Fig. 1. Principle and experimental block diagram of terahertz-wave frequency up-conversion detection

Fig. 2. The evolution of all waves within a DAST crystal in ideal conditions. (a) Only DFG considered; (b) only SFG considered; (c) coexistence of DFG and SFG

Fig. 3. Photon numbers of signal lights as functions of crystal thickness for weak terahertz-wave detection

Fig. 4. Photon numbers of signal lights at different pumping intensities. (a) DFG and SFG signal lights versus crystal thickness under the coexistence of DFG and SFG; (b) total photon numbers of signal lights as functions of THz photon numbers

Fig. 5. The number of signal photons and the SNR as functions of the pumping power density in terahertz single-photon detection