Self-Compression of Laser Pulses in Plasma

We study self-compression of weakly relativistically intense laser pulses in subcritical plasmas using one- (1D) and three-dimensional (3D) direct particle-in-cell (PIC) simulations. The self-compression works in the density window from $1/4$ critical to slightly below critical density, where the Raman instability is prohibited. An analytical model is developed to describe the self-compression. The model admits pulsing Gaussian solutions and a long-lived running soliton solution. The 1D PIC results agree well with the analytical model, and compressions by an order of magnitude are observed. In the 3D geometry, the longitudinal self-compression competes with the transverse self-focusing/filamentation. To damp the filamentation we use a periodic plasma-vacuum structure. The 3D PIC simulations suggest that a 30 fs long laser pulse is efficiently compressed to 5 fs.

Figure 1

The 1D PIC simulation of pulse compression. The laser pulse intensity, $|a{|}^2$ vs $\psi$, is given in the pulse comoving frame at different times: dashed line at $t=0$; dotted line at $t=290\lambda /c$; solid line at $t=890\lambda /c$. The initial pulse parameters: $a_0=0.12$, $T_0/2\pi =10$; plasma density $n/n_c=0.3$.

Figure 2

Maximum pulse intensity $|a{|}^2$ vs the propagated distance $z/\lambda$ in plasma with density $n=0.3n_c$. Solid lines correspond to the results of 1D PIC simulation: lines $A$ and $C$ are Gaussian pulses with initial amplitudes $a_0=0.1$ and $a_0=0.12$ correspondingly and $T_0/2\pi =10$. $B$ is the soliton pulse with $a_s=0.16$ and $T_s/2\pi =2.15$. The dashed line is the analytical prediction for a Gaussian pulse with the initial parameters $a_0=0.1$, $T_0/2\pi =10$.

Figure 3

The 3D PIC simulation results of pulse compression in periodic plasma-vacuum structure. (a) On-axis intensity $a^2$ of the initial laser pulse; (b) $(ZX)$ section of the initial pulse intensity; (c) on-axis intensity of the compressed pulse; (d) $(ZX)$ section of the compressed pulse intensity. The initial pulse amplitude $a_0=0.14$, duration $T/2\pi =11$, and the power 26 TW. The compressed pulse is 5 fs long and has the peak power of more than 100 TW.