On-chip spectroscopy that could realize real-time fingerprinting withlabel-free and high-throughput detection of trace molecules is one of the 'holygrails" of sensing. Such miniaturized spectrometers would greatly enableapplications in chemistry, bio-medicine, material science or spaceinstrumentation, such as hyperspectral microscopy of live cells orpharmaceutical quality control. Dual-comb spectroscopy (DCS), a recenttechnique of Fourier transform spectroscopy without moving parts, isparticularly promising since it measures high-precision spectra in the gasphase using only a single detector. Here, we present a microresonator-basedplatform designed for mid-infrared (mid-IR) DCS. A single continuous-wave (CW)low-power pump source generates two mutually coherent mode-locked frequencycombs spanning from 2.6 $\mu$m to 4.1 $\mu$m in two silicon micro-resonators.Thermal control and free-carrier injection control modelocking of each comb andtune the dual-comb parameters. The large line spacing of the combs (127 GHz)and its precise tuning over tens of MHz, unique features of chip-scale combgenerators, are exploited for a proof-of-principle experiment of vibrationalabsorption DCS in the liquid phase, with spectra of acetone spanning from 2870nm to 3170 nm at 127-GHz (4.2-cm$^{-1}$) resolution. We take a significant steptowards a broadband, mid-IR spectroscopy instrument on a chip. With furthersystem development, our concept holds promise for real-time and time-resolvedspectral acquisition on the nanosecond time scale.
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