Unique X-ray emission characteristics from volumetrically heated nanowire array plasmas

Highly anisotropic emission of hard X-ray radiation (h$\nu $ \textgreater 10 keV) is observed when arrays of ordered nanowires (50 nm diameter wires of Au or Ni [1]) are volumetrically heated by normal incidence irradiation with high contrast 50-60 fs laser pulses of relativistic intensity. The annular emission is in contrast with angular distribution of softer X-rays (h$\nu $ \textgreater 1 KeV) from these targets and with the X-ray radiation emitted by polished flat targets, both of which are nearly isotropic. Model computations that make use the electron energy distribution computed by particle-in-cell simulations show that the unexpected annular distribution of the hard x-rays is the result of bremsstrahlung from fast electrons. Volumetric heating of Au nanowire arrays irradiated with an intensity of 2 x 10 19 W cm-2 is measured to convert laser energy into h$\nu $\textgreater 1KeV photons with a record efficiency of \textgreater 8 percent into 2$\pi$, creating a bright picosecond X-ray source for applications. \\[4pt] [1] M.Purvis et al. Nature Photonics 7, 796 (2013).