Huan Zhao

An ordinary free-electron laser is like a musical instrument with a single string: it can play only one note until you change its design. To get a different wavelength, you have to retune magnets or accelerate electrons harder. That's expensive and bulky.

A shooter aims at a target, pulls the trigger, and hears two shots: one loud, one quiet. That's how the new setup works, except instead of bullets, there's an electron bunch, and instead of shots, there are two flashes of light. One flash is X-ray, the other is terahertz. They fly to the same point with perfect timing.