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.
 |
| An
infrared laser pulse (left) hits a gas target, where it stimulates
atoms to create x-ray radiation. Using a grid, the beam can be split up
into a spectrum of different wavelengths (right). |
Laser Light: Photons Oscillating in SyncIn 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. Infrared Light
Makes Atoms Emit X-RaysThe 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.
5000 Photons Combined to One Single PhotonThe 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.
Working with Invisible Radiation“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.
Laser Labs Instead of Particle AcceleratorsThis 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.
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