X-ray generation from femtosecond-laser-produced plasma on nanostructure-array targets

Abstract
In order to increase the soft x-ray conversion efficiency for femtosecond-laser-produced plasma, we adopted nanostructure-array targets. One structure is a nanohole-array structure. This structure was made by utilizing the anodic oxidation of an aluminum plate. Around a 30-fold x-ray fluence enhancement was achieved in the 5-20 nm wavelength range when the nanohole diameters were about 90 nm. The enhancement increases as the ionization level of Al becomes higher and the x-ray wavelength becomes shorter. Over a 50-fold enhancement was obtained at a soft x-ray wavelength around 6 nm, which corresponds to the emission from Al⁸⁺⁹⁺ ions. Another structure is a nanocylinder-array structure. After filling up the alumina nanoholes (mean cell size : 100 nm, pore diameter : 70-90 nm) with Au by electrodeposition, the alumina part was removed. Soft x-ray fluence from these targets increases as the Au cylinder height increases. Around a 15-fold enhancement compared with a gold foil target was obtained in the 7-20 nm wavelength range when the cylinder height was 18 µm. Larger enhancement is expected by optimizing the cylinder size. Soft x-ray (> 0.1 keV) pulse durations on both targets were 17 ps, which is much shorter than that obtained by using a prepulse technique.