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 now developed a method that makes it possible to tune the frequency continuously.
Using a six-metre-long gas-filled waveguide containing the appropriate gas, the researchers can “detune” the light frequencies in such a way that, in the end, exactly the X-ray frequency required 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 now solved this problem. The researchers developed a method that allows the frequencies to be shifted continuously until the entire gap between two neighbouring teeth of the frequency comb is covered.
“For decades, high-harmonic light has behaved a little like a guitar with fixed frets,” says Prof. Tenio Popmintchev from 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. If the X-ray frequencies can be thought of as “overtones” – integer multiples of the laser frequency – then even a small change in the frequency of the driving laser produces a much larger change in the resulting X-ray frequencies. A comparatively small shift in the laser wavelength can therefore be enough to bridge the entire gap to the next harmonic in the X-ray range.
Technically, however, this is challenging. The team used an ytterbium infrared laser whose light is sent through a six-metre-long hollow waveguide filled with gas. Inside the waveguide, a strong nonlinear interaction between the light and the gas modifies the spectrum of the pulse. This modified infrared laser light is then converted into visible light with the help of a crystal, and only this specially prepared visible light, with a colour range from blue to yellow-green, is then directed at helium atoms.
Compared with the infrared light normally used for this purpose, this visible light has crucial advantages: it can be controlled very accurately, so that it oscillates at a pace that fits the emerging X-rays. Only if this timing precision is maintained (in physical terms: when phase matching of electromagnetic waves is obtained) can the illuminated atoms jointly produce a brighter, narrower-linewidth X-ray beam, rather than uncoordinated X-ray waves that partially cancel each other out. As a bonus, each atom produces more intense harmonics: with visible-light driving, its electrons are accelerated and recombine within a shorter interval than with infrared pulses — fast enough that quantum diffusion of the electron wavepacket has less time to reduce the efficiency.
“Spectral broadening, such as the kind we produce in our gas-filled waveguide, would normally mean losing control of the pulse,” says Dr. Dimitar Popmintchev from the Institute of Photonics at TU Wien, first author of the study. “We realised that exactly the same nonlinearity can also be used to steer the pulse in a controlled way.” Depending on the gas used, the spectrum can be shifted in different directions. Atomic gases broaden it towards both higher and lower frequencies. “Molecular gases can produce a stronger shift towards longer wavelengths through energy exchange with their internal degrees of freedom. The type of gas and the pressure can therefore be used to control both the direction and the magnitude of the frequency shift,” explains Dr. Paolo Carpeggiani from TU Wien. Crucially, the X-ray emission remains bright across the entire tuning range while the narrow spectral lines are preserved.
The new method is particularly interesting for experiments in which the light frequency must be tuned precisely to a specific resonance. This is crucial for research areas like precision spectroscopy, nuclear clocks, and semiconductor metrology. “Nature does not place its resonances where our harmonics happen to fall,” says Tenio Popmintchev. “If we can scan continuously across an entire energy range instead of simply jumping back and forth between fixed frequencies, many experiments become practically feasible in the first place, using laboratory-scale apparatus – much smaller than large-scale photon factories such as synchrotrons and free electron lasers.”
The vision is to create a light source that can be tailored as flexibly as possible: instead of adapting the experiment to whatever properties of the radiation happen to be available, the radiation itself can be adapted to the experiment.
Communications Physics
Experimental study
Not applicable
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