Control of transmission of right circularly polarized laser light in overdense plasma by applied magnetic field pulses

The effect of a transient magnetic field on right-hand circularly polarized (RHCP) laser light propagation in overcritical-density plasma is investigated. When the electron gyrofrequency is larger than the wave frequency, RHCP light can propagate along the external magnetic field in an overcritical density plasma without resonance or cutoff. However, when the magnetic field falls to below the cyclotron resonance point, the propagating laser pulse will be truncated and the local plasma electrons resonantly heated. Particle-in-cell simulation shows that when applied to a thin slab, the process can produce intense two-cycle light pulses as well as long-lasting self-magnetic fields.

Figure 1

(a) Evolution of the component $E_y$ of the (incident, reflected, and transmitted) RHCP wave electric fields. Interference between the incident and reflected waves can be clearly seen in the front vacuum region. The black dashed lines mark the turning-on and -off times (including the ramps) of the applied magnetic field. (b) The $z$ component of the wave and self-generated magnetic fields. (c) The magnitude $\left|\mathit{E}\right|$ of the wave electric fields. (d) The magnitude $\left|\mathit{B}\right|$ of the wave and self-generated magnetic fields. Note that the self-generated field exists only inside the target, appearing as a thin red region (line) in $2{\lambda }_L\le x\le 2.055{\lambda }_L$, and it persists even after the laser-plasma interaction has ended. The laser parameters are $a_0=0.1$ and $\tau =20$.

Figure 2

(a) Evolution of the amplitudes of the incident (blue dashed curve) and transmitted (red solid curve) RHCP pulses. The maximum transmitted light wave electric field $\left|E\right|\sim 0.05$ appearing at $t\sim 10$ corresponds to the light intensity $I\sim 5\times {10}^{15}{\mathrm{W}\mathrm{cm}}^{-2}$. (b) The corresponding spectra. The black dotted curves for the transmitted pulse are obtained from the Fresnel formula. The spectra curves have been normalized by their own maxima. The laser and plasma parameters are the same as in Fig. 1.

Figure 3

(a) Evolution of the $y$ component $E_y\left(t\right)$ (red solid curve) and the magnitude $\left|E\right(t\left)\right|$ (blue solid curve) of the electric field of the transmitted wave at $x=2.1{\lambda }_L$, just outside the right edge of the target, and (b) the corresponding SPD (in arbitrary units, as obtained from short-time Fourier transform with a two-cycle Chebyshev window). (c) The corresponding SPD at $t=9.08T_L$ (magenta), $t=9.88T_L$ (red), and $t=10.88T_L$ (cyan).

Figure 4

(a) The $y$ component $E_y(x,t)$ of the wave electric field. (b) The total magnetic field $B_z(x,t)$ in the $z$ direction, consisting of the oscillating light wave and the static self-generated magnetic fields. (c) The electron density $n_e(x,t)$. (d) The $y$ component $J_y(x,t)$ of the current density. The density depression appearing in (c) for $t\gtrsim 20$ is a result of the unstable bipolar, thus sheared, current distribution in (d).