Just in time before the start of realization of the three pillars of the European ELI project (Extreme Light Infrastructure: ultra-high power laser systems with special emphasis on Beamlines in Prague, Attosecond Science in Szeged and Nuclear Physics in Bucharest-Magurele), Prof. Gerard Mourou and Prof. Toshiki Tajima have evaluated the potential of large-scale laser facilities with respect to the production of ultra-intense ultra-short pulses of coherent high-energy X-ray and γ-ray beams in the current issue of "Science".
Gerard Mourou is Professor at the Ecole Polytechnique and Director of the Institut de Lumière Extrême in Palaiseau and inventor of the chirped pulse amplification technique, an important milestone in high-power laser technology. Toshiki Tajima, professor at the Ludwig-Maximilian-University (LMU) Munich and member of the DFG cluster of excellence "Munich-Centre for Advanced Photonics" (MAP), is the "father" of laser-driven particle acceleration. Both work together in coordinating the ELI project.
In their article the two experts show that not only are short laser pulses a method to create very high intensities at still manageable pulse energies, but also the reverse is true: high intensities are required in order to produce very short pulses. The key point is that shorter pulses need a broader spectrum before compression. A milestone was the creation of 2.6 fs (2.6-15
seconds) pulses, corresponding to a single wavelength at 800 nm. Shorter pulses require higher frequencies that can be produced by high-harmonic generation in a gas jet. This technique allowed Prof. Ferenc Krausz (LMU, Max-Planck Institute of Quantum Optics) to achieve a world record of laser pulses with a duration of only 80 attoseconds (10-18
seconds). Still shorter pulses demand higher intensities. In the high-energy relativistic regime beyond 10-18
, electrons oscillate at the target surface with changing mass according to their varying velocity. This "oscillating mirror" modulates reflected laser light and such creates very high harmonics (3200th order experimentally verified).
The authors showed in theory and simulation that by shaping relativistic mirrors (i.e. very dense bunches of electrons) laser intensities of 1022
could produce few-attosecond backscattered X-ray or γ-ray pulses. High-density relativistic flying mirrors could be produced by imploding spherical targets with very intense laser pulses. By backscattering laser light from such mirrors, laser intensities of 1024
could ultimately produce even γ-ray pulses of approx. 100 yoctoseconds (10-22
s) duration. In this way ELI class laser systems have the potential to create the shortest coherent pulses, suitable to probe the vacuum and take a look into the atomic nucleus. Thus the future of high-field science and that of ultrafast optical science are now merged. It is anticipated that there will be an emerging brand new cross-fertilized interdiscipline, such as the ultrafast streaking of vacuum structure going one step beyond atomic streaking.