text: There are a great variety of techniques to generate THz radiation and to detect THz fields. One can, e.g., use an antenna, a quantum-cascade laser, a free-electron laser, or optical rectification to produce well-defined THz sources. The resulting THz field can be characterized via its electric field E(t). Present-day experiments can already output E(t) that has a peak value in the range of MV/cm (megavolts per centimeter). To estimate how strong such fields are, one can compute the level of energy change such fields induce to an electron over microscopic distance of one nanometer (nm), i.e., L = 1 nm. One simply multiplies the peak E(t) with elementary charge e and L to obtain e E(t) L = 100 meV. In other words, such fields have a major effect on electronic systems because the mere field strength of E(t) can induce electronic transitions over microscopic scales. One possibility is to use such THz fields to study Bloch oscillations where semiconductor electrons move through the Brillouin zone, just to return to where they started,  giving rise to the Bloch oscillations.
  The THz sources can be also extremely short, down to single cycle of THz fields oscillation. For one THz, that means duration in the range of one picosecond (ps). Consequently, one can use THz fields to monitor and control ultrafast processes in semiconductors or to produce ultrafast switching in semiconductor components. Obviously, the combination of ultrafast duration and strong peak E(t') provides vast new possibilities to systematic studies in semiconductors. 
  Besides the strength and duration of E(t), the THz field's photon energy plays a vital role in semiconductor investigations because it can be made resonant with several intriguing many-body transitions. For example, electrons in conduction band and holes, i.e., electronic vacancies, in valence band attract each other via the Coulomb interaction. Under suitable conditions, electrons and holes can be bound to excitons that are hydrogen-like states of matter. At the same time, the exciton binding energy is few to hundreds of meV that can be matched energetically with a THz photon. Therefore, the presence of excitons can be uniquely detected based on the absorption spectrum of a weak THz field. Also simple states, such as plasma and correlated electron–hole plasma can be monitored or modified by THz fields.