Electro-optically sampled broadband dual-comb spectroscopy across mid-IR to terahertz (6.6–200 µm) spectral range

Ultrabroadband Electro-Optic Sampling (EOS) with few-cycle optical pulses is known to be an exceptionally 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 (10-80 MHz, 0.0003-0.0027 cm-1) spectroscopic measurements across the whole frequency range of 1.5 to 45 THz (6.6–200 μm), excluding the strongly absorbing Reststrahlen band of lattice resonances at 4.5–9 THz, with an instantaneous spectral coverage exceeding an octave (e.g., 9–22 μm). As a pump source, we use a pair of mutually coherent low-noise frequency combs centered at 2.35 μm produced by mode-locked solid-state Cr:ZnS lasers. To produce a molecular 'sensing' comb in the long-wave infrared region, one of the two driving combs is frequency down-converted via Intrapulse Difference Frequency Generation (IDFG) in ZGP or GaSe nonlinear crystals. The second driving comb is frequency doubled in a GaSe crystal to produce a near-IR comb for EOS. A low intensity and phase noise of our dual-comb system allows capturing a vast amount of comb-mode resolved (mode spacing 80 MHz) spectral information (⪆200,000 comb lines) at up to a video rate of 69 Hz. This result was also facilitated by high IDFG conversion efficiency (e.g., ⪆10% in ZGP crystal). Our long-wavelength IR measurements with low-pressure gases: ethanol, isoprene, and dimethyl sulfide reveal spectroscopic features that had never been explored before.