Ultra-bright XUV Emission from Laser Wakefields in Underdense Plasma

By use of particle-in-cell (PIC) simulation, we observe ultra-bright XUV radiation pules emerging from a laser wakefield driven above the wave-breaking thereshold in underdense plasma, which is as short as as few hundred attoseconds in one-dimentsional simulations and a few femtoseconds in two-dimensional simulations. It is concerned with the formation of a transverse narrow current layer co-moving with the intense laser pulse. This current layer is formed by an electron density spike with trapped electrons in the wakefield with certain transverse kinetic momentum of electrons left behind the laser pulse. This net momentum appears when the driving laser pulse undergoes strong self-modulation and subsequent pulse steepening at the front. In the multi-dimensional simulations, the XUV emission is found only near the laser axis with a much smaller spot size than the laser pulse due to the transverse dynamics of the trapped electrons. The present scheme provides an alternative method to produce ultra-bright XUV pulses for ultrafast applications.