Production of 100-terawatt single attosecond X-ray pulse

The birth of attosecond light sources has opened a window into the fastest atomic-scale electronic dynamics. Performing true attosecond-pump-attosecond-probe experiments requires a single attosecond pulse at high intensity and large photon energy, a challenge that has yet to be conquered so far. Here we show that 100-terawatt single attosecond X-ray pulse with unprecedented intensity 10²¹W/cm² and duration 8.0as can be produced by intense laser irradiating a capacitor-nanofoil target composed of two separate nanofoils. In the interaction, a strong electrostatic potential develops between two foils, which drags electrons out of the second foil and piles them up in vacuum, forming an ultradense relativistic electron nanobunch. This nanobunch reaches both high density and high energy in only half a laser cycle and smears out in others, resulting in coherent synchrotron emission of a single, intense attosecond pulse.