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.
Physicists from the team behind the new work have found a way to make the instrument play overtones — the very harmonics that make a violin's sound richer. They modulated the electron beam density to suppress the fundamental frequency and allow the third and fifth harmonics to amplify. The result was a stable laser pulse with energy up to 540 microjoules — twice as broad in frequency as before, without changing the setup.
This approach promises compact X-ray sources for laboratories where there's no room for giant accelerators. Instead of a new setup for each task — one, but with tunable overtones.
The pulse energy of 540 microjoules is comparable to the kinetic energy of a flying mosquito, but compressed into billionths of a second and focused into a micron-sized spot, it can vaporize matter.