Ultrafast Dephasing of Strongly Coupled Plasmon-Exciton States

在化学系化学结构、动力学和机制A(CSDM-A)计划的支持下，圣母大学的Gregory Hartland教授和Masaru Koo教授正在使用光学显微镜研究激子-等离子激子极化子在单个半导体-金属纳米结构中的传播。极化子是一种不寻常的状态，它是通过将金属中光激发产生的表面等离子体与分子或半导体的激发态相耦合而产生的。极化子的寿命非常短，移动速度非常快，这使得使用传统技术研究它们变得困难。哈特兰、库诺教授和他们的学生将使用复杂的光散射和超快显微镜技术来测量激子-等离子激子极化子的寿命，以及它们沿着单个半导体-金属纳米结构传播的距离。该项目的发现可能导致更好地了解纳米材料中极化子的性质，以及太阳能发电的新策略。高中教师和学生将从宾夕法尼亚-哈里斯-麦迪逊学校公司招聘，该公司是当地的一个学区，拥有大约12,000名学生。他们将参与光散射实验，以及开发一种低成本显微镜的项目，用于检测和表征环境中的微塑料。该项目将以这种方式促进国家科学队伍的发展，并为研究生和本科生提供研究机会。要了解激子-等离子体激子极化子的性质，以及它们是否可以用于太阳能转换等应用，了解它们的动力学是很重要的。因为这些态的寿命通常很短(100 FS)，所以测量它们很有挑战性。在这个项目中，我们将用光散射和超快瞬时吸收显微镜研究由单个金属纳米结构的传播表面等离子体(PSP)与半导体的激子跃迁耦合而产生的极化子态的寿命。在光散射实验中，将使用实空间成像和背焦面成像相结合的方法，分别测量激子-PSP极化子态的传播长度和群速度。这两个量给出了极化子的寿命。重要的是，这些实验可以用来询问寿命非常短的系统。对于动力学相对较慢(50-100飞秒)的系统，寿命也将通过超快的单粒子瞬时吸收实验直接测量。将对连接到银或金纳米结构的不同半导体以及非传统等离子体系统，如氮化钛(TiN)进行测量。拟议的工作有几个更广泛的技术影响。例如，表面等离子激元的传播长度比激子态的传播长度要长得多。因此，将激子耦合到SPP可以显著增加它们的传播长度，并潜在地改善太阳能转换设备的性能。该奖项反映了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。

With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Professors Gregory Hartland and Masaru Kuno of the University of Notre Dame are using optical microscopy to study the propagation of exciton-plasmon polaritons in individual semiconductor-metal nanostructures. Polaritons are unusual states that are produced by coupling the surface plasmons created by optical excitation in metals to the excited states of molecules or semiconductors. Polaritons have very short lifetimes and move very quickly, making them difficult to study using conventional techniques. Professors Hartland, Kuno and their students will use sophisticated light scattering and ultrafast microscopy techniques to measure the lifetimes of exciton-plasmon polaritons, as well as the distances they travel along individual semiconductor-metal nanostructures. Discoveries from this project could lead to a better understanding of the properties of polaritons in nanomaterials, and new strategies for solar energy generation. High school teachers and students will be recruited for this project from the Penn-Harris-Madison School Corporation, a local school district with a population of approximately 12,000 students. They will participate in the light scattering experiments, as well as a proect to develop a low-cost microscope for detecting and characterizing microplastics in the environment. The project will contribute to the development of the Nation's scientific workforce in this way as well as by providing research opportunities for graduate and undergraduate students. To understand the properties of exciton-plasmon polaritons, and whether they can be used for applications such as solar energy conversion, it is important to understand their dynamics. Because the lifetimes of these states are typically very short (100 fs), they are challenging to measure. In this project the lifetimes of polariton states created by coupling propagating surface plasmons (PSPs) of single metal nanostructures to the exciton transitions of semiconductors will be investigated by light scattering and ultrafast transient absorption microscopy. In the light scattering experiments a combination of real space and back-focal plane imaging will be used to measure the propagation lengths and group velocities, respectively, of the exciton-PSP polariton states. These two quantities give the polariton lifetime. Importantly, these experiments can be used to interrogate systems with very short lifetimes. For systems where the dynamics are relatively slow (50-100 fs), the lifetimes will also be directly measured by ultrafast single-particle transient absorption experiments. Measurements will be performed for different semiconductors coupled to Ag or Au nanostructures, as well as for non-traditional plasmonic systems, such as titanium nitride (TiN). There are several technological broader impacts of the proposed work. For example, the propagation lengths for SPPs (surface plasmon polarities) are much longer than those for exciton states. Thus, coupling excitons to SPPs can significantly increase their propagation lengths, and potentially improve the performance of solar energy conversion devices.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.