  We demonstrated combination of gapless terahertz (THz) comb with dual-comb
spectroscopy, namely gapless dual-THz-comb spectroscopy, to achieve the
spectral resolution equal to width of the THz comb tooth. The gapless THz comb
was realized by interpolating frequency gaps between the comb teeth with
sweeping of a laser mode-locked frequency. The demonstration of low-pressure
gas spectroscopy with gapless dual-THz-comb spectroscopy clearly indicated that
the spectral resolution was decreased down to 2.5-MHz width of the comb tooth
and the spectral accuracy was enhanced to 10-6 within the spectral range of
1THz. The proposed method will be a powerful tool to simultaneously achieve
high resolution, high accuracy, and broad spectral coverage in THz
spectroscopy.
