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、动力学和机理(CSDM-A)项目的支持下，普渡大学的黄丽柏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。该项目使用被困在一对间隔非常近的镜子之间的光来控制分子之间的能量转移。最近的发展表明，当数百万个分子被放置在适当设计的腔体中时，它们可以共同作用于单一的光模式。尽管分子通常表现为独立摆动的钟摆(代表被激发的电子-空穴对，称为激子)，但与腔内光的相互作用导致钟摆的相关运动，就像它们都连接到一个驱动杆上一样。当光子和激子(即光和钟摆)之间的能量交换速率比其他能量损失途径更快时，就形成了具有混合光-物质特性的新的量子态--极化子。极化子的有效质量很低，可以使能量以极高的速度传播，比激子本身快许多个数量级。因此，极化子有可能提高分子材料中的能量迁移速度。然而，极化子态是非常脆弱和短暂的，因为数百万分子的同步很容易被破坏。当相干被破坏时，能量就集中在几个分子上，而不是被数百万个分子共享。理解相干和局部化之间的竞争是利用极化子力量的关键。黄教授领导的研究小组正在通过开发显微技术来记录极化子的传播，以应对这一科学挑战，分辨率为15飞秒(一飞秒是千分之一秒)和50纳米(一纳米是十亿分之一米)。该团队还拍摄了传播方向的图像，以区分极化子和激子的贡献。研究活动与K-12、本科生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别的量子化学实验模块，并与科学超级英雄YouTube频道合作，就与研究相关的主题为K-12学生和普通公众制作短视频。分子激子和腔光子之间的强耦合为实现长程相干能量输运提供了新的范例。一种单模的光可以与数百万甚至数十亿的分子共同作用，形成宏观上相干的光-物质混合态，即所谓的极化子。极化子的形成可以极大地改变分子的能量格局和动力学。极化子的光子性质使其能够快速传播和远距离离域，这有利于增强激子的输运。然而，由于分子系统固有的不均匀性和大的振动耦合导致超快的退相、高密度的暗态和无序诱导的局域化，在利用强的光-物质相互作用进行长距离能量传输方面仍然存在挑战。该项目旨在开发实空间和傅立叶空间的超快显微镜工具，以全面了解分子聚集体中相干极化子传输和退相干过程之间的竞争。通过将超快泵浦-探测显微镜与傅里叶滤波相结合，测量了分子极化子的输运，同时具有高的空间(~50 nm)和时间(~15fs)分辨率以及动量选择性。利用可调谐拉比分裂的Fabry-Perot微腔中的分子聚集体，研究了振子耦合、无序和暗态在从相干输运到扩散输运的转变中的作用，以及由集体相干导致的宏观长度尺度上的远程能量转移的直接可视化。除了通过这些测量获得的基本见解之外，与与理论小组合作的量子化学计算进行比较还提供了有价值的信息，有助于发展用于模拟分子极化子的量子力学理论。该项目的更广泛的影响通过教育和外展活动进一步加强，使研究向广大受众开放。这一奖项反映了NSF的法定使命，并通过使用基金会的智力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