Dense Polarized Positrons from Laser-Irradiated Foil Targets in the QED Regime

Many works have shown that dense positrons can be effectively generated from laser-solid interactions in the strong-field quantum electrodynamics (QED) regime. Whether these positrons are polarized has not yet been reported, limiting their potential applications. Here, by polarized QED particle-in-cell simulations including electron-positron spin and photon polarization effects, we investigate a typical laser-solid setup that an ultraintense linearly polarized laser irradiates a foil target with micrometer-scale-length preplasmas. We find that once the positron yield becomes appreciable with the laser intensity exceeding ${10}^{24}\mathrm{W}/{\mathrm{cm}}^2$, the positrons are obviously polarized. Around 30 nC positrons can acquire $>30\%$ polarization degree with a flux of ${10}^{12}{\mathrm{sr}}^{-1}$. The angle-dependent polarization is attributed to the asymmetrical laser fields that positrons undergo near the skin layer of overdense plasmas, where radiative spin flip and radiation reaction play significant roles. The polarization mechanism is robust and could generally appear in future 100-PW-class laser-solid experiments.

Figure 1

(a) Schematic for generating polarized positrons in laser-solid interactions, where a linearly polarized laser pulse impinges on a foil target with micrometer-scale-length preplasmas. Positrons of two polarizations ${\overline{S}}_z>0$ and ${\overline{S}}_z<0$ are deflected along opposite angles ${\theta }_y<0$ and ${\theta }_y>0$, respectively, where ${\theta }_y=\arctan (p_y/|p_x|)$. (b) and (c) are distributions of positron number $f_{+}({\theta }_y,{ϵ}_{+})$ and polarization ${\overline{S}}_z$ versus ${\theta }_y$ and energy ${ϵ}_{+}$ at the end of interactions, where $f_{+}({\theta }_y)$ and ${\overline{S}}_z$ versus ${\theta }_y$ are plotted by curves.

Figure 2

(a) Spatiotemporal ($x\text{−}t$) evolution of electron density $n_e$ and positron creation rate ${\mathrm{\Gamma }}_{+}$, where ${\mathrm{\Gamma }}_{+}$ represents positrons created per unit length and unit time. Contour lines of $|B_z/B_c|-|E_y/E_c|=300$ (MFDR) and $-300$ (EFDR) are plotted, where $E_c(B_c)=m_{e}c{\omega }_0/|e|$. (b) Magnetic field $B_z$ and radiation power $P_{\mathrm{rad}}$ of marked positrons created in the elliptical zone marked in (a). Several marked positrons are tracked, indicating they can be classified into two bunches, denoted as bunch I and bunch II. (c)–(e) Evolution of a typical bunch-II positron [the orange trajectory in (b)]: (c) experienced fields $E_y$ and $B_z$, (d) momenta $p_x$ and $p_y$, and (e) QED parameter ${\chi }_e$ and spin component $S_z$, where light-gray and dark-gray regions denote EFDR and MFDR, respectively, and the red star in (e) represents a strong radiation event.

Figure 3

(a)–(f) show positrons created in the negative half cycle [elliptical zone marked in Fig. 2]; (g) and (h) are those in the next positive half cycle for comparison. The positron number $f_{+}({\theta }_y)$ and polarization ${\overline{S}}_z$ versus the deflection angle ${\theta }_y$ [(a),(g)] at birth and [(b),(h)] at the end of interactions. [(c),(e)] $f_{+}({\theta }_y,{ϵ}_{\mathrm{rad}})$ and [(d),(f)] ${\overline{S}}_z$ versus ${\theta }_y$ and ${ϵ}_{\mathrm{rad}}$ at the end of interactions for [(c),(d)] bunch I and [(e),(f)] bunch II, where ${ϵ}_{\mathrm{rad}}$ is the total radiation energy per positron. $f_{+}({\theta }_y)$ versus ${\theta }_y$ at birth and at the end of interactions are also plotted by curves in (c) and (e).

Figure 4

The positron yield $N_{+}$ (solid triangle) and polarization ${\overline{S}}_z$ (dashed circle) at ${\theta }_y>20^{\circ }$ versus (a) the preplasma scale length $L$ and (b) laser amplitude $a_0$, respectively, where we take $a_0=1500$ in (a) and $L=1.5\mu \mathrm{m}$ in (b). Other parameters are the same as Fig. 1.

Figure 5

3D PIC simulation results. (a) Densities of the electron $n_e$ and positron $n_{+}$. (b) Central slice of positron polarization ${\overline{S}}_z$ in the $x\text{−}y$ plane. (c) Positron flux and two positron polarization components (d) ${\overline{S}}_z$ and (e) ${\overline{S}}_y$ versus the polar angle $\theta$ and azimuthal angle $\varphi$, where the laser is polarized along $\varphi =0$, 180° and propagates along $\theta =0$.