  Ultrabroadband electro-optic sampling with few-cycle optical pulses is known
to be an extremely sensitive technique to detect electric field amplitudes. By
combining this method with dual-comb spectroscopy and with a new class of
ultrafast lasers, we perform high-resolution (<10 MHz, 0.0003 wavenumbers)
spectroscopic measurements across the whole frequency range of 1.5 to 45 THz
(6.6-200 microns) with an instantaneous spectral coverage exceeding an octave
(e.g., 9-22 microns). As a driving source, we use a pair of highly
mutually-coherent low-noise frequency combs centered at 2.35 microns produced
by mode-locked solid-state Cr: ZnS lasers. One of the two combs is frequency
downconverted via intrapulse difference frequency generation to produce a
molecular sensing comb, while the second comb is frequency doubled to produce a
near-IR comb for electro-optic sampling (EOS). An ultra-low intensity and phase
noise of our dual-comb system allows capturing a vast amount of longwave
spectral information (>200,000 comb-mode spectral lines) at up to a video rate
of 69 Hz and with the high dynamic range limited by the shot noise of the
near-IR EOS balanced detection. Our long-wavelength IR measurements with
low-pressure gases: ethanol, isoprene, and dimethyl sulfide reveal
spectroscopic features that had never been explored before.
