  Two semiconductor optical frequency combs consuming less than 1 W of
electrical power are used to demonstrate high-sensitivity mid-infrared
dual-comb spectroscopy in the important 3-4 $\mu$m spectral region. The devices
are 4 millimeters long by 4 microns wide, and each emits 8 mW of average
optical power. The spectroscopic sensing performance is demonstrated by
measurements of methane and hydrogen chloride with a spectral coverage of 33
cm$^{-1}$ (1 THz), 0.32 cm$^{-1}$ (9.7 GHz) frequency sampling interval, and
peak signal-to-noise ratio of ~100 at 100 $\mu$s integration time. The
monolithic design, low drive power, and direct generation of mid-infrared
radiation are highly attractive for portable broadband spectroscopic
instrumentation in future terrestrial and space applications.
