Guest Speaker – Physical (CHEM 781) – April 5, 2023

Bio
Joel Yuen-Zhou is Associate Professor in the Department of Chemistry and Biochemistry at UC San Diego. He holds a BSc in Chemistry and Mathematics from MIT (2007) and a PhD in Chemical Physics from Harvard University (2012) under the supervision of Alán Aspuru-Guzik. Yuen-Zhou started his independent career at UC San Diego in 2015 and obtained tenure in 2021. Yuen-Zhou is a pioneer of theories and computational tools to predict and understand the chemical dynamics and spectroscopy of molecular polaritons. His work has been recognized with awards such as NSF CAREER, DOE Early Career, Sloan, Camille Dreyfus, ACS Open Eye, and JPC Lectureship.

Abstract
The strong coupling of molecular transitions to photonic modes in optical microcavities gives rise to hybrid light-matter modes known as polaritons. In recent years, there has been much interest in polariton chemistry, namely, the use of polaritons to control chemical and material properties and processes. However, the mechanisms whereby polaritons can control chemistry are still rather opaque. In this talk, I will discuss the “large N problem” in polariton chemistry, where the formation of two polariton modes occurs at the expense of having N-1 ~10⁶−10¹² dark modes which under many circumstances have similar behavior as bare molecular excitations.^​1​ I will showcase a class of models that benefit from the existence of polaritons even at thermal equilibrium and offer some hypotheses about what might be happening in experiments^​2,3​.
Next, I will discuss strategies to ameliorate the large N problem by using concepts of (a) cavity optomechanics^​4​, (b) polariton condensates, and (c) non-equilibrium initialization of polaritons. In particular, I will emphasize a powerful and elegant computational tool^​6​ that we have recently developed to efficiently simulate the polariton chemistry problem, and showcase interesting experimental scenarios of polariton mediated photophysics and photochemistry^​7,8​ where these tools offer tremendous insights.

​​^{1. Campos-Gonzalez-Angulo, J. A., Poh, Y. R., Du, M. & Yuen-Zhou, J. Swinging between shine and shadow: Theoretical advances on thermally-activated vibropolaritonic chemistry (a perspective). (2022) doi:10.48550/arxiv.2212.04017.
​2. Campos-Gonzalez-Angulo, J. A., Ribeiro, R. F. & Yuen-Zhou, J. Resonant catalysis of thermally activated chemical reactions with vibrational polaritons. Nat Commun 10, (2019).
​3. Du, M. & Yuen-Zhou, J. Catalysis by Dark States in Vibropolaritonic Chemistry. Phys Rev Lett 128, (2022).
​4. Koner, A., Du, M., Pannir-Sivajothi, S., Goldsmith, R. H. & Yuen-Zhou, J. Thermodynamic coupling of reactions via few-molecule vibrational polaritons. arxiv.org (2023).
​5. Pannir-Sivajothi, S., Campos-Gonzalez-Angulo, J. A., Martínez-Martínez, L. A., Sinha, S. & Yuen-Zhou, J. Driving chemical reactions with polariton condensates. Nature Communications 2022 13:1 13, 1–9 (2022).
​6. Pérez-Sánchez, J. B., Koner, A., Stern, N. P. & Yuen-Zhou, J. Collective dynamics Using Truncated Equations (CUT-E): simulating the collective strong coupling regime with few-molecule models. (2022) doi:10.48550/arxiv.2209.04955.
​7. Xiang, B. et al. Intermolecular vibrational energy transfer enabled by microcavity strong light–matter coupling. Science (1979) 368, 665–667 (2020).
​8. Chen, T. T., Du, M., Yang, Z., Yuen-Zhou, J. & Xiong, W. Cavity-enabled enhancement of ultrafast intramolecular vibrational redistribution over pseudorotation. Science (1979) 378, 790–794 (2022).}