Principles of THz Generation, Chapter 2 in "Semiconductor Terahertz Technology: Devices and Systems at Room-Temperature Operation"

The THz frequency range (100 GHz-10 THz) is situated between microwaves and infrared optics. For a long time, it was called the THz gap, since there were no efficient sources and detectors available in contrast to the neighbouring microwave and optical domains. In the mean time, a multitude of means to generate THz radiation has been developed in order to close this gap. For highest THz power levels, i.e. tens of Watt level average power and tens of J level pulse energies, factory hall sized free electron lasers and synchrotrons have been constructed. A heavily accelerated, relativistic electron beam is guided into an undulator, a structure where the electron is deflected back and forth by alternating magnetic fields. The acceleration of the relativistic electrons perpendicular to their main direction of propagation results in dipole radiation along the propagation axis. In free electron lasers, the undulator is usually situated inside a THz cavity, The cavity field acts back on the electron beam, resulting in micro bunching, i.e. dicing the electron beam into packets. Constructive interference of all electron packets occurs, resulting in a laser-like behaviour.