Compact terahertz multiheterodyne spectroscopy using a Y-shape dual-comb configuration.

Due to the fast and high resolution characteristics, the dual-comb spectroscopy attracts more and more interest since the first demonstration. In the terahertz frequency range where abundant absorption lines (finger-prints) of molecules are located, the multiheterodyne spectroscopy employing the dual-comb technique shows an advantage in real-time spectral detection over the traditional Fourier transform infrared or time domain spectroscopies. Here, we demonstrate the first compact terahertz multiheterodyne spectroscopy using a Y-shape dual-comb configuration without a need of additional fast detectors. With only $\sim$270 nW terahertz power coupled into a terahertz QCL comb (no optics for alignment), the down-converted dual-comb spectra are successfully obtained at different carrier frequencies. To prove the spectroscopic ability, we further demonstrate that the compact dual-comb system can be used to calibrate the relative humidity in the air and to measure the transmission of a GaAs etalon. Due to the small optical coupling aperture (150 $\mu$m), it is also potential to use the dual-comb technique for terahertz imaging.