$ waveguides, pumped at 1550 nm, can provide spectrally smooth, broadband light for gas spectroscopy in the important 2 $\mu$m to 2.5 $\mu$m atmospheric water window, which is only partially accessible with silica-fiber based systems. By combining Er+:fiber frequency combs and supercontinuum generation in tailored Si${}_3$N${}_4$ waveguides, high signal-to-noise dual-comb spectroscopy (DCS) spanning 2 $\mu$m to 2.5 $\mu$m is demonstrated. Acquired broadband dual-comb spectra of CO and CO${}_2$ agree well with database line shape models and have a spectral-signal-to-noise as high as 48$\mathrm{/}\sqrt{\mathrm{s}}$, showing that the high coherence between the two combs is retained in the Si${}_3$N${}_4$ supercontinuum generation. The DCS figure of merit is 6$\times 10^6/\sqrt{$\mathrm{s}$}$,equivalenttothatofall-fiberDCSsystemsinthe1.6${$\mu$}$m band. Based on these results, future DCS can combine fiber comb technology with Si${}_3$N${}_4$ waveguides to access new spectral windows in a robust non-laboratory platform.