Phys. Rev. Lett. 115, 184801 (2015)

Free-electron lasers (FELs) use magnetic undulators to 'wiggle' electron bunches, creating photons whose parameters are highly tunable. Ying Wu and colleagues from the USA, Russia and China have now reported two-colour operation in a ring-based FEL. Using three undulators and dual-band dielectric mirrors they achieved simultaneous lasing at 720 nm in the near-infrared and 360 nm in the ultraviolet. Importantly, the arrangement yielded a similar level of gain at each wavelength, which is a challenging task. The ability to keep one lasing wavelength fixed while tuning the other is also important for exploitation of two-colour FELs. The ultraviolet lasing wavelength was able to be held at 360.05 ± 0.04 nm while the infrared lasing wavelength was swept (with 5-nm step size) from 675 nm to 735 nm. The team also achieved a fixed infrared wavelength of 720.00 ± 0.09 nm during ultraviolet wavelength tuning from 374 nm to 350 nm.