Mechanism studies for relativistic attosecond electron bunches from laser-illuminated nanotargets

To find a way to control the electron-bunching process and the bunch-emitting directions when an ultraintense, linearly polarized laser pulse interacts with a nanoscale target, we explored the mechanisms for the periodical generation of relativistic attosecond electron bunches. By comparing the simulation results of three different target geometries, the results show that for nanofoil target, limiting the transverse target size to a small value and increasing the longitudinal size to a certain extent is an effective way to improve the total electron quantity in a single bunch. Then the subfemtosecond electronic dynamics when an ultrashort ultraintense laser grazing propagates along a nanofoil target was analyzed through particle-in-cell simulations and semiclassical analyses, which shows the detailed dynamics of the electron acceleration, radiation, and bunching process in the laser field. The analyses also show that the charge separation field produced by the ions plays a key role in the generation of electron bunches, which can be used to control the quantity of the corresponding attosecond radiation bunches by adjusting the length of the nanofoil target.

Figure 1

Three different target geometries used in the simulation.

Figure 2

The spatial distribution of the generated electrons for mass-limited foil target with a different transverse length of (a) 98 nm, (b)200 nm, (c) 393 nm, and (d) the integrated results along the $Y$ axis. The colorbar indicates the value of the electron density with arbitrary units.

Figure 3

The spatial distribution of the generated electrons for nanosphere targets with diameters of (a) 36 nm, (b) 50 nm, (c) 71 nm, (d) 100 nm, nanofoil target with changeable longitudinal length of (e) 50 nm, (f) 98 nm, (g) 200 nm, (h) 393 nm, and (i), (j), (k), (l) the corresponding integrated results along the $Y$ axis. The colorbar indicates the value of the electron density with arbitrary units.

Figure 4

(a) Schematic drawing of the interaction between the relativistic laser pulse and nanofoil target. The angle from the electron-emitting direction to the $+X$ direction is defined as $\theta$. During the continuous electron ejection from the target, a continuous increasing charge separation field (CSF) is formed. (b) The generated fanlike electron bunches where the black arrows indicate the emitting direction of the bunches. (c) The averaged electron bunch duration drawn from the black rectangle in panel (b). The colorbar indicates the value of the electron density with arbitrary units.

Figure 5

(a) The trajectories for the electrons with initial $x$ location within [1600, 1650] nm. (b) The trajectories for the electrons with initial $x$ location within [1950, 2000] nm. (c) The trajectories for the electrons with initial $x$ location within [2550, 2600] nm. (d) The trajectories for the electrons with initial $x$ location within [3550, 3600] nm. The colorbar indicates the logarithm value of the electron spatial density with arbitrary units.

Figure 6

The evolution of the $X$ component of velocity $V_x$ and energy $\gamma$ for the electrons with initial $x$ location within (a), (e) [1600, 1650] nm, (b), (f) [1950, 2000] nm, (c), (g) [2550, 2600] nm, and (d), (h) [3550, 3600] nm. The colorbar indicates the logarithm value of the electron spatial density with arbitrary units.

Figure 7

(a) The two-dimensional spatial distribution of electric field $E_x$ (V/m) near the target obtained from simulation results at $t=12T$, where $T$ is laser period. The rectangle denotes the outline of the target with $L=2000$ nm, $S=20$ nm, and $n_e=75n_c$. (b) The one-dimensional distribution of the electric field $E_x$ along the target central axis obtained from the simulation results and analytical formula Eq. (2).

Figure 8

(a) The relation between the effective target length $L_{\text{eff}}$ and maximum electron initial Lorentz factor ${\gamma }_0$. (b) The trajectory for an electron with initial Lorentz factor ${\gamma }_0=2.46$ calculated using Eq. (5).

Figure 9

From the trajectory in Fig. 8, the (a) $X$ and $Y$ position, (b) electron velocity, (c) acceleration, (d) radiation power, and (e) electron moving direction $\theta$ related to time can be obtained. Red solid lines in (a)–(c) are for the $X$ component, blue dashed lines in (a)–(c) are for the $Y$ component.

Figure 10

The relation between the electron's initial kinetic energy ${\gamma }_0$ and the electron moving direction at the first, second, and third extreme points.

Figure 11

The returning points (black circle) and the extreme points (orange square). The four lines are the $X$ position evolution of the laser electric peaks. To prevent the symbols from overlapping with each other, many overlapped symbols are omitted in this figure. The black arrows denote the increase of ${\gamma }_0$.

Figure 12

The superimposed radiation evolution arrays $P(\theta , t$) (a.u.) for electrons within the initial location in the target of (a) 1600 to 1850 nm, (b) 1850 to 2100 nm, (c) 2100 to 2350 nm, (d) 2350 to 2600 nm, (e) 2600 to 2850 nm, (f) 2850 to 3100 nm, (g) 3100 to 3350 nm, (h) 3350 to 3600 nm.

Figure 13

The schematic drawing of the interaction between the relativistic laser pulse and nanoscale target. The angle between the electron moving direction and $+X$ direction is defined as $\theta$.

Figure 14

The obtained $X$ component of electric fields $E_x$(V/m) for targets with the length of (a) 200 nm, (b) 400 nm, (c) 800 nm, (d) 1200 nm, (e) 1600 nm, (f) 2000 nm, at $t=12T$. The colorbar indicates the intensity of the electric field with arbitrary units.

Figure 15

The coordinate system and target position.