Laser-Driven Ultrafast Field Propagation on Solid Surfaces

The interaction of a $3\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ laser pulse with a metallic wire has been investigated using proton radiography. The pulse is observed to drive the propagation of a highly transient field along the wire at the speed of light. Within a temporal window of 20 ps, the current driven by this field rises to its peak magnitude $\sim {10}^4\mathrm{A}$ before decaying to below measurable levels. Supported by particle-in-cell simulation results and simple theoretical reasoning, the transient field measured is interpreted as a charge-neutralizing disturbance propagated away from the interaction region as a result of the permanent loss of a small fraction of the laser-accelerated hot electron population to vacuum.

Figure 1

(a) Schematic describing the experimental setup used on shot $A$. (b) The corresponding setup used on shot $B$. In both cases, ${\mathrm{CPA}}_2$ strikes into the page.

Figure 2

Samples of RCF data from shot $A$ describing the ${\mathrm{CPA}}_2$ interaction on a vertical $125\mu \mathrm{m}$ Au wire. The dashed red line marks the zero level of the probe proton beam. Corresponding proton density lineouts taken across the zero level are shown below each RCF image. Relative to the time of arrival of the ${\mathrm{CPA}}_2$ interaction pulse, the probing times depicted by each layer are (i) 0, (ii) 10, and (iii) 25 ps.

Figure 3

(a) Selection of experimental (top) and simulated (bottom) RCF data detailing the propagation of the charging front away from interaction point on shot $B$. The front position is indicated by the red arrows. The ${\mathrm{CPA}}_2$ interaction plasma is visible to the bottom right of each experimental image as a circular area of proton depletion. (b) Measurement of distance from interaction point ($d_2$) as a function of absolute probing time (relative to the acceleration of the proton beam at the source). The velocity at which the charging front moves along the wire is measured to be $(0.95\pm 0.05)c$.

Figure 4

PIC simulation results detailing $E_x$ and $J_y$ contours near the target rear surface for two different times (in femtoseconds) relative to the arrival of the pulse peak at the target front surface. The rear surface is located at $x=5$. $E_x$ is in units of $3.05\times {10}^8\mathrm{V}/\mathrm{cm}$ and $J_y$ in units of $4.8\times {10}^{13}\mathrm{A}/{\mathrm{cm}}^2$.