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.
Usually, such flashes are produced on different machines, and synchronizing them is like trying to get two drummers to play in rhythm without seeing each other. Here, a single electron bunch passes first through an X-ray amplifier, then through a terahertz one. The first takes energy for the X-rays, the second for the terahertz. The bunch doesn't have time to spread out, so both flashes are born from the same "shot."
The scientists measured the energy of both flashes. It turned out that the X-ray flash was brighter than if the bunch hadn't been pre-"sliced" into thin layers. These layers are like the teeth of a comb, setting the rhythm for both the X-rays and the terahertz. The delay between the flashes is always the same because it's determined by the bunch itself, not by an external timer.
Such a tool is needed to watch materials in motion. X-rays show where atoms are, and terahertz shows how they vibrate. Together, they give the full picture: like filming a movie of a crystal's life, where every frame is sharp and synchronized.
Terahertz radiation is sometimes called the "terahertz gap": for a long time, there were no sources or detectors capable of working in this range. Now it's used in airport security scanners.
In the series "Star Trek," the medical tricorder uses something similar to terahertz scanning: it sees through materials and tissues without damaging them.