Fig. 1. Scheme of the pre-structured target and the p-polarized laser pulse. The laser is normally incident, and the structure period and depth are both $\unicode[STIX]{x1D706}_{0}$, where $\unicode[STIX]{x1D706}_{0}$ is the laser wavelength.

Fig. 2. The electric field $E_{y}$ and the Poynting flux $S_{x}=E_{y}B_{z}$ at $t=30T_{0}$ ($t=25T_{0}$ for (d)). (a) and (b) are the distributions of $E_{y}$ for the pre-structured and flat targets, respectively. (c) and (d) are the distributions along the $x$ axis for the pre-structured and flat targets, respectively. (e) and (f) are the distributions of $S_{x}$ behind the target for the pre-structured and flat targets, respectively. In (a)–(c), (e) and (f), the laser normalized vector potential is $a_{0}=3$ and the electron density is $n_{e}=25n_{c}$. In (d), the laser normalized vector potential is $a_{0}=5$ and the electron density is $n_{e}=900n_{c}$. In (c) and (d), the red dashed line and the black solid line represent the pre-structured and flat target cases, respectively. In this figure, $E_{0}=m_{e}\unicode[STIX]{x1D714}_{0}c/e\approx 3.22\times 10^{12}~\text{V}/\text{m}$. The electric fields in (a) and (b) are both normalized by $E_{0}$.

Fig. 3. Time-space evolution of $E_{y}$ and snapshots of the electron density distribution at $t=30T_{0}$ for both targets. (a) is the evolution of $E_{y}$ on $y=y_{1}$, (b) is the evolution of $E_{y}$ on $y=y_{0}$, (c) is the density on the pre-structured target and (d) is the density on the flat target. Here, $E_{y}$ is also normalized by $E_{0}$ and the electron densities (normalized by $n_{c}$) are on a logarithmic scale.

Fig. 4. Time evolution of the averaged momentum of the electrons near the target back surface, snapshot of the SPW and time evolution of the electric fields at a point $(x_{1},y_{0})$ near the front surface. (a) is the evolution of $p_{x}$, (b) is the evolution of $p_{y}$, (c) is the SPW $E_{x}/E_{0}$ (the electrostatic field $\langle E_{x}/E_{0}\rangle$, which is calculated by averaging $E_{x}/E_{0}$ in 5 laser cycles, is omitted) and (d) is the evolution of the electric fields $E_{x}$ (red solid line) and $E_{y}$ (black dashed line). In (a) and (b), the momentum is calculated by $\langle p_{\unicode[STIX]{x1D6FC}}\rangle =\sum p_{\unicode[STIX]{x1D6FC}i}/N$ ($\unicode[STIX]{x1D6FC}=x,y$), where $p_{\unicode[STIX]{x1D6FC}i}$ is the $p_{\unicode[STIX]{x1D6FC}}$ of the $i$th electron in an area $x\in (x_{0},x_{0}+\unicode[STIX]{x1D6FF}x)$ and $y\in (y_{0},y_{0}+\unicode[STIX]{x1D6FF}y)$, and $N$ is the total number.

Fig. 5. Comparison between the cases with and without a pre-plasma. (a) is the initial electron density with a pre-plasma, (b) is the electric field $E_{y}$ on the $x$ axis for cases with a pre-plasma (black solid line) and without a pre-plasma (red dashed line), (c) is the spectra of the electric fields shown in (b) (the black solid line also represents the case with a pre-plasma) and (d) is a snapshot of the electric field $E_{x}$, with $\langle E_{x}/E_{0}\rangle$ omitted. In the inserted figure of (a), the electron density distribution along the $x$ axis is shown on $y=0$ (blue line) and on $y=\unicode[STIX]{x1D706}_{0}/2$ (red line).

Fig. 6. Spectra of the transmitted radiation behind the pre-structured target (red dashed line) and the flat target (black solid line). In this case $a_{0}=12$ and $n_{e}=400n_{c}$.