Low-pressure gas spectroscopy using terahertz frequency synthesizer traceable to microwave frequency standard via dual optical comb

We constructed a widely and continuously tunable terahertz frequency synthesizer traceable to a microwave frequency standard. Photomixing of two optical frequency synthesizers, linked to the frequency standard via dual optical frequency combs, makes this THz synthesizer traceable to the frequency standard. To demonstrate the potential of wide and continuous tunability in the THz synthesizer, we tuned its output frequency by 120 GHz continuously around 0.65 THz by using a uni-traveling-carrier photodiode as a photomixer and applied it for precise THz spectroscopy of low-pressure molecular gas. This THz synthesizer will be a powerful tool for broadband, high-precision THz spectroscopy.