Electromagnetic waves at terahertz (THz) frequencies are in the region of the electromagnetic spectrum between 0.3 and 3 THz. Terahertz waves are safe to use because they are non-ionizing and can pass through clothing, skin, paper, cardboard and plastic, wood, plastic and ceramics. They can also penetrate fog and clouds, but cannot penetrate metal, concrete or water.
Despite many valuable useful applications, the adoption of terahertz waves has been slow because of the limited output power from currently available sources. Terahertz waves lie between the optical and the microwave spectrum and cannot be efficiently generated by either scaling down optical sources like lasers or scaling up conventional microwave sources such as klystrons. Current moderate size terahertz sources can only generate a few milliwatts of average power and hence require expensive and complicated schemes for detection. Therefore, the lack of commercially available sources and detector for this frequency region lead to the term ‘Terahertz Gap’.