Fig. 4: Particle-in-cell simulations results.

a–c Nitrogen target: snapshots when the two lasers collide at z = 500 μm and at later times during acceleration. Electrons originating from the scissor-cross ionization injection are shown in green, while background plasma electrons are displayed in gray. The contribution from the scissor-cross ionization injection accounts for 88% of the total injected charge. d–f Same plots for helium. The red-blue colormap shows the electric field of the laser. The quantities ρ_e and ρ_e,ion denote, respectively, the charge density of the pre-ionized electrons and the charge density of the newly ionized (inner-shell) electrons; both are normalized to en_p, where e is the elementary charge and n_p is the unperturbed background plasma density. The oblique trigger-laser field is not displayed because it is not defined on the simulation grid. Instead, it is realized by analytically defining the external electromagnetic field⁶⁸. All the colorbars refer to panels (a–f). g Simulated electron-energy spectra under different gas/laser conditions; the spectra for “helium with DL + TL” and “nitrogen with DL” are multiplied by 20 for visibility due to the very small injected amount.