Efficient Picosecond X-Ray Sources Generated by Highly Relativistic Irradiation of Vertically Aligned Nanostructures

The irradiation of high aspect ratio, vertically aligned nanoscale rods with ultrahigh contrast
$(> 10^{12})$
, femtosecond laser pulses at highly relativistic intensities
$(> 10^{21}\text{Wcm}^{-2})$
provides a unique combination of drastically enhanced light penetration deep into the near solid density target. This results in the material being volumetrically heated to depths two orders of magnitude greater than the skin depth of the material, creating a large volume, ultrahigh energy density
$(> 10^{9}\text{Jcm}^{-3})$
plasma
¹
. Time integrated spectroscopic measurements of Au and Ni nanowire plasmas at an intensity of
$5\mathrm{x}10^{18}\text{Wcm}^{-2}(\mathrm{a}_{\mathrm{o}}=1)$
demonstrated the creation of multi- keV, near solid density plasmas ionized to the Au
$52+$
and
$\text{Ni}^{26+}$
charge states2. The radiative lifetime of these plasmas is significantly decreased due to their high electron density while their hydrodynamic expansion time is greatly increased due to their large size. The result is a large increase in the conversion of the optical laser light into hv > 1keV x-rays. Previously, the conversion efficiency has been shown to exceed 20% into 4 pi steradians when irradiating 80nm diameter, 12% of solid density, Au nanowires at an intensity of
$4\mathrm{x}10^{19}\text{Wcm}^{-2\ 3}$
. Here we present the first results of Au nanowires irradiated at highly relativistic intensity of
$5\mathrm{x}10^{21}\text{Wcm}^{-2}$
corresponding to
$\mathrm{a}_{\mathrm{o}}=17$
. Results of experiments conducted with a variety of nanowire diameters and at various fractions of solid density will be presented and compared to the results of particle in cell simulations.