In our setup, a femtosecond fiber laser (MenloSystem Inc., Martinsried, Germany, F-780A), which operates with a 100-MHz repetition rate, is used to provide 100-fs laser pulses with a central wavelength of 780 nm. The laser pulses are split into a pump beam and a probe beam by a beam splitter. The pump beam is incident on the photoconductive emitter to produce electron–hole pairs, which are then accelerated by a biased electric field. According to Maxwell’s theory, these accelerated electron–hole pairs will generate a transient electric field and emit a transient electromagnetic wave. Because the duration of the excited laser pulse is 100 fs and the lifetime of the photoexcited carriers in the antenna substrate (low-temperature GaAs) is about 360 fs, the duration of the transient electromagnetic wave is on the picosecond order, lying in the THz band. The THz pulses are collimated to the sample by a pair of silver-coated off-axis parabolic mirrors, and then the pulses carrying information about the samples are guided to the THz detector by another pair of off-axis parabolic mirrors, changing the refractive index of ZnTe. This change will be encoded by the probe beam, which reaches the crystal synchronously, and will be detected by a balanced photodetector. After lock-in amplification, the signal will be recorded, and the waveform of the THz pulse will be measured by time-domain sampling. In order to remove the effect of water vapor, a sample chamber, which can be purged with nitrogen, was used to reduce the relative humidity down below 5% during the whole experiment. Moreover, each sampled data point in the THz pulse is measured five times and averaged to increase the signal-to-noise ratio.