Coherent synchrotron emission in transmission from ultrathin relativistic laser plasmas

Abstract

Relativistic laser plasmas have been shown to provide a robust platform for the generation of bright attosecond pulses via the relativistically oscillating mirror and coherent wake emission mechanisms. Theoretical work, however, has shown an alternative method for achieving this goal: dense nanobunch formation and acceleration on timescales of less than an optical laser cycle (∼10−15 s) during relativistic laser–plasma interactions. This opens up the exciting potential for developing a new bright ultrafast extreme ultraviolet XUV/x-ray source. Here we demonstrate, using a previously unexplored geometry, coherent synchrotron emission generated during relativistically intense laser–ultrathin foil interactions which extends to ∼1 keV photon energies. Particle-in-cell code simulations reveal how periodic sub-laser cycle acceleration of dense nanobunches of electrons formed during normal incidence interactions result in bursts of bright attosecond radiation in transmission and how these pulses relate to plasma density scalelength. This work shows clear potential for a novel, intense source of attosecond XUV (∼10−18 s) radiation. Experimentally, high order (n) harmonic spectra (I(n)) are characterized by a slow decay (n−1.62 ) before a rapid efficiency rollover. Such a microscopic coherent synchrotron source (<5 × 10−6 m) has the potential to significantly increase XUV pulse brightness significantly over current sources.