TU Vienna improves technology for generating X-ray laser pulses

26 Jan 2023

Starting point is an ytterbium laser – rather than the conventional Ti-sapphire laser.

Intense, extremely short-wave X-ray pulses in the nanometer wavelength range are difficult to produce, but now a new, simpler method has been developed at TU Wien (Technical University of Vienna, Austria): the starting point is not a titanium-sapphire laser, which had mostly been used for this purpose, but an ytterbium laser. The crucial trick is that the light is then sent through a gas in order to change its properties.

The work is described in ACS Photonics.

The wavelength of a laser beam depends on the material in which it is generated: In the atoms or molecules involved, electrons change from one state to another state with lower energy. This causes a photon to be emitted – its wavelength depends on how much energy the electron lost during its change of state. In this way, different laser colours can be produced – from red to violet.

However, for creating laser beams with even much smaller wavelengths, special tricks have to be employed: First, laser beams with a long wavelength are created and shot at atoms. An electron is ripped away from the atom and accelerated in the electric field of the laser. It then turns back and collides again with the atom from which it came – and thereby it can produce short-wave X-rays. This technique is known as “high harmonic generation.”

“At first glance, the situation seems somewhat counter-intuitive,” said Paolo Carpeggiani from the Institute of Photonics at TU Wien. “It turns out, in fact, that the larger the wavelength of the original laser beam, the smaller the wavelengths you can achieve in the end.”

However, the efficiency of X-ray radiation production also decreases in the process: if the aim is to produce very short-wave radiation, its intensity becomes very low.

Yb instead of Ti:sapphire; gas instead of crystal

Until now, this technique was almost always employed by using titanium-sapphire lasers and then increasing the wavelength of their radiation with special crystals in order to generate the shortest possible X-ray radiation through High Harmonic Generation.

However, the team at TU Wien has now developed a simpler and at the same time more powerful method: they used an ytterbium laser. Ytterbium lasers are simpler, cheaper and more powerful than titanium-sapphire lasers, but until now, their performance in x-ray production was much lower.

At TU Wien, the wavelength of the ytterbium laser’s radiation was first increased – not by sending this radiation through a crystal as usual, but by sending it through a molecular gas. “This increases the efficiency quite dramatically,” said Paolo Carpeggiani. “Instead of the 20% we used to get, we get around 80%.”