“…However, these methods are not used widely because of their relatively poor operability, integration limitations, and high cost. Recently, our team designed a horn-shaped tapered parallel plate waveguide (HSTPPW), which can enhance the electrical field of the incident THz wave at its central position ( Hou et al., 2021 ; Shi et al., 2021 ). Based on HSTPPW, the THz spectral information of substances in an aqueous solution can be obtained by the THz time-domain spectroscopy (THz-TDS) system with high accuracy, although the method is expensive and it is a little difficult to control the thickness of the sample dropped into the waveguide.…”