A radiation source comprising a frequency conversion member (1) configured to emit a beam of emitted radiation (7) with at least one frequency in the range from 0.1 THz to 100 THz, in response to irradiation with an input beam (3) with a frequency different to that of the emitted radiation (7), the source being subjected to a magnetic field (9) which has a component parallel to that of the emitted beam of radiation (7). The radiation source may be optimised by using both the fluence of the input beam and the magnetic field or may comprise a magnetically doped frequency conversion member or also have means for applying an electric field.