Terahertz Time-Domain Spectroscopy (THz-TDS) is a spectroscopic technique that uses broadband pulses of terahertz radiation to probe the properties of materials. It can be used in transmission or reflection mode to receive information after the sample is irradiated. After fast Fourier transform, the THz-TDS signal can simultaneously obtain two types of signals, intensity and phase of the electric field, both of which are very sensitive to material properties. It can directly measure the relative permittivity of the material, and can also calculate the material's optical coefficients (such as absorption, refractive index, reflectance, etc.) and structural characteristics (such as porosity, crystallinity, water content, etc.).

Using Terahertz Time of Flight (THz-TOF), the sample can be scanned and imaged to obtain terahertz signals from different layers within the sample. Through data analysis, not only can a 3D graph be obtained, but also the homogeneity of different layers can be analyzed. Traditional Fourier transform spectroscopy in the visible range analysis can only measure optical intensity, and the information obtained is very limited.

In addition, if the material is birefringent, the terahertz polarization properties can be utilized to distinguish and study the properties of materials as well.