Coupling between a laser and a prestructured target with an arbitrary structure period

The coupling between a laser and a prestructured target with an arbitrary structure period is investigated with the help of two-dimensional (2D) particle-in-cell (PIC) simulations. It is shown that new electromagnetic (e.m.) waves will be generated after the coupling. Since the coupling is a resonant process, strong surface currents will be generated, which result in the generation of strong quasistatic magnetic fields. The frequencies which the newly excited e.m. waves contain are harmonics of the laser frequency and the frequencies can be controlled by the structure period. Also, the propagation directions of the newly excited e.m. waves are well controlled by the ratio between the structure period and the laser wavelength, which means e.m. waves excited by lasers with different frequencies have different propagation directions. As a result, the prestructured target can act as a new kind of optical gratings which can be used to split superintense laser pulses. The controlling of both the harmonic frequencies and the propagation directions can be explained by matching condition of the coupling, which is a formula resembling but physically different from the diffraction grating equation. The quasistatic magnetic fields are on the target front surface and as strong as hundreds of teslas, but the amplitude will be decreased by the newly excited e.m. waves when they are propagating along the target surface. Since the propagation directions are controlled by the structure period, with an optimal structure period, the prestructured target can also act as a source of strong quasistatic magnetic fields.

Figure 1

Geometry of laser interaction with prestructured target. The $p$-polarized laser is propagating in the $x$ direction and its electric field lies in the $y$ direction. The structure is periodically distributed along the $y$ axis and is uniform along the $z$ axis.

Figure 2

Snapshots of the longitudinal electric field $E_x$ for four different structure periods at $t=38T_0$. (a) ${\lambda }_s={\lambda }_0$, (b) ${\lambda }_s=\sqrt{2}{\lambda }_0$, (c) ${\lambda }_s=1.5{\lambda }_0$, and (d) ${\lambda }_s=2{\lambda }_0$. The electric field is normalized by $m_e{\omega }_{0}c/e$.

Figure 3

Wave-number spectra of the electric fields shown in Fig. 2. The colorbar is in an arbitrary unit. In this figure, the red dashed lines in the transverse direction mark the directions of the target surfaces, the red dashed lines in the longitudinal direction mark the directions of the target normal and the red dashed lines in the oblique directions mark the propagation directions of the newly generated e.m. waves. And the noted angles are the angles between the target surfaces and the propagations directions of the newly generated e.m. waves.

Figure 4

Wave-number spectrum of $E_x$ of the case ${\lambda }_s={\lambda }_0/\sqrt{2}$. In this figure, the dashed lines are plotted for the same purpose as Fig. 3.

Figure 5

Snapshots of the quasistatic magnetic field $\overline{B_x}$ for the four structure periods at $t=38T_0$. The magnetic field is achieved by averaging the magnetic field $B_x$ within three laser cycles. And the magnetic field is normalized by $m_e{\omega }_0/e$.

Figure 6

Snapshots of the phase spaces $y-p_y$ for the four structure periods at $t=38T_0$. The four structure periods are ${\lambda }_s={\lambda }_0$ (a), ${\lambda }_s=\sqrt{2}{\lambda }_0$ (b), ${\lambda }_s=1.5{\lambda }_0$ (c), and ${\lambda }_s=2{\lambda }_0$ (d). The space is normalized by the laser wavelength ${\lambda }_0$ and the momentum is normalized by the electron momentum at light speed $m_{e}c$. The colorbar is the number of the electrons in logarithmic scale.

Figure 7

Snapshots of the electron energy spectra for the four structure periods at $t=38T_0$.