A breakthrough in laser science was achieved in Vienna: In the labs of the Photonics Institute at the Vienna University of Technology, a new method of producing bright laser pulses at x-ray energies was developed. The radiation covers a broad energy spectrum and can therefore be used for a wide range of applications, from materials science to medicine. Up until now, similar kinds of radiation could only be produced in particle accelerators (synchrotrons), but now a laser laboratory can also achieve this. The new laser technology was presented in the current issue of the magazine “Science”.
In a laser beam, all the photons oscillate in perfect unison. The wave crests are aligned – this kind of radiation is called “coherent”. The coherent light created in the labs of Professor Andrius Baltuska’s team (Photonics Institute, TU Vienna) has very special properties: It is composed from photons of very different energies – extending to x-ray radiation with very short wavelengths and high energy.
A double slit experiment proofs that the x-ray radiation is coherent - the photons are in a collective quantum state, like the photons in visible laser light.
The energy for this kind of radiation is supplied by short infrared laser pulses. They are fired at noble gas, where they rip electrons out of the atoms. These electrons are then accelerated by the infrared light and return to their atoms, where they convert their kinetic energy into x-ray radiation. That way, long-wave infrared photons are converted into short-wave x-ray photons. When the atoms in the gas container all do this dance with their electrons in the right rhythm and all the x-ray-waves add up perfectly, a beam of laser-like x-rays is created. Research groups from several universities were involved in this experiment: Vienna University of Technology, University of Colorado, Columbia University and the University of Salamanca.
The idea of combining several photons to a single photon with higher energy is not new: In 1961, two photons from a red ruby laser were combined to one blue photon. The new experiment however combines more than 5000 photons of low energy to one high-energy x-ray photon.
The infrared photons have a rather low energy – but for the experiment, a large number of them is needed. That is why the infrared source has to be very strong. A unique infrared laser was used, specially developed at the Vienna University of Technology, with a peak power of 100 gigawatts. This corresponds to the power of several hundred hydroelectric power plants – but only during the short laser pulse, which lasts for femtoseconds (10^-15 seconds). The team from the University of Colorado contributed know-now on the creation of x-rays in noble gas at high pressure. The theory groups from Cornell and Salamanca studied the phenomenon using numerical calculations.
“Together we discussed how to combine the technological know how of our research teams, and finally we chose the most challenging path”, says Audrius Pugzlys (TU Vienna). The team decided to use infrared radiation with a very long wavelength of four micrometers. This kind of radiation is invisible to the human eye and it is hard to trace even with technological tools. This makes the experiments very challenging, but it allows for higher x-ray energies. The effort finally paid off: “Our coherent x-ray radiation opens the door to very precise spectroscopy, which can be used to research new materials, to advance electronics or to analyze biomolecules”, says Audrius Pugzlys.
This kind of radiation used to be available only in expensive particle accelerators (synchrotrons). The new table top x-ray light source, however, can be assembled in a small laser lab. “Synchrotrons still deliver more photons per second than our beam does, but for many applications, our light source will be very useful”, says Audrius Pugzlys. The hard x-ray regime of photons with extremely high energy cannot yet be reached, but the energy of the photons in the x-ray beam is much higher than in any other light-powered tabletop device. Now the team is trying to reduce the time interval between the laser pulses. This should drastically increase the average intensity of the beam.
Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers. Tenio Popmintchev, Ming-Chang Chen, Dimitar Popmintchev, Paul Arpin, Susannah Brown, Skirmantas Ališauskas, Giedrius Andriukaitis, Tadas Balčiunas, Oliver D. Mücke, Audrius Pugzlys, Andrius Baltuška, Bonggu Shim, Samuel E. Schrauth, Alexander Gaeta, Carlos Hernández-García, Luis Plaja, Andreas Becker, Agnieszka Jaron-Becker, Margaret M. Murnane, Henry C. Kapteyn. Science 8 June 2012: Vol. 336 no. 6086 pp. 1287-1291 DOI: 10.1126/science.1218497
Vienna University of Technology