We present experimental results of vacuum laser acceleration (VLA) of electrons using radially polarized laser pulses interacting with a plasma mirror. Tightly focused, radially polarized laser pulses have been proposed for electron acceleration because of their strong longitudinal electric field, making them ideal for VLA. However, experimental results have been limited until now because injecting electrons into the laser field has remained a considerable challenge. Here, we demonstrate experimentally that using a plasma mirror as an injector solves this problem and permits us to inject electrons at the ideal phase of the laser, resulting in the acceleration of electrons along the laser propagation direction while reducing the electron beam divergence compared to the linear polarization case. We obtain electron bunches with few-MeV energies and a 200-pC charge, thus demonstrating, for the first time, electron acceleration to relativistic energies using a radially polarized laser. High-harmonic generation from the plasma surface is also measured, and it provides additional insight into the injection of electrons into the laser field upon its reflection on the plasma mirror. Detailed comparisons between experimental results and full 3D simulations unravel the complex physics of electron injection and acceleration in this new regime: We find that electrons are injected into the radially polarized pulse in the form of two spatially separated bunches emitted from the p-polarized regions of the focus. Finally, we leverage on the insight brought by this study to propose and validate a more optimal experimental configuration that can lead to extremely peaked electron angular distributions and higher energy beams.
We study both experimentally and numerically the emission of energetic electrons during the reflection of a relativistic few-cycle laser pulse off a plasma mirror with controlled electron density gradient. A weak prepulse is used to trigger plasma expansion on a solid density target (optical grade fused silica) and electron emission is measured for different plasma scale lengths using a time-delayed relativistic-intensity few-cycle laser pulse with duration tunable from 24fs down to 3.5fs (1.5 cycle at the 719-nm carrier wave). Two distinct acceleration regimes are identified, for which the electron ejection mechanisms are radically different. On the one hand, when the plasma-vacuum interface is sharp, an attosecond electron bunch is emitted from the plasma at each laser optical cycle [1,2]. These electrons can then be efficiently accelerated in vacuum by the reflected laser field (vacuum laser acceleration or VLA) [3]. On the other hand, when the plasma scale length is larger, on the scale of a few laser wavelengths, a different regime is identified in which we observe what appears to be a collimated laser wakefield accelerated electron beam. Back-acceleration of energetic electrons can be explained by ionization injection of the rotating plasma waves inside the inhomogeneous electron density gradient formed at the plasma mirror surface [4]. These electrons are only detected when the laser pulse duration is shorter than 10 fs, clearly showing that new and unexpected laser-plasma interaction regimes become observable in the few-cycle regime.
[1] M. Bocoum et al., Anti-correlated emission of high harmonics and fast electron beams from plasma mirrors, Physical Review Letters 116, 185001 (2016)
[2] M. Thévenet et al., On the physics of electron ejection from laser-irradiated overdense plasmas, Physics of Plasmas 23, 063119 (2016)
[3] M. Thévenet, et al. Vacuum laser acceleration of relativistic electrons using plasma mirror injectors, Nature Physics 12, 355–360 (2015)
[4] N. Zaïm, et al. Laser wakefield acceleration driven by few-cycle laser pulses in overdense plasmas, manuscript in preparation
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