An orchestra where every musician plays at their own tempo sounds like a mess. But when the conductor raises the baton, all instruments blend into a harmonious melody where every note is in its place. Physicists face a similar challenge when stabilizing the frequency of a laser. A laser doesn't emit a continuous beam but a series of ultrashort pulses, like a drumroll. If the gaps between the beats 'drift,' the roll turns into noise. Stabilization is the conductor that makes every pulse arrive exactly on time.
In the new work, scientists took a special type of laser — a thin-disk laser, where light is amplified as it passes through a thin crystal resembling a small mirror. This laser delivers an average power of 180 watts — roughly equivalent to 18 incandescent bulbs, but packed into pulses lasting millionths of a billionth of a second. Inside the laser, the peak power reaches 500 megawatts, like the instantaneous flash of an entire power plant. To tame this power, the scientists used a clever trick: they passed part of the light through a yttrium aluminum garnet crystal, which broadens the spectrum just as a prism splits white light into a rainbow. Then they brought the two edges of this rainbow together, and by their interference, a pattern of bright and dark fringes, they could detect the slightest jitter in the pulse rhythm.
After such stabilization, the laser compresses its pulses to a peak power of 0.9 gigawatts: that's like a thousand power plants flashing for an instant. Such a beam can knock electrons out of noble gas atoms, producing ultraviolet and even extreme ultraviolet radiation. This radiation, in turn, can be turned into a 'frequency comb'—a set of thousands of precisely measured colors used in the most accurate clocks and in spectroscopy. Previously, such devices were bulky and finicky. Now, thanks to the compact thin-disk laser, there's a chance to create a tabletop device that delivers a stable comb in the ultraviolet. And that opens new possibilities for studying fast processes in atoms and molecules, for precise navigation, and possibly for future quantum technologies.
One pulse from this laser lasts about 200 femtoseconds. In that time, light travels a distance less than the thickness of a human hair, and the pulse itself carries enough energy to vaporize a drop of water.
The idea of a precise frequency comb evokes 'lightsabers' from science fiction, but in reality it's a tool for ultra-precise measurements that could help build quantum computers.