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.

中文翻译：

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超强径向极化激光脉冲与等离子镜的相互作用

我们介绍了使用与等离子反射镜相互作用的径向偏振激光脉冲对电子进行真空激光加速（VLA）的实验结果。紧密聚焦的径向偏振激光脉冲由于其强大的纵向电场而被提出用于电子加速，使其非常适合VLA。但是，到目前为止，由于将电子注入激光场仍然是一个巨大的挑战，因此实验结果一直受到限制。在这里，我们通过实验证明，使用等离子镜作为注入器可以解决此问题，并允许我们在激光的理想相位处注入电子，从而导致沿激光传播方向的电子加速，而与线性极化情况。我们获得了几束MeV能量和200 pC电荷的电子束，因此首次证明了使用径向偏振激光将电子加速到相对论能量。还测量了从等离子体表面产生的高谐波，它提供了电子在等离子反射镜上反射后向激光场中注入电子的更多信息。实验结果与完整的3D模拟之间的详细比较揭示了在这种新机制下电子注入和加速的复杂物理原理：我们发现电子以从p极化区域的两个空间上分开的束的形式注入到径向极化脉冲中。焦点。最后，