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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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