Figure 1.

Principle of dual THz comb spectroscopy. λ₁-comb light (frequency spacing = f_rep1) is incident on a photoconductive antenna for THz detection (PCA detector), resulting in generation of a photocarrier THz comb (PC-THz comb, frequency spacing = f_rep1) in the PCA detector. λ₂-comb light (frequency spacing = f_rep2) is incident on a photoconductive antenna for THz generation (PCA emitter), resulting in radiation of an electromagnetic THz comb (EM-THz comb, frequency spacing = f_rep2) from the PCA emitter. When the free-space-propagating EM-THz comb is detected by the PCA detector having the PC-THz comb, a secondary frequency comb in the RF region (RF comb, frequency spacing = ∆f_rep = f_rep1 − f_rep2) is generated as a current signal from the PCA detector via multi-frequency-heterodyning photoconductive detection between the EM-THz comb and PC-THz comb. The RF comb is a replica of the EM-THz comb whose frequency spacing is downscaled from f_rep2 to ∆f_rep by a conversion factor f_rep2/∆f_rep.