Turning solid aluminium transparent by intense soft X-ray photo-ionization

J. Chalupský, L. Juha, V. Hájková, T. Burian and J. Cihelka from the Department of Radiation and Chemical Physics, Institute of Physics ASCR, v.v.i., and M. Kozlová from Department of Ultra-intense Lasers, Institute of Physics ASCR, v.v.i., carried out in a wide international collaboration lead by a group from the Clarendon Laboratory (Oxford) several series of experiments at the FLASH facility (HASYLAB/DESY, Hamburg) demonstrating of a new finding of self-induced trasparence of aluminium in the soft X-ray spectral region at an extreme irradiance level. Results of these experiments have been published recently:

B. Nagler, U. Zastrau, R. Fäustlin, S. M. Vinko, T. Whitcher, A. J. Nelson, R. Sobierajski, J. Krzywinski, J. Chalupsky, E. Abreu, S. Bajt, T. Bornath, T. Burian, H. Chapman, J. Cihelka, T. Döppner, S. Düsterer, T. Dzelzainis, M. Fajardo, E. Förster, C. Fortmann, E. J. Galtier, S. H. Glenzer, S. Göde, G. Gregori, V. Hajkova, P. Heimann, L. Juha, M. Jurek, F. Y. Khattak, A. R. Khorsand, D. Klinger, M. Kozlova, T. Laarmann, H. J. Lee, R. Lee, K.-H. Meiwes-Broer, P. Mercere, W. J. Murphy, A. Przystawik, R. Redmer, H. Reinholz, D. Riley, G. Röpke, F. Rosmej, K. Saksl, R. Schott, R. Thiele, J. Tiggesbäumker, S. Toleikis, T. Tschentscher, I. Uschmann, H. J. Vollmer, J. Wark: Turning solid aluminium transparent by intense soft X-ray photo-ionization, Nature Physics 5, 693-696 (2009).

In addition to their contribution to performing the experiments, the co-authors belonging to the Institute of Physics ASCR, v.v.i., applied their own, original techniques of irradiance determination based on ablation crater analysis [1].

The experiment mentioned above has demonstrated a significant increase of transparency for the radiation at high intensities in the soft x-ray spectral region. The experiment is made possible by focusing of ultra-short pulses generated by FLASH on the surface of a thin Al foil to a focal spot with the diameter of only a few hundreds nm [2]. The laser was tuned to a wavelength capable of photo-ionizing the L-shell electrons of Al. The radiation intensity in the focus was so high (> 10¹⁶ W/cm ²) 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. 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.

References:
[1] J. Chalupský et al.: Characteristics of focused soft X-ray free-electron laser beam determined by ablation of organic molecular solids, Opt. Express 15, 6036 (2007).
[2] A. J. Nelson et al.: Soft x-ray free electron laser microfocus for exploring matter under extreme conditions, Opt. Express 17, 18271 (2009).