7. Optical continuous THz sources

This section describes THz lasers and optical frequency mixing sources. Recall here that lasers require stimulated emission between a high-energy level and a lower-energy level, the latter needing to be less populated than the high-energy level for amplification to occur. The emission wavelength corresponds to the energy difference between these two levels. For THz radiation, these levels must be separated by a few meV. Either the rotovibrational levels of molecules in gaseous form must be used, or artificial structures must be created whose energy levels are defined by their geometric shape.