X-ray emission from nanostructured targets irradiated by a relativistically intense mid-infrared driver

Abstract

We report the first experimental results on multi-keV X-ray generation from relativistic laser-solid interaction using a mid-IR (3.9 μm) high power femtosecond laser source and nanostructured solid targets. Regimes of relativistic laser-plasma interaction with a long wavelength driver qualitatively differ from experiments with conventional near-IR or visible laser sources. The dynamics of the laser-particle interaction by relativistic dimensionless parameter a 0 ∝ √Iλ where I — the laser intensity, λ — the wavelength. The same number can be achieved for a long wavelength laser source with much less energy fluence than for a short-wavelength one. This scaling results in high efficiency of X-ray generation from solid plasmas at rather moderate intensities [1]. Also, the relatively small photon energy of the mid-IR radiation reduces the multi-photon absorption probability, and sample with the bandgap of several eV will be ionized closer to the peak intensities, whereas for near-IR or visible laser sources a few photon ionization will occur on the leading front of the laser pulse. We investigate these new regimes of relativistic laser-plasma interaction for different nanostructured solid targets, where the morphology enables an efficient volumetric heating mechanism of solid density plasma [2].