Energy Penetration Into Arrays of Aligned Nanowires

This [Bargsten et al. Sci. Adv. 2017; 3 : e1601558] Paper proposes a new method to realize energy densities of 10^22 W/cm^2. For this, a compound consisting of two-segmented nanowire arrays are irradiated with ultrahigh-intensity-contrast 400-nm wavelength, 0.6-J laser pulses at 55-fs FWHM. These were grown by electrodeposition of metallic ions onto a porous template, the density of the pores determining the average density of nanowire array. The template is then dissolved away, leaving only freestanding aligned nanowire arrays.

The contrast of the laser preserves the nanowires until the arrival of the intense laser pulse. Both segments of the material (Ni, Co) have a high Young's modulus. The Co-segment is grown first, the Ni-segment is grown on top of it. In the spectra, the height of the Co-peak will give information about how far the energy penetrates down the length of the nanowires.

Simulations of the x-ray spectra resemble the experimental data, which provides plasma conditions for increasing irradiation intensity. For hot electron gases, energy density and pressure are determined by integrals of energy and the momentum-velocity product, weighted by distribution. The energy density emerges directly from the PIC simulation, and assuming a relativistic gas at equilibrium gives the pressure. The laser pulse's strong electric field strips electrons off the material, creating ionization up to Z = 18, which are rapidly accelerated into the gaps. On impact, electrons may produce higher ionization states, as the hot electrons deposit their energy deep into the nanowire cores. Generally, the stripped electrons accelerate toward the substrate. The resulting strong quasi-static self-generated azimuthal magnetic field pinches the nanowires into a hot, extremely dense plasma at the tips. As the pulse propagates down the length of the wires, the wires expand and the gaps in between are filled with a supercritical-density plasma, homogenized by ensemble collisions. This forms a uniform plasma layer of several micrometers. The wires expand until the gaps are filled up. A thermalized electron temperature of about 14 keV emerges over the plasma volume, with an average electron density beyond 3*10^23/cm^3. Higher densities should be realizeable through irradiating the arrays with higher wire-filling factors, arrays of higher Z-material, or both. For higher Z-material, the nanowires show extreme degrees of ionization in the simulations.