[论文解读] Controlling Sub-Cycle Optical Chirality in the Photoionization of Chiral Molecules

Shaked Rozen, Antoine Comby+9 more|arXiv (Cornell University)|Jun 26, 2019

Laser-Matter Interactions and Applications参考文献 52被引用 60

一句话总结

The paper combines theory and experiment to shape sub-cycle optical chirality using a two-color field (fundamental plus phase-locked second harmonic) to induce enantiosensitive, sub-cycle photoionization (ESCARGOT) in chiral molecules, revealing forward/backward and up/down asymmetries that depend on two-color delay.

ABSTRACT

Controlling the polarization state of electromagnetic radiation enables the investigation of fundamental symmetry properties of matter through chiroptical processes. Many strategies have been developed to reveal structural or dynamical information about chiral molecules, from the microwave to the extreme ultraviolet range. Most schemes employ circularly or elliptically polarized radiation, and more sophisticated configurations involve, for instance, light pulses with time-varying polarization states. In all these schemes, the polarization state of light is always considered as constant over one optical cycle. In this study, we zoom into the optical cycle in order to resolve and control a subcyle attosecond chiroptical process. We engineer an electric field whose instantaneous chirality can be controlled within the optical cycle, by combining two phase-locked orthogonally polarized fundamental and second harmonic fields. While the composite field has zero net ellipticity, it shows an instantaneous optical chirality which can be controlled via the two-color delay. We theoretically and experimentally investigate the photoionization of chiral molecules with this controlled chiral field. We find that electrons are preferentially ejected forward or backward relative to the laser propagation direction depending on the molecular handedness, similarly to the well-established photoelectron circular dichroism process. However, since the instantaneous chirality switches sign from one half cycle to the next, electrons ionized from two consecutive half cycles of the laser show opposite forward/backward asymmetries. This chiral signal provides a unique insight into the influence of instantaneous chirality in the dynamical photoionization process. Our results demonstrate the important role of sub-cycle polarization shaping of electric fields, as a new route to study and manipulate chiroptical processes.

研究动机与目标

• 激励在亚轮周期内对光的瞬时手性进行控制，以在阿托秒尺度探测手性光学动力学。
• 开发并测试一种两色场（基频 + 相位锁定的二阶高次谐波）以产生瞬时手性且净偏振椭圆度为零。
• 从实验和理论上证明瞬时手性驱动对手性敏感的光离子化信号（ESCARGOT）。
• 同时探索多光子和强场电离区间，并将其与传统的 PECD 概念联系起来。

提出的方法

• 设计一个两色电场 E(t)=E0 cos(ωt) ŷ + rE0 cos(2ωt+φ) x̂，且 r≈0.3。
• 从 E(t) 和 B(t) 定义瞬时椭圆率 ε(t) 和瞬时手性 C(t)，以量化亚轮周期场的性质。
• 通过在双色场中求解时间依赖薛定谔方程(TDSE)，对一个简化的手性分子进行光电化，提取离化部分和动量空间分布。
• 使用欧拉旋转在分子坐标系与实验坐标系之间转换，并映射到 VMI 动量分布以便与实验比较。
• 将 VMI 分布相对于传播轴分解为对称部分和反对称部分，以分离 ESCARGOT 信号。

实验结果

研究问题

• RQ1 在一个光学循环内的瞬时手性是否能够在净场手性在一个循环内为零时驱动光离化中的对手性敏感信号？
• RQ2 ESCARGOT 信号如何在多光子与强场区域中随两色延迟 φ 和激光强度变化？
• RQ3 场形（C 形与 8 形）在产生对手性敏感的亚轮周期响应中的作用是什么？
• RQ4 在 fen chone 和 camphor 的实验测量在多大程度上与基于 TDSE 的 toy-model 对亚轮周期手性效应的预测一致？

主要发现

• ESCARGOT 信号（反对称的光电子分布）在所有两色延迟以及所有研究的电离区间中出现。
• 在多光子区间，ESCARGOT 达到约 1.5%，并表现出带有交替符号的结构化角分布。
• 在强场区间，ESCARGOT 持续存在但降至约0.5%，并在偏振平面附近变尖，具有与 ATI 特征相关的环结构。
• C 形场可以产生显著的前后和上下不对称性，这是由于场的时间导数（矢势）在连续两个半周期中赋予相反的旋转。
• 实验的 fenchone 数据显示的 ESCARGOT 信号与 TDSE 预测一致，包括相位相关的符号变化，而 camphor 的结果揭示分子特异性差异，与 toy 模型的一致性较差。
• 该工作强调亚轮周期手性光电离对瞬时场手性的敏感性，并展示了一条控制手性光学过程的新途径。