  On-chip spectroscopy that could realize real-time fingerprinting with
label-free and high-throughput detection of trace molecules is one of the 'holy
grails" of sensing. Such miniaturized spectrometers would greatly enable
applications in chemistry, bio-medicine, material science or space
instrumentation, such as hyperspectral microscopy of live cells or
pharmaceutical quality control. Dual-comb spectroscopy (DCS), a recent
technique of Fourier transform spectroscopy without moving parts, is
particularly promising since it measures high-precision spectra in the gas
phase using only a single detector. Here, we present a microresonator-based
platform designed for mid-infrared (mid-IR) DCS. A single continuous-wave (CW)
low-power pump source generates two mutually coherent mode-locked frequency
combs 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 and
tune 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 comb
generators, are exploited for a proof-of-principle experiment of vibrational
absorption DCS in the liquid phase, with spectra of acetone spanning from 2870
nm to 3170 nm at 127-GHz (4.2-cm$^{-1}$) resolution. We take a significant step
towards a broadband, mid-IR spectroscopy instrument on a chip. With further
system development, our concept holds promise for real-time and time-resolved
spectral acquisition on the nanosecond time scale.
