Real-time observation of laser-driven electron acceleration

Electron acceleration by laser-driven plasma waves(1,2) is capable of 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(7). Future applications, such as the development of laboratory X-ray sources with unprecedented peak brilliance(8,9) or ultrafast time-resolved measurements(10) 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(-2.1)(+1.9) fs electron bunch duration full-width at half-maximum (2.5(-0.9)(+0.8) 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-angstrom-wavelength ultrafast electron and photon sources for diffraction imaging with atomic resolution in space and time(11).