Coherent Control and Analysis of Atomic Multi-Photon Processes

相干控制旨在利用光来操纵量子过程的结果，例如原子的激发或电离、化学反应或集体多粒子过程。应用范围从医学成像技术(磁共振成像)到大生物分子的结构和动力学分析(磁共振光谱学)，再到量子信息处理(量子计算)。目前的技术依赖于光的微观属性的成形，例如光脉冲的时间结构以及分别对应于光子的能量、自旋极化和密度的电场频率、方向和强度。在这个项目中，高级研究人员和研究生将提出已建立的相干控制方案，并开发新的方案来控制最简单的原子系统之一-只有一个活跃电子的锂原子-光吸收过程中的电子动力学。在新的实验中，将通过塑造激光的宏观特性来克服先前的限制。具体地说，激光场波前将被改变，产生所谓的电磁“涡旋”激光光束，这种光束在传播方向携带轨道角动量，从而打开在传统光吸收过程中被“禁止”的额外电离路径。塑造激光脉冲的所有性质，包括它们的波前，将极大地扩展现有的科学工具箱，以诱导和观察特定的量子路径，并将为控制自然界中许多过程的电子动力学的相干控制打开新的方向盘。在更高的技术层面上，锂原子被困在一个全光学陷阱中，并受到飞秒激光脉冲的影响。在第一步中，将例如通过创建具有可变偏振的双色激光脉冲的可调谐序列来改变飞秒激光场的微观属性。这将使增强特定的多光子吸收路径、研究依赖时间的里德堡动力学或创建奇异的电子波包成为可能。在第二步中，激光光束的宏观性质将被整形，并将使用全息术产生携带内轨道角动量的光学涡旋光束。我们的目标是将角动量耦合到电子，从而打开偶极禁止的电离路径。我们将用“反应显微镜”分析抛射电子波包上光学涡旋的印记，从而测量原子碎片的动量矢量。尽管最近有理论预测，但到目前为止，关于光电离中的非偶极跃迁，即束缚-自由跃迁的实验证据是完全没有的。考虑到准确地将(微观)原子定位在螺旋激光光束中心的艰巨实验挑战，这种数据的缺乏并不令人惊讶。新的实验旨在克服这一障碍，并提供关于光致电离过程中轨道角动量转移的第一个证据，从而为理论描述提供基准。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Coherent control aims at manipulating the outcome of quantum processes such as the excitation or ionization of atoms, chemical reactions, or collective many-particle processes by using light. Applications range from medical imaging techniques (magnetic resonance imaging) to structural and dynamical analysis of large biomolecules (magnetic resonance spectroscopy) to quantum information processing (quantum computing). Present techniques rely on the shaping of the microscopic properties of the light, such as the temporal structure of the light pulses as well as the electric field frequency, direction, and strength corresponding to the photons' energy, spin polarization, and density, respectively. In this project, the senior investigators and graduate students will advance established coherent control schemes and develop new ones to control the electron dynamics in photo-absorption processes in one of the simplest atomic systems – a lithium atom with a single active electron. In the new experiments, previous limitations will be overcome by shaping the macroscopic properties of the laser beam. Specifically, the laser field wavefronts will be altered, creating so-called electromagnetic "vortex" laser beams, which carry orbital angular momentum in the direction of propagation, thereby opening additional ionization pathways which are “forbidden” in conventional photo-absorption processes. Shaping ALL properties of laser pulses, including their wavefronts, will significantly expand the available scientific toolbox to induce and observe specific quantum pathways and it will unlock new dials for the coherent control of electron dynamics governing many processes in nature. On a more technical level, lithium atoms are trapped in an all-optical trap and subjected to femtosecond laser pulses. In a first step, the microscopic properties of the femtosecond laser fields will be altered, e.g., by creating tunable sequences of bichromatic laser pulses with variable polarization. This will enable enhancing specific multiphoton absorption pathways, studying time-dependent Rydberg dynamics, or creating exotic electronic wave packets. In the second step, the macroscopic properties of the laser beam will be shaped, and optical vortex beams carrying internal orbital angular momentum will be generated using holography. The goal is to couple this angular momentum to the electrons, thereby opening dipole-forbidden ionization pathways. The imprint of the optical vortices on the ejected electron wave packet will be analyzed employing a "reaction microscope", allowing the measurement of the momentum vectors of atomic fragments. Despite recent theoretical predictions, experimental evidence on non-dipole transitions in photoionization, i.e., on bound-free transitions, are, to date, entirely absent. This lack of data is not surprising considering the formidable experimental challenge to position a (microscopic) atom accurately in the center of a helical laser beam. The new experiments aim to overcome this obstacle and provide the first evidence on orbital angular momentum transfer in photoionization processes, thereby providing a benchmark for theoretical descriptions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.