CAREER: Molecular polaritonics: new opportunities for spectroscopy and control of charge and energy transport

加州大学圣地亚哥分校(UCSD)的乔尔·袁周获得了化学系化学理论、模型和计算方法项目以及材料研究部凝聚态和材料理论项目的支持，以开发理论和计算方法来研究光(光子)相互作用的激发分子态(激子)的混合量子力学状态。这些被称为极化子的轻物质混合态，具有不只存在于单独成分中的浮现特性。这项研究利用极化子的二元性来设计一种方法，在这种方法中，可以利用光与物质的相互作用来获得操纵电荷传输和光收集能量的新方法。通常，从阳光中捕获的能量以激子的形式在分子天线上传输，激子是在分子之间传输的中性纳米级激发。了解和控制分子材料中的激子输运是凝聚态物理和化学理论的重大挑战，也是光合作用研究和有机太阳能电池等新的集光技术的主要关注点。该项目培养研究生和博士后，熟悉理论化学、纳米光子学和凝聚态理论的交叉研究。它还通过一系列模仿拓扑材料中集体现象的表演编排，为学校学生制定了一个广泛的外展计划。周元洲博士还参与开发了一种新的定向策略，以适应他所在的UCSD系不断增长的国际研究生人数。极化子继承了光的波状属性，如扩展的空间相干性，但也继承了物质属性，这些属性允许能量局部化能量并引发化学反应或带电粒子之间的强相互作用。这项研究利用极化子的二元性来设计光-物质相互作用的方式，以产生不是单独通过每个组件发生的现象。这方面的一个例子是纳米和介观尺度中增强的电荷传输或拓扑保护的能量传输。在这里，拓扑学指的是不需要微调参数就能生存的特性，而是依赖于对材料缺陷和杂质具有健壮性的全局特性。本文主要研究了分子极化子的异常离域激子态。通过注意到偶极光-物质相互作用是各向异性的，并且与自旋-轨道耦合同构，该研究利用后者来诱导类似于拓扑绝缘体中的奇异极化子效应，从而产生对无序健壮的纳米和介观尺度能量流的空间和方向控制。这些想法首先在卟啉阵列的纯激子系统上进行测试，然后在受限电磁环境中的类似发色团系统上进行测试。在石墨烯等二维材料中与狄拉克系统的类比证明是卓有成效的，并产生了尚未探索的分子聚集体的前沿。袁周小组还处理了最近的观察结果，表明受限电磁场可以通过形成离域极化子来增强有机聚合物的导电性，为控制分子材料中的电子传输开辟了新的途径。尽管该项目本质上是理论上的，但它有一个强大的光谱成分，旨在模拟实验者可能产生的测量结果。与实验者的合作是这项工作的重要组成部分。袁周和他的同事正在提供第一个理论和计算框架来描述拓扑带结构和振动自由度之间的相互作用，包括振动退相干的有害或有益的影响。他们还在研究极化激元辅助电荷传输的综合理论。这些研究为纳米和介观尺度的能量和电荷流的可控性提供了洞察力，并为光收集技术和光学逻辑器件的设计提供了新的范例。科学部分的发展伴随着在跨学科环境中对研究生和博士后研究员的培训。在完成他们的工作后，受训者准备好在学术和工业环境中面对各种各样的当代科学挑战。该项目还包括一个多方面的教育项目，目的是向广大受众普及物质拓扑相的抽象概念。这包括一系列名为Top-Dance的体验式和解释性舞蹈活动，高中生参加集体编舞，目的是在准二维材料中重新创造能量和电荷传输，如项目中提到的那些。这些事件在事件中被记录、分析、消化，并在社交媒体上分发，以提供直观的替代方法来可视化上述概念。最后，为了回应最近加州大学圣迭戈分校化学和生物化学系国际研究生的崛起，周元洲教授还重新设计了迎新课程，目的是培养这些学生更有效地融入具有挑战性的学术和研究环境。实现这一目标的途径是举办一系列讲习班，讨论学术领导能力和多样性问题，并在系内建立一个辅导支助网络。

Joel Yuen-Zhou, of the University of California San Diego (UCSD), is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry and the Condensed Matter and Materials Theory program in the Division of Materials Research to develop theoretical and computational methods to study hybrid quantum mechanical states of light (photons) interacting excited molecular states (excitons). These light-matter hybrid states, known as polaritons, have emergent properties that do not exist in the separate components alone. This research harnesses the duality of polaritons to design ways in which light-matter interactions may be exploited to obtain novel ways to manipulate transport of charge and light-harvested energy. Typically, energy captured from sunlight moves across a molecular antenna in the form of excitons, neutral nanoscale excitations transported across molecules. Understanding and controlling exciton transport in molecular materials is a significant challenge for the theory of condensed phases physics and chemistry, as well as a primary concern for studies of photosynthesis and new light-harvesting technologies, such as organic solar cells. This project trains graduate students and postdoctoral fellows to be conversant on research at the intersection of theoretical chemistry, nano-photonics, and condensed matter theory. It also develops a broad outreach plan to school students via a series of performance choreographies mimicking collective phenomena in topological materials. Dr. Yuen-Zhou is also involved in developing a new orientation strategy to accommodate the increasing international graduate student population in his department at UCSD.Polaritons inherit wavelike properties from light such as extended spatial coherence, but also matter properties which allow energy to localize energy and give rise to a chemical reaction or to strong interactions between charged particles. This research harnesses the duality of polaritons to design ways in which light-matter interaction interact to produce phenomena that do not occur via each of the components alone. An example of this is enhanced charge transport or topologically-protected energy transport in the nano- and mesoscales. Here, topological refers to properties that do not require fine-tuning of parameters to survive, but rather, rely on global characteristics which are robust to material imperfections and impurities. This investigation focuses on the unusually delocalized exciton states of molecular polaritons. By noting that dipolar light-matter interaction is anisotropic and isomorphic to spin-orbit coupling, the research harnesses the latter to induce exotic polaritonic effects akin to those found in topological insulators, giving rise to spatial and directional control of nano- and mesoscale energy flow which is robust to disorder. These ideas are first tested on purely excitonic systems of porphyrin arrays and then on similar systems of chromophores in confined electromagnetic environments. Analogies to Dirac systems in two-dimensional materials such as graphene prove fruitful and give rise to unexplored frontiers of molecular aggregates. The Yuen-Zhou group also addresses recent observations suggesting that confined electromagnetic fields can enhance the conductivity of organic polymers owing to the formation of delocalized polaritons, opening new avenues for control of electron transport in molecular materials. Even though the project is theoretical in nature, it has a strong spectroscopic component aimed at simulating measurements that experimentalists might produce. Collaborations with experimentalists are a strong component of the work. Yuen-Zhou and coworkers are providing the first theoretical and computational framework to describe the interplay between topological band structures and vibronic degrees of freedom, including the deleterious or beneficial effects of vibrational decoherence. They are also studying a comprehensive theory for polaritons-assisted charge transport. These studies provide insights on the limits of controllability of energy and charge flow in the nano- and mesoscales, as well as new paradigms for the design light-harvesting technologies and optical logic devices. The development of the scientific component is accompanied by the training of graduate students and postdoctoral fellows in an interdisciplinary environment. Upon completion of their work, trainees are prepared to face a wide variety of contemporary scientific challenges, both in academic and industrial settings. The project also includes a multifaceted educational project with the goal of popularizing abstract concepts of topological phases of matter to a broad audience. This consists of a series of experiential and interpretative dance events termed Top-Dances, where high-school students participate in collective choreographies that aim to recreate energy and charge transport in quasi-two-dimensional materials such as those addressed in the project. These events are recorded, analyzed, digested in the events, and distributed in social media to provide alternative and intuitive ways to visualize the aforementioned concepts. Finally, as a response to the recent rise of international graduate students in the Department of Chemistry and Biochemistry of UCSD, Professor Yuen-Zhou also redesigns an orientation program with the goal of fostering a more effective integration of these students into a challenging academic and research environment. This goal is carried out through a series of workshops addressing issues of academic leadership and diversity, as well as the installation of a mentoring support network within the department.