The X-rays used to diagnose a broken leg in the hospital are easy to make. In industry, however, an entirely different type of X-ray radiation is needed – for example, X-ray laser pulses that are as short and as powerful as possible. It is used, for example, in the production of nanostructures and electronic devices, as well as monitoring chemical reactions in real time.
Very powerful, very short X-rays of the nanometer wavelength are difficult to produce, but now a new, simple method has been developed at TU Wien (Vienna): the starting point is not a titanium-sapphire laser, which was often used for this purpose, but a ytterbium laser. The key trick is that light is sent through the gas to change its shape.
Long waves lead to short waves
The length of the laser beam depends on the material: In the atoms or molecules involved, the electrons change from one state to another with less energy. This results in the emission of a photon – its wavelength (and therefore its color) depends on the energy the electron lost during its transition. In this way, different laser colors can be produced – from red to violet.
However, to create laser beams with very short wavelengths, special tricks must be used: First, laser beams with long wavelengths are created and fired with atoms. An electron is removed from the atom and accelerated in the electric field of the laser. It then turns around and collides with the atom it came from – and in doing so can produce short X-rays. This process is called “higher harmonic generation.”
Paolo Carpeggiani from the Institute of Photonics at TU Wien said: “In fact, it seems that the larger the length of the initial laser beam, the lower the wavelength you can achieve in the end.” However, the intensity of X-ray radiation also decreases during this period: if you want to emit very short radiation, the intensity is very low.
Ytterbium instead of titanium sapphire, gas instead of crystal
Until now, this method has always been used using titanium-sapphire lasers and increasing the length of their radiation with special crystals to produce very short X-ray radiation through High Harmonic Generation. However, the TU Wien team has now developed a simple and at the same time very powerful method: they used a ytterbium laser. Ytterbium lasers are simpler, cheaper and more powerful than titanium-sapphire lasers, but until now, their performance in producing x-rays was very low.
At TU Wien, the wavelength of the ytterbium laser radiation was increased for the first time – not by sending the radiation through the crystal as usual, but by sending it through molecular gas. Paolo Carpeggiani says: “This increases efficiency a lot. “Instead of the 20% we used to get, we get about 80%.
