It is the dawning of a new era of X-ray lasers – at least according to the plans of Professor Tenio Popmintchev. In recent years, the researcher at the University of California San Diego, previously at the University of Colorado at Boulder, has raised a lot of attention, for example with an experiment he performed together with TU Wien, in which more than 5000 photons of low energy were combined to one single X-ray photon of much higher energy. Now Popmintchev was awarded one of the prestigious ERC Starting Grants by the European Research Council. The ERC project XSTREAM will be based at TU Wien and Popmintchev will split his time on two continents - between Vienna and San Diego.
X-Rays and Lasers
The first laser light ever created was bright red. Today, there are many different solid state lasers, based on different materials, emitting light in different colours. However, in order to create laser light in the regime of extremely short-wavelengths, for instance X-rays, different physical phenomena have to be employed.
One possibility is creating an extreme version of the so-called “high harmonic generation” process – this is being done in Tenio Popmintchev’s labs, as well as at the Photonics institute at TU Wien. When a very intense laser pulse is fired onto a suitable target, this target can be forced to emit laser radiation itself, with a much shorter wavelength than the original laser.
This can only be explained on an atomic scale: the laser pulse rips an electron out of each atom. Then the electrons are accelerated in the electric field of the laser and return to their parent atoms, carrying a lot more energy than they had before. Whenever an electron recombines with an atom, a photon can be emitted with an energy many times higher than the energy of the photons in the laser beam. During its very fast journey, the electron can absorb a large number of photons and eventually it can convert them into one photon with high energy, corresponding to a very short wavelength.
Using this technique, spectacular records have been set again and again over the last few decades. Tenio Popmintchev has succeeded in extending this technique further, creating bright laser pulses in the X-ray regime for the first time. Usually, huge facilities are required to produce intense X-ray laser beams, but now this becomes possible using nothing more than a tabletop-scale apparatus.
It’s all about the Phase
A crucial point is making sure that all the photons oscillate in perfect unison, so that they do not interact destructively with each other but add up to one bright, intense beam. This property, called “phase matching”, can in certain cases be achieved by carefully selecting parameters such as the wavelength of the driving laser pulse and the target material.
“In fact, this stable phase relationship between the photons of the driving laser and the emitted X-ray light is essential for transferring the laser properties of the driving light to the generated light”, says Tenio Popmintchev. “This means that instead of having a simple X-ray light bulb, radiating in all directions like a Roentgen X-ray tube, the generated X-rays emitted in a well-directed beam with low divergence.”
“Furthermore, this phase relationship allows for the design of X-rays with arbitrary spectral, spatial, and temporal shapes, as well as with tailored quantum properties with no classical analogue – a level of design which is impossible in the Roentgen X-ray tube. And it is all done instantly, right at the moment the X-rays are created, just by controlling the electrons, without the need for any post processing”, says Popmintchev.
The physical limitations of the high harmonic process are not yet clear. In the Photonics Institute at TU Wien, Tenio Popmintchev is going to explore several novel approaches, probing the extremes of what is feasible. Some of the unique state-of-the-art laser systems at TU Wien, as well as newly developed ones, will be used to produce designer X-ray laser beams with extremely short wavelengths and unique properties.
Tenio Popmintchev was born in Bulgaria in 1977. He studied theoretical physics at Sofia University and earned his doctorate in experimental Atomic, Molecular and Optical Physics at the University of Colorado at Boulder. Today, he is working as an Assistant Professor at UC San Diego. Starting in the summer of 2017, he will have a joint appointment in Vienna. The ERC project XSTREAM will be entirely based in Vienna, with support from UC San Diego. One of the scientists Popmintchev is going to collaborate with at TU Wien is Professor Andrius Baltuska who was also awarded an ERC Starting Grant for advanced laser research in 2011.