Ultrafast Dephasing of Strongly Coupled Plasmon-Exciton States

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.