In a free electron laser (FEL), the frequency scales as the square of the beam voltage divided by the wiggler period. Because of the favorable scaling with voltage, FELs show promise for operating at high frequencies.
P. E. Latham,
"Harmonic operation of a free electron laser", Proc. SPIE 1514, 15th International Conference on Infrared and Millimeter Waves, 15145N (1 December 1990); doi: 10.1117/12.2301613; https://doi.org/10.1117/12.2301613