Studying the Dynamics of Relativistic Laser-Plasma Interaction on Thin Foils by Means of Fourier-Transform Spectral Interferometry

We apply Fourier-transform spectral interferometry (FTSI) to study the interaction of intense laser pulses with ultrathin targets. Ultrathin submicrometer-thick solid CH targets were shot at the PHELIX laser facility with an intensity in the mid to upper $10^{19}\mathrm{W}/{\mathrm{cm}}^2$ range using an innovative double-pulse structure. The transmitted pulse structure was analyzed by FTSI and shows a transition from a relativistic transparency-dominated regime for targets thinner than 500 nm to a hole-boring-dominated laser-plasma interaction for thicker targets. The results also confirm that the inevitable preplasma expansion happening during the rising slope of the pulse, a few picoseconds before the maximum of the pulse is reached, cannot be neglected and plays a dominant role in laser-plasma interaction with ultrathin solid targets.

Figure 1

Experimental setup. OAP denotes an off-axis parabola.

Figure 2

Measured (blue) and reconstructed (green) spectra based of the spectrum of shot 13992 (red) and a modulation given by Eq. (4). The data used are $\tau =3.0896\mathrm{ps}$ and $\eta =0.090$.

Figure 3

Phase measurement before and after the target.

Figure 4

Delay change between the main and post pulse as a function of the target thickness.

Figure 5

Simulation of the target hydrodynamical preexpansion during the rising slope of the pulse (until 3 ps before the maximum intensity is reached), supposing various initial target thicknesses. The electron density profiles are given on the laser axis at the end of the hydrodynamical simulation. The dotted horizontal lines at $n_c$ and $n_0$ indicate the plasma critical density and the density at roughly solid-state density, respectively.