The terahertz region of the electromagnetic spectrum - loosely defined as the frequency range from 0.3 to 20 THz (1 mm – 15 µm wavelength, 10 – 600 1/cm) - is a frontier area for photonics research. This part of the electromagnetic spectrum has been less well explored and has been called the "terahertz gap" because of the lack of high-power, room-temperature sources and of efficient detectors. The generation and detection of radiation in this frequency range is difficult; scaling and extrapolating known concepts of the microwave or infrared ranges is not possible. Up-scaling of microwave devices suffers from the transit time limitation of classical devices. Down-scaling of photonic concepts conflicts with the fact that the energy of a THz photon is substantially smaller than the thermal energy.

Despite these difficulties, the terahertz region has attracted enormous attention in recent years as a number of applications with a potentially large impact have been identified, particularly in the area of imaging. Historically, the fields of astronomy and chemistry have been a driving force for developing sources and detectors in the terahertz regime as the rotational and vibrational resonances of molecules as well as thermal emission in molecular spectra play an important role here. Recent advances however make terahertz radiation attractive for a much wider range of activities.