Attosecond bunches of gamma photons and positrons generated in nanostructure targets

The subatomic experimental exploration of physical processes on extremely short time scales has become possible by the generation of high-quality electron bunches and x-ray pulses with subfemtosecond durations. Increasing the photon energy from the x-ray to gamma-ray regime makes probing of extremely small space-time domains accessible. Here, a mechanism for generating attosecond gamma photon and positron bunches with small divergence using laser intensities below ${10}^{23}\mathrm{W}/{\mathrm{cm}}^2$ is proposed. In contrast with previous works, in our scheme a single laser pulse is sufficient instead of two counterpropagating pulses. Numerical simulations are used to formulate the conditions for confined radiation and to characterize the generated photon and positron bunches.

Figure 1

(a) Laser electric field ($P$ polarized) and (b) electron density distribution at the beginning of the interaction ($t=24$ fs). The laser spot diameter is $2\mu \mathrm{m}$ and it propagates from left to right with the central axis at $y=0\mu \mathrm{m}$. The flat foil is placed at $x=11\mu \mathrm{m}$, where both nanorods end. In (c) a schematic view of the laser field (blue continuous line) and longitudinal force (red dashed line) acting on the electrons between the nanorods. The vertical dashed lines and corresponding labels are explained in the text.

Figure 2

Snapshots of the laser field, electrons, and photons at different time instances of the simulation. The color (gray) scale shows the background electric field. The dashed lines in (a) indicate the phase positions ${\xi }_0, {\xi }_1, {\xi }_2$ which are shown in Fig. 1 and described in the text. Electrons with energy higher than 300 MeV are shown by green (light gray) dots, while photons with ${\epsilon }_{\text{ph}}>200$ MeV are shown in (a), (b) and with ${\epsilon }_{\text{ph}}>300$ MeV in (c) by blue (dark) dots.

Figure 3

(a) Zoom on the electron density (color scale) at $t=48$ fs and the photons are shown with blue (black) dots (${\epsilon }_{\text{ph}}>400$ MeV). (b) Angular-energy distribution of all photons at the same time instance.

Figure 4

(a) Evolution of photon density along the line $y=0$. At a later time, secondary peaks also appear beside the primary peaks. (b) Positron density along the $y=0$ axis. (c) Longitudinal momentum spectrum of positrons and their angular distribution shown in the inset.

Figure 5

(a) Bunching factor (in units of nm, solid lines) and rms angle (in radian, dashed lines) of photons vs energy at different time instances. Photons near the axis are included: $-0.4\mu \mathrm{m}<y<0.4\mu \mathrm{m}$. (b) The same as in (a), but for different laser intensities at the given time instance.