Relativistic electrons are prodigious sources of photons. Beyond classical accelerators, ultra-intense laser interactions are of particular interest as they allow the coherent motion of relativistic electrons to be controlled and exploited as sources
of radiation. Under extreme laser conditions theory predicts that isolated free relativistic electron sheets (FRES) can be produced and exploited for the production of a new class of radiation - unipolar extreme ultraviolet(XUV) pulses. However, the combination of extremely rapid rise-time and highest peak intensity in these simulations is still beyond current laser technology. We demonstrate a route to isolated FRES with existing lasers by exploiting relativistic transparency to produce an ultra-intense pulse with a steep rise time. When such an FRES interacts with a second, oblique target foil the electron sheet is rapidly accelerated (kicked). The radiation signature and simulations demonstrate that a single, nanometer thick FRES was produced. The experimental observations together with our theoretical modeling suggest the production of the first unipolar (half-cycle) pulse in the XUV - an achievement that has so far only been realized in the terahertz spectral domain.
We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2 degree are observed
in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from micrometer thick targets. We demonstrate that this reduction arises from a steep longitudinal electron density gradient and an exponentially decaying transverse profile at the rear side of the ultrathin foils. Agreements are found both in an analytical model and in particle-in-cell simulations. Those novel features make nm foils an attractive alternative for high flux experiments relevant for fundamental research in nuclear and warm dense matter physics.
