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By irradiating a flat Al target with femtosecond laser pulses at moderate intensities of $\ensuremath{\sim}{10}^{17}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$, we obtained stable collimated quasimonoenergetic electrons in the specular direction but deviated somewhat toward the target normal. An associated local minimum located on the other side of the specular direction seems to indicate that the peak actually results from the deflection of the collimated electrons from their initial ejection direction. We have proposed a two-step model in which some laser-accelerated electrons are able to leave the plasma in a narrow phase-locked window of the moving wave interference pattern, and are then steered toward the target normal by the ponderomotive force of the interference field. The periodic repetition of the electron emission leads to a pulse train of collimated quasimonoenergetic electrons with subcycle duration.

We report on near-GeV electron beam generation from an all-optical cascaded laser wakefield accelerator (LWFA). Electron injection and acceleration are successfully separated and controlled in different LWFA stages by employing two gas cells filled with a $\mathrm{He}/{\mathrm{O}}_{2}$ mixture and pure He gas, respectively. Electrons with a Maxwellian spectrum, generated from the first LWFA assisted by ionization-induced injection, were seeded into the second LWFA with a 3-mm-thick gas cell and accelerated to be a 0.8-GeV quasimonoenergetic electron beam, corresponding to an acceleration gradient of $187\text{ }\text{ }\mathrm{GV}/\mathrm{m}$. The demonstrated scheme paves the way towards the multi-GeV laser accelerators.