Energy deposition of highly relativistic laser pulses into solid and near solid density high Z plasmas*

The irradiation of near solid density, high Z nanostructure arrays at moderately relativistic intensities (a$_{\mathrm{o}}=$ 1) offers nearly complete absorption of the laser light, which penetrates deep into the array [1]. As the intensity increases, however at highly relativistic intensities (a$_{\mathrm{o}}=$ 20), the wires explode before the peak of the laser pulse forming a plasma exceeding even the relativistically corrected electron density, on the order of 10$^{\mathrm{23}}$ electrons/cc. Despite the formation of this supercritical density surface, the laser energy is still deposited deep into the nanowires by accelerated high energy electrons that ionize gold up to Au$^{\mathrm{+69}}$ (Ne-like Au). Ionized K shell spectroscopy of buried Ni tracers reveals the heat penetration depth in solid density Au slab targets and near solid density Au nanowire plasmas is \textgreater 1 micron and \textgreater 5 micron, respectively. These experimental results are in agreement with fully relativistic three dimensional particle in cell simulations. [1] M.A. Purvis et al Nature Photonics 7, 796 (2013)