Ultrafast Imaging of Molecular Polariton Transport: Competition between Coherence and Localization

分子极化子传输的超快成像：相干性和定位之间的竞争

• 批准号: 2154388
• 负责人: Libai Huang
• 金额: $ 48万
• 依托单位: Purdue University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-07-01 至 2025-06-30
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2154388&HistoricalAwards=false
• 关键词: Ultrafast; Imaging; Molecular; Polariton; Transport

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professor Libai Huang and her research group at Purdue University are studying new ways to use light to control the transfer of energy between confined molecules. The project uses light trapped between a pair of very closely spaced mirrors to control the migration of energy between the molecules. Recent developments demonstrate that millions of molecules can collectively interact with a single mode of light when they are placed in a properly designed cavity. Although the molecules normally act like independently oscillating pendula (representing excited electron-hole pairs called excitons), the interaction with light inside the cavity results in a correlated motion of the pendula as if they are all attached to a single driving rod. When the energy exchange rate between the photons and the excitons (i.e., the light and the pendula) is faster than other energy loss pathways, new quantum states known as polaritons are formed with mixed light-matter characteristics. Polaritons have a low effective mass that allows the energy to travel at an extremely high speed, many orders of magnitude faster than the excitons alone. Thus, polaritons have the potential to enhance the speed of energy migration in molecular materials. However, the polariton states are very fragile and short-lived because the synchronization of millions of molecules can be easily disrupted. When the coherence is disrupted, energy becomes localized on a few molecules instead of being shared by millions of molecules. Understanding the competition between coherence and localization holds the key to harnessing the power of polaritons. The research team led by Prof. Huang is addressing this scientific challenge by developing microscopy techniques to record the propagation of polaritons with a resolution of 15 femtoseconds (a femtosecond is one quadrillionth of a second) and of 50 nanometers (a nanometer is one-billionth of a meter). The team also images the direction of propagation to differentiate polariton and exciton contributions. The research activities are integrated with K-12, undergraduate, and graduate science education. Specific educational and outreach activities include developing an undergraduate-level quantum chemistry lab module on strong light-matter coupling and partnering with the Superheroes of Science YouTube channel to make short videos for K-12 students and the general public on topics related to the research. Strong coupling between molecular excitons and cavity photons provides a new paradigm for achieving long-range coherent energy transport. A single mode of light can collectively interact with millions, or even billions, of molecules to form macroscopically coherent light-matter hybrid states known as polaritons. The formation of polaritons can dramatically alter the energy landscape and dynamics of the molecules. The photonic nature of polaritons enables fast propagation and long-range delocalization, which are beneficial for enhancing exciton transport. However, challenges remain in exploiting strong light-matter interactions for long-range energy transport in molecular systems, due to the inherent inhomogeneities and large vibronic coupling that result in ultrafast dephasing, a high density of dark states, and disorder-induced localization. This project aims to develop ultrafast microscopy tools in both real- and Fourier-space to provide a comprehensive picture of the competition between coherent polariton transport and decoherence processes in molecular aggregates. The measurements probe the transport of molecular polaritons with simultaneously high spatial (~50 nm) and temporal (~15 fs) resolution as well as momentum selectivity by combining ultrafast pump-probe microscopy with Fourier filtering. Molecular aggregates in Fabry-Perot microcavities with tunable Rabi splitting are used to investigate the role of vibronic coupling, disorder, and dark states in the transition from coherent to diffusive transport regime, as well as the direct visualization of remote energy transfer on macroscopic length scales resulting from the collective coherence. Beyond the fundamental insights made possible by these measurements, comparison with quantum chemistry calculations in collaboration with theory groups provides valuable information to aid the development of quantum mechanical theories for modeling molecular polaritons. The broader impacts of the project are further enhanced through educational and outreach activities that make the research accessible to a wide audience.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.

在化学系化学结构，动力学和机制-A（CSDM-A）计划的支持下，普渡大学的黄立白教授和她的研究小组正在研究利用光来控制受限分子之间能量转移的新方法。该项目利用被困在一对非常接近的镜子之间的光来控制分子之间的能量迁移。最近的发展表明，当数百万个分子被放置在适当设计的腔中时，它们可以共同与单一模式的光相互作用。虽然分子通常表现得像独立振荡的钟摆（代表被称为激子的激发电子-空穴对），但与腔内光的相互作用导致钟摆的相关运动，就好像它们都连接到单个驱动杆上。当光子和激子之间的能量交换速率（即，光和摆）比其他能量损失途径更快，形成具有混合光-物质特性的称为极化激元的新量子态。极化激元具有低有效质量，允许能量以极高的速度传播，比单独的激子快许多数量级。因此，极化激元具有提高分子材料中能量迁移速度的潜力。然而，极化激元态是非常脆弱和短暂的，因为数百万个分子的同步很容易被破坏。当相干性被破坏时，能量就局限在几个分子上，而不是被数百万个分子共享。理解相干性和局域化之间的竞争是利用极化激元的关键。黄教授领导的研究小组正在通过开发显微镜技术来应对这一科学挑战，以15飞秒（一飞秒是一秒的千万亿分之一）和50纳米（一纳米是十亿分之一米）的分辨率记录极化激元的传播。该团队还对传播方向进行成像，以区分极化子和激子的贡献。研究活动与K-12，本科和研究生科学教育相结合。具体的教育和推广活动包括开发一个关于强光物质耦合的本科生级量子化学实验室模块，并与YouTube科学超级英雄频道合作，为K-12学生和公众制作与研究相关主题的短视频。分子激子和腔光子之间的强耦合为实现长距离相干能量输运提供了新的范例。一种单模光可以与数百万甚至数十亿的分子共同作用，形成宏观相干的光-物质混合态，称为极化激元。极化激元的形成可以极大地改变分子的能量景观和动力学。极化激元的光子性质使得能够实现快速传播和远程离域，这有利于增强激子输运。然而，挑战仍然存在于利用强的光-物质相互作用在分子系统中的远程能量传输，由于固有的不均匀性和大的电子振动耦合，导致超快退相，高密度的暗态，和无序诱导的本地化。该项目旨在开发超快显微镜工具，在真实的和傅立叶空间，提供一个全面的图片之间的竞争相干极化激元传输和退相干过程中的分子聚集体。测量探测分子极化激元的传输，同时具有高的空间（~50 nm）和时间（~15 fs）分辨率以及动量选择性相结合的超快泵浦-探测显微镜与傅立叶滤波。利用可调谐拉比分裂的法布里-珀罗微腔中的分子聚集体，研究了电子振动耦合、无序和暗态在从相干到扩散输运的转变中的作用，以及由集体相干引起的宏观尺度上的远程能量转移的直接可视化.除了这些测量所带来的基本见解之外，与理论小组合作的量子化学计算的比较提供了有价值的信息，以帮助发展用于模拟分子极化激元的量子力学理论。通过教育和推广活动，使研究成果更广泛地为广大受众所接受，进一步增强了该项目的广泛影响。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Enhanced Two-Dimensional Exciton Propagation via Strong Light–Matter Coupling with Surface Lattice Plasmons

• DOI: 10.1021/acsphotonics.3c00466
• 发表时间: 2023-05
• 期刊: ACS Photonics
• 影响因子: 7
• 作者: Linrui Jin;Alexander D. Sample;Dewei Sun;Yao Gao;Shibin Deng;Ran Li;L. Dou;Teri W. Odom;Libai Huang