Orientation-Dependent Ion Acceleration from Laser-Irradiated Rectangular Nanorings

Abstract

Laser-driven ion acceleration from nanostructured targets offers a promising route to compact, high-energy ion sources. In this work, we demonstrate through particle-in-cell simulations that rectangular nanoring targets significantly enhance energy absorption and increase the cutoff energy of laser-accelerated ions. The nanoring geometry enables strong field confinement within its hollow core when optimally oriented relative to the laser polarization, leading to hotter electron populations and more robust sheath acceleration. These results demonstrate that rectangular nanorings offer a versatile platform for controlling laser-plasma interactions at solid densities and advancing compact, high-repetition-rate particle sources.

Figures (6)

• Figure 1: Schematic of the nanoring geometry. The nanoring has an inner hollow rectangular region and an outer rectangular profile.
• Figure 2: Comparison of simulation results for two target geometries: $200$ nm$\times400$ nm ($S_\parallel$, left column) and $400$ nm$\times200$ nm ($S_\perp$, right column). (a,b) Instantaneous electric field $E_y$ in the $x$--$y$ plane at $z=0$ and $t = 1.66$ fs. The color scale is symmetric and normalized to the same maximum absolute value for both panels; the dashed rectangles outline the target positions. (c,d) Streak images of the electron energy spectra: $dN_e/d\mathcal{E}_e$ (cm$^{-3}$ eV$^{-1}$) versus electron energy $\mathcal{E}_e$ (MeV) and time (fs). The color scale is logarithmic and identical for both cases. (e,f) Spatio-temporal evolution of $E_y$ at the polarization axis [$(x,z) =(0,0)$]: transverse profiles along $y$ ($\mu$m) as a function of time. The field is normalized to the incident amplitude $E_0$ and is identical for both cases.
• Figure 3: Proton acceleration from rectangular plastic nanorings of varying geometry and orientation. Four configurations are compared: $200$ nm$\times400$ nm ($S_\parallel$, blue solid), $400$ nm$\times200$ nm ($S_\perp$, green dash-dotted), $200\times200$ nm square (red dashed), and $400\times400$ nm square (purple dotted). (a) Maximum normalized electric field amplitude $|E_y|/E_0$ along the laser axis ($y=0$) as a function of $x$. The shaded region ($-0.2\,\mu\mathrm{m}<x<0.2\,\mu\mathrm{m}$) indicates the position inside the nanoring. (b) Average electron kinetic energy $\langle \mathcal{E}_e \rangle$ versus time. (c) Peak electric field amplitude $|E_y|$ versus time. (d) Average proton kinetic energy $\langle \mathcal{E}_p \rangle$ versus time. (e) Proton cutoff energy $\mathcal{E}_p^{\text{max}}$ versus time. (f) Proton energy spectra $dN_p/d\mathcal{E}_p$ versus proton energy at $t = 95.2$ fs.
• Figure 4: Dependence of proton acceleration on nanoring length $l$. Four geometries with different lengths are compared, all with inner cross section $200$ nm$\times400$ nm and shell thickness $40$ nm. (a) Average electron kinetic energy $\langle \mathcal{E}_e \rangle$ versus time. (b) Average proton kinetic energy $\langle \mathcal{E}_p \rangle$ versus time. (c) Proton energy spectra $dN_p/d\mathcal{E}_p$ at $t = 95.2$ fs.
• Figure 5: Dependence of proton acceleration on inner radius $r_a$ for circular nanorings. Four geometries are compared, with fixed length $l = 400$ nm and shell thickness $40$ nm. $r_a = 0$ corresponds to a solid rod (no hollow core). (a) Average electron kinetic energy $\langle \mathcal{E}_e \rangle$ versus time. (b) Average proton kinetic energy $\langle \mathcal{E}_p \rangle$ versus time. (c) Proton energy spectra $dN_p/d\mathcal{E}_p$ at $t = 95.2$ fs.