[论文解读] Time- and angle-resolved photoelectron spectroscopy of strong-field light-dressed solids: Prevalence of the adiabatic band picture

Ofer Neufeld, Wenwen Mao+5 more|arXiv (Cornell University)|Apr 29, 2022

Laser-Matter Interactions and Applications参考文献 118被引用 19

一句话总结

本研究通过从头算的时域和角分辨光电子能谱（Tr-ARPES）模拟，研究了强场激光作用下固体的电子能带结构，发现即使在高强场（~10¹² W/cm²）和宽波长范围（800–4000 nm）下，绝热（瞬时）能带图像依然保持稳健，Keldysh参数最高达~7。非绝热效应虽较小但可探测，尤其在双色光场探测时更为明显。

ABSTRACT

In recent years, strong-field physics in condensed-matter was pioneered as a novel approach for controlling material properties through laser-dressing, as well as for ultrafast spectroscopy via nonlinear light-matter interactions (e.g. harmonic generation). A potential controversy arising from these advancements is that it is sometimes vague which band-picture should be used to interpret strong-field experiments: the field-free bands, the adiabatic (instantaneous) field-dressed bands, Floquet bands, or some other intermediate picture. We here try to resolve this issue by performing 'theoretical experiments' of time- and angle-resolved photoelectron spectroscopy (Tr-ARPES) for a strong-field laser-pumped solid, which should give access to the actual observable bands of the irradiated material. To our surprise, we find that the adiabatic band-picture survives quite well, up to high field intensities (~10^12 W/cm^2), and in a wide frequency range (driving wavelengths of 4000 to 800nm, with Keldysh parameters ranging up to ~7). We conclude that to first order, the adiabatic instantaneous bands should be the standard blueprint for interpreting ultrafast electron dynamics in solids when the field is highly off-resonant with characteristic energy scales of the material. We then discuss weaker effects of modifications of the bands beyond this picture that are non-adiabatic, showing that by using bi-chromatic fields the deviations from the standard picture can be probed with enhanced sensitivity. Our work outlines a clear band picture for the physics of strong-field interactions in solids, which should be useful for designing and analyzing strong-field experimental observables and also to formulate simpler semi-empirical models.

研究动机与目标

• 解决强场激光作用下固体的能带图像解释中的模糊性。
• 确定绝热（瞬时）能带图像在何种条件下依然有效。
• 识别并表征绝热图像之外的非绝热偏离。
• 证明Tr-ARPES可作为强驱动体系中可观测电子能带的直接探测手段。
• 通过建立可靠的能带框架，实现更简单半经验模型的设计。

提出的方法

• 对单层六方氮化硼（hBN）在强场激光辐照下的时间依赖密度泛函理论（TDDFT）进行从头算模拟。
• 采用时间依赖表面通量（T-SURFF）方法，以高动量分辨率计算光电子发射谱。
• 使用复吸收边界条件以隔离光电子信号并最小化连续态传播的伪影。
• 通过泵浦-探测延迟模拟Tr-ARPES，以获取瞬时（绝热）和时间平均（Floquet）能带结构。
• 将完整TDDFT结果与独立粒子近似（IPA）进行比较，以分离电子-电子关联的作用。
• 采用双色激光场以增强对绝热图像之外非绝热效应的探测灵敏度。

实验结果

研究问题

• RQ1 在强场激光作用下，即使激光强度高达~10¹² W/cm²，绝热能带图像是否依然有效？
• RQ2 所观测的Tr-ARPES谱与绝热、Floquet及中间能带图像的预测相比如何？
• RQ3 非绝热效应在多大程度上改变了瞬时图像之外的能带结构？
• RQ4 双色光场是否能增强Tr-ARPES中对非绝热偏离的探测能力？
• RQ5 动力学电子-电子关联是否是导致偏离绝热能带描述的重要因素？

主要发现

• 绝热能带图像在激光强度达~10¹² W/cm²、Keldysh参数最高达~7的条件下依然高度准确，表明其稳健性远超传统绝热区域。
• 即使产生了高达~25 eV的高次谐波，也未观察到能带结构的明显崩塌或能量尺度的显著改变。
• 激光场的主要效应是通过瞬时矢势移动能带原点，并调节电子相位因子，与绝热理论一致。
• 非绝热修正确实存在但较微弱，仅在使用双色光场增强探测灵敏度时才可被检测到。
• 独立粒子近似（IPA）能极好地复现完整TDDFT结果，表明电子-电子关联并未显著改变Tr-ARPES谱，也未导致明显偏离绝热性。
• 在瞬时区域能谱中，Tr-ARPES可通过将电离时间映射到矢势引起的能带位移，实现对光电子发射时间延迟的精确测量。