This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Ultrashort laser pulses can be used to generate X-rays. Normally, however, only certain specific frequencies are produced.
A team from TU Wien and the University of California San Diego has developed a method that makes it possible to tune the frequency continuously. Using a six-meter-long gas-filled waveguide (20 feet long) containing the appropriate gas, the researchers can "detune" the light frequencies so that, in the end, exactly the required X-ray frequency is produced. The results have been published in the journal Communications Physics.
For many years, researchers have used an atomic physics trick to generate high-frequency X-rays: Atoms are irradiated with a laser, which can cause them to produce a whole series of frequencies themselves—namely, integer multiples of the original frequency. These are known as "high harmonics." This is reminiscent of musical instruments: Acoustic frequencies, too, are often generated not individually but in entire series. If, for example, a violin plays a note at 440 hertz, overtones at 880 hertz and still higher multiples of the fundamental frequency are automatically produced as well.
In laser physics, the term "frequency comb" is often used because the resulting frequencies are always separated by a specific interval, like the teeth of a comb. The advantage is that a single experimental setup can generate a whole range of frequencies. The disadvantage is that if the exact wavelength needed lies between two of these "frequency teeth," the setup is of little use.
This is a problem, for example, when trying to excite an atomic resonance that requires one very specific frequency. A team from TU Wien and the University of California San Diego has solved this problem. The researchers developed a method that allows the frequencies to be shifted continuously until the entire gap between two neighboring teeth of the frequency comb is covered.
"For decades, high-harmonic light has behaved a little like a guitar with fixed frets," says Tenio Popmintchev, a professor at the Institute of Photonics at TU Wien. "You get certain notes, at exactly the intervals dictated by physics. What we have now built is more like a slide guitar in the X-ray range: We can move continuously between the harmonics, in either direction, and select exactly the energy we need." The trick is not to modify the X-rays directly, but rather the laser used to generate them.
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