Terahertz Physics Yan Zhang, Capital Normal University Terahertz radiation is a special range sandwiched between the microwave and infrared in the electromagnetic spectrum. It is found that many wood, ceramic, plastic, and paper-based materials are highly transparent to terahertz radiation. In combination with the relatively short terahertz wavelengths, this transparency presents an opportunity for high-resolution, potentially noninvasive imaging for security or quality-control applications. The low energy of the terahertz photons and extreme sensitivity to water molecules absorption provide another change to in-vivo characterization and imaging of biological cells. The relatively short wavelength and large bandwidth are suitable for highly directive and secure point-to-point communication, with the potential benefit of enormous data rates. Furthermore, the terahertz spectrum also hosts a range of important microscopic phenomena, such as the inter/intra-molecular motions and Debye relaxation processes. Semiconductors with moderate carrier concentration have a plasma frequency and damping rate between 0.1 and 2.0 THz. All of this translates into the ability to sense the presence or character of a wide range of materials in ways that are inaccessible to other frequency bands. This unique property of sensing has now been utilized to probe the properties and dynamics of semiconductors, superconductors and other correlated electron materials. In this presentation, the main applications of the terahertz radiation, spectroscopy and imaging, are introduced. Some piratical applications, such as explosives detection, are demonstrated and further problems are proposed.