Fyzikální ústav Akademie věd ČR

Induced transparency observed during FEL irradiation of Al foil

The experiment performed with a free electron laser FLASH (Free-Electron LASer in Hamburg) by a group of scientists of 10 countries including a team of the Laser plasma department IP ASCR and of the Department of x-ray lasers of the same Institute, demonstrated a marked increase of transparency for the radiation at high intensities in the soft x-ray spectral region. The experiment consisted in focusing ultra short (tens of fs) pulses generated by the FEL on the surface of a thin Al foil to a focal spot with the diameter of only a few hundreds nm. The laser was tuned to a wavelength capable of exciting the L-shell electrons of Al to the conduction band. The radiation intensity in the focus was so high (> 10¹⁶ W/cm²) that the photons are able to photoionize all the atoms in the beam path and the rest of the pulse then can pass through the sample with no interaction whatever. Hence, the number of photons passed through the foil is growing steeply with the radiation intensity and the sample becomes transparent in the soft x-ray region. Beside the participation in the experiment itself the team of IP contributed to the success by applying their original method of characterization of radiation field of laser imprint on a suitable material examined by AFM and Nomarski microscopy. This method can be applied to finding an ideal focus and to establish the intensity profile in the focus with a great precision. The impact of this discovery can be expected in the planetology and the astrophysics as well as in the field of controlled fusion and other branches of science requiring large energy densities under laboratory conditions. The original communication on the results appeared in the September edition of Nature Physics [1] and it has been reported also in [2]. [1] Nagler, B. et al. (Czech co-authors: Chalupský, J., Burian, T., Cihelka, J., Hájková, V., Kozlová, M. a Juha, L.): Turning solid aluminium transparent by intense soft X-ray photo-ionization, Nature Physics 5, 693 (2009). [2] Battersby, S.: Transparent metal could be the stuff of giant planets, New Scientist 203 (2719), 20 (2009);