S. Cousens

Coherent synchrotron emission in transmission from ultrathin relativistic laser plasmas

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 1keV 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

Bright Subcycle Extreme Ultraviolet Bursts from a Single Dense Relativistic Electron Sheet

Double-foil targets separated by a low density plasma and irradiated by a petawatt-class laser are shown to be a copious source of coherent broadband radiation. Simulations show that a dense sheet of relativistic electrons is formed during the interaction of the laser with the tenuous plasma between the two foils. The coherent motion of the electron sheet as it transits the second foil results in strong broadband emission in the extreme ultraviolet, consistent with our experimental observations.

Noncollinear Polarization Gating of Attosecond Pulse Trains in the Relativistic Regime

High order harmonics generated at relativistic intensities have long been recognized as a route to the most powerful extreme ultraviolet pulses. Reliably generating isolated attosecond pulses requires gating to only a single dominant optical cycle, but techniques developed for lower power lasers have not been readily transferable. We present a novel method to temporally gate attosecond pulse trains by combining noncollinear and polarization gating. This scheme uses a split beam configuration which allows pulse gating to be implemented at the high beam fluence typical of multi-TW to PW class laser systems. Scalings for the gate width demonstrate that isolated attosecond pulses are possible even for modest pulse durations achievable for existing and planned future ultrashort high-power laser systems. Experimental results demonstrating the spectral effects of temporal gating on harmonic spectra generated by a relativistic laser plasma interaction are shown.

Enhanced Laser-Driven Ion Acceleration by Superponderomotive Electrons Generated from Near-Critical-Density Plasma

We report on the experimental studies of laser driven ion acceleration from a double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer-thin diamondlike carbon foil. A significant enhancement of proton maximum energies from 12 to ∼30  MeV is observed when a relativistic laser pulse impinges on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.

Polarization Gating in Relativistic Laser-Solid Interactions

High order harmonic generation from relativistic laser-solid interactions (focused intensity of \(>10^{18}~\)Wcm\(^{-2}\)) has the potential to serve as a source of bright attosecond radiation. One key mechanism that can generate such radiation is the Relativistically Oscillating Mirror (ROM) where the overdense plasma surface oscillates at relativistic velocities leading to a Doppler upshift of the reflected laser radiation. A major obstacle to the application of such a harmonic source is that the radiation is emitted as a periodic pulse train with the frequency of the driving laser. One route to limiting this emission to a single pulse is to exploit the ellipticity dependence of these mechanisms by forming a pulse whose polarisation varies from circular to linear to circular—a technique known as polarization gating. At small angles of incidence it is expected that the efficiency of the ROM mechanism drops dramatically for circular polarization. Here we present a novel method of implementing this technique for high power laser pulses along with proof of principle experimental results.

Generation of sub-cycle attosecond pulses from a single laser-driven relativistic electron sheet

Techniques that produce bright isolated attosecond pulses are very attractive for attosecond science. Here we report recent experimental results on generation of sub-cycle attosecond pulses from a signle laser-driven relativistic electron sheet.