Evidence of Resonant Surface-Wave Excitation in the Relativistic Regime through Measurements of Proton Acceleration from Grating Targets

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast ($\sim {10}^{12}$) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity $>{10}^{19}\mathrm{W}/{\mathrm{cm}}^2$. A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.

Figure 1

Schematic top view of the experimental setup. The Thomson parabola and the frosted glass screen were located along the target normal direction and the laser specular directions, respectively.

Figure 2

Frames (a) and (b): first two layers of the exposed RCF at high laser contrast conditions, showing the typical proton forward (30°) and backward ($-150°$) proton spots as well as two smaller spots ($\simeq -125°$ and $\simeq -155°$) attributed to laser reflection at 0 and $+1$ grating diffraction orders. The position 0° corresponds to the incident laser axis. The parabolic “shadows” are due to the boundary of the target holder, which screens diffuse radiation (such as electrons and hard x-rays) from the plasma. Frame (c): same as (a), but for an angle of incidence different by 15°, leading to a shift of the diffraction spots. Frame (d): first RCF layer in low laser contrast conditions, for which both the backward proton and the diffraction spots disappear. All sets of RCFs have been exposed to three laser shots.

Figure 3

Maximum proton energy (filled data points) and reflected light signal (empty data points) as a function of incidence angle $\alpha$. Left and right frames correspond to $20\mu \mathrm{m}$ thick plane targets and to $23\mu \mathrm{m}$ thick grating targets, respectively. Filled circles and triangles correspond to 0.5 and $0.3\mu \mathrm{m}$ deep gratings, respectively. The (red) dashed line is proportional to ${sin}^2\alpha /\cos \alpha$. The other lines are guides for the eye.

Figure 4

2D PIC simulation results: (a) cutoff energy of protons; (b) fractional absorption. Empty and filled symbols refer to PTs and GTs, respectively. Squares are data from emi2d (EMI) simulations with grating depth $\delta =0.53\mu \mathrm{m}$ for GT, and circles and triangles are data from aladyn (ALA) simulations for $\delta =0.4$ and $0.2\mu \mathrm{m}$, respectively. The data correspond to a time $t=350\mathrm{fs}$ for emi2d and $t=200\mathrm{fs}$ for aladyn, respectively, relative to the time $t=0$ at which the pulse peak reaches the target.

Figure 5

2D PIC simulation results: snapshots at $t=75\mathrm{fs}$ of the magnetic field $B_y$ (normal to the simulation plane) in the interaction at 30° with both plane (PT) and grating (GT) targets of $0.8\mu \mathrm{m}$ thickness. The laser axis of incidence is marked by the dashed arrows. For the GT, a wave propagating near the target surface (on the left upper side) and reflection at several diffraction orders are apparent, whereas the PT plot is dominated by the specularly reflected pulse. The thick arrows give the propagation direction of the surface (for GT) and reflected (for PT) waves. See the text for parameters.