Terahertz (THz) waves, with the spectrum located between microwaves/millimeter-waves and infrared radiation, were historically difficult to be generated and detected and known as “terahertz gap”. However, Since the 1980s, the rapid development of semiconductor optoelectronics, ultrafast optics and ultrafast electronics has greatly promoted the research on generation, detection and application of THz waves. Many breakthroughs were achieved in the past decades, including high performance sources, sensitive detectors and interaction of THz wave with matters. THz waves have low photon energy and are non-ionizing. They can penetrate through most non-conductive materials. Many complex molecules have intra-/inter-molecule rotational or vibrational modes in THz frequencies. Due to these unique features, THz waves have great scientific value and have found an increasingly wide variety of applications in molecular fingerprint detection, diagnostic imaging, security and anti-terrorism, broadband communication, astronomical research, etc.