The reason for people's interest in THz technology is that THz rays are universally available and are an effective clue and tool for people to understand the natural world. But compared to other bands of electromagnetic waves such as infrared and microwave, there is a dearth of knowledge and application of it. Second, THz rays have their own characteristics.
The typical pulse width of THz pulses is on the order of picoseconds, which not only facilitates time-resolved studies, but also effectively suppresses the interference of far-infrared background noise through sampling measurement techniques. At present, pulsed THz radiation usually has only a low average power of THz rays, but it has a high signal-to-noise ratio due to the high peak power of THz pulses and the fact that the real-time power of THz pulses rather than the average power is obtained by using coherent detection techniques. Currently, signal-to-noise ratios in time-domain spectroscopy systems are 10^5 or higher.
THz pulse sources usually contain only a number of cycles of electromagnetic oscillations, and the frequency band of a single pulse can cover a range from GHz up to a few tens of THz. The vibrational and rotational energy levels of many biomolecules, and the phonon vibrational levels of dielectrics, semiconductors, superconducting materials, and thin-film materials, etc., fall in the THz band range. Therefore, THz time-domain spectroscopy as an effective means of detecting material information in the THz band is very suitable for measuring material absorption spectra, which can be used for qualitative identification work.
THz photons have low energy, and the energy of a photon with a frequency of 1 THz is only about 4 millielectron volts, so it is not easy to destroy the detected material.
Many non-metallic, non-polar materials absorb THz rays less, making it possible to detect internal information in combination with the appropriate technology. For example, ceramics, cardboard, plastics, foam, etc. are transparent to THz radiation, so THz technology can be used as a non-ionizing and coherent complementary source of x-rays for security monitoring at airports, stations, etc., such as for detecting hidden smuggled items including firearms, explosives, drugs, etc., as well as for detecting soldering of integrated circuits. Polar substances absorb THz electromagnetic radiation more strongly, especially water, and THz spectroscopy should take various measures to avoid the influence of moisture, but in THz imaging technology, this feature can be used to distinguish different states of biological tissues, such as the distribution of fat and muscle in animal tissues, the diagnosis of the degree of damage to human burns, and the distribution of water content in plant leaf tissues. Terahertz imaging technology and other bands of imaging technology, it is obtained by the detection of the image resolution and depth of field have increased significantly (ultrasound, infrared, X-ray technology can also improve the image resolution, but millimeter-wave technology has not been significantly improved). In addition, terahertz technology has many unique properties, such as less scattering in non-homogeneous substances, the ability to detect and measure water vapor content, and so on.
Terahertz spectroscopy is not only a high signal-to-noise ratio, can quickly analyze and identify subtle changes in the composition of the sample, and terahertz spectroscopy is a non-contact measurement technology, making it possible to semiconductors, dielectric thin films and physical information of the body of the material to carry out rapid and accurate measurements. In view of the characteristics of THz rays, it will certainly bring far-reaching impact to the fields of communication, radar, astronomy, medical imaging, biochemical identification, material science, security checks, etc., which will in turn change people's production and life.