Strong-field ionization of nanoscale clusters provides excellent opportunities to study the complex correlated electronic and nuclear dynamics of near-solid density plasmas. Yet, monitoring ultrafast, nanoscopic dynamics in real-time is challenging, which often complicates a direct comparison between theory and experiment. Here, near-infrared laser-induced plasma dynamics in $\sim 600\mathrm{nm}$ diameter helium droplets are studied by femtosecond time-resolved x-ray coherent diffractive imaging. An anisotropic, $\sim 20\mathrm{nm}$ wide surface region, defined as the range where the density lies between 10% and 90% of the core value, is established within $\sim 100\mathrm{fs}$, in qualitative agreement with theoretical predictions. At longer timescales, however, the width of this region remains largely constant while the radius of the dense plasma core shrinks at average rates of $\approx 71\mathrm{nm}/\mathrm{ps}$ along and $\approx 33\mathrm{nm}/\mathrm{ps}$ perpendicular to the laser polarization. These dynamics are not captured by previous plasma expansion models. The observations are phenomenologically described within a numerical simulation; details of the underlying physics, however, remain to be explored.

中文翻译：

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强场诱导纳米等离子体演化过程中的各向异性表面展宽和核心耗尽

纳米级团簇的强场电离为研究近固体密度等离子体的复杂相关电子和核动力学提供了极好的机会。然而，实时监测超快纳米级动力学具有挑战性，这通常会使理论和实验之间的直接比较变得复杂。在这里，近红外激光诱导的等离子体动力学在$～600\mathrm{纳米}$通过飞秒时间分辨 X 射线相干衍射成像研究直径氦液滴。各向异性，$～20\mathrm{纳米}$宽表面区域，定义为密度在核心值的 10% 到 90% 之间的范围，建立在$～100\mathrm{fs}$，与理论预测定性一致。然而，在较长的时间尺度上，该区域的宽度基本保持不变，而致密等离子体核心的半径以平均$\approx 71\mathrm{纳米}/\mathrm{ps}$沿着和$\approx 33\mathrm{纳米}/\mathrm{ps}$垂直于激光偏振。以前的等离子膨胀模型没有捕捉到这些动态。在数值模拟中对观察结果进行了现象学描述；然而，基础物理学的细节仍有待探索。