The coupling of laser energy to electrons is fundamental to almost all topics in intense laser-plasma interactions, including laser-driven particle and radiation generation, relativistic optics, inertial confinement fusion and laboratory astrophysics. We report measurements of total energy absorption in foil targets ranging in thickness from 20 μm, for which the target remains opaque and surface interactions dominate, to 40 nm, for which expansion enables relativistic-induced transparency and volumetric interactions. We measure a total peak absorption of ∼80% at an optimum thickness of ∼380 nm. For thinner targets, for which some degree of transparency occurs, although the total absorption decreases, the number of energetic electrons escaping the target increases. 2D particle-in-cell simulations indicate that this results from direct laser acceleration of electrons as the intense laser pulse propagates within the target volume. The results point to a trade-off between total energy coupling to electrons and efficient acceleration to higher energies.

中文翻译：

---

在从表面主导到体积主导的强激光-等离子体相互作用机制的转变中，能量吸收和与电子的耦合

激光能量与电子的耦合是强烈激光等离子体相互作用中几乎所有主题的基础，包括激光驱动的粒子和辐射生成、相对论光学、惯性约束聚变和实验室天体物理学。我们报告了箔目标总能量吸收的测量结果，厚度范围从 20 μm（目标保持不透明且表面相互作用占主导地位）到 40 nm（其膨胀能够实现相对论诱导的透明度和体积相互作用）。我们在约 380 nm 的最佳厚度处测量了约 80% 的总吸收峰。对于较薄的目标，其具有一定程度的透明度，尽管总吸收减少，但逃离目标的高能电子数量增加。2D 细胞内粒子模拟表明，这是由于强激光脉冲在目标体积内传播时电子的直接激光加速所致。结果表明在与电子耦合的总能量和对更高能量的有效加速之间进行权衡。