Real-time observation of laser-driven electron acceleration

Electron acceleration by laser-driven plasma waves ^1,2 is capableof producing ultra-relativistic, quasi-monoenergetic electron bunches ^3-5 with orders of magnitude higher accelerating gradients and much shorter electron pulses than state-of-the-art radio-frequency accelerators. Recent developments have shown peak energies reaching into the GeV range ⁶ and improved stability and control over the energy spectrum and charge. Future applications, such as the development of laboratory X-ray sources with unprecedented peak brilliance or ultrafast time-resolved measurements critically rely on a temporal characterization of the acceleration process and the electron bunch. Here, we report the first real-time observation of the accelerated electron pulse and the accelerating plasma wave. Our time-resolved study allows a single-shot measurement of the 5.8 ^+1.9 _-2.1 fs electron bunch full-width at half-maximum (2.5 ^+0.8 _-0.9 fs root mean square) as well as the plasma wave with a density-dependent period of 12-22 fs and reveals the evolution of the bunch, its position in the surrounding plasma wave and the wake dynamics. The results afford promise for brilliant, sub-ångström-wavelength ultrafast electron and photon sources for diffraction imaging with atomic resolution in space and time.