Events

Spectroscopy and Topological Phases for Organic Excitons

April 21, 2015 at 4:30 PM/ RLE Haus 36-428

Joel Yuen
The Research Laboratory of Electronics, Massachusetts Institute of Technology

abstract
The understanding and control of energy flow at the nanoscale via exciton dynamics is of fundamental chemical and physical interest, but is also technologically relevant for the design of novel light-harvesting materials. In the first part of my talk, I will explain some of our work designing spectroscopic protocols to understand exciton dynamics under coherent illumination via ultrafast Quantum Process Tomography (QPT), a technique which retrieves the time evolution of the quantum state of excitons via nonlinear spectroscopy (1,2). As an application, I will describe the first ultrafast QPT experiment carried out with the Nelson and Bawendi groups at MIT on a nanotubular J-aggregate system at room temperature. I will also clarify the possible relevance of strongly coupled chromophores in natural light-harvesting under incoherent illumination from sunlight (3). Then, I will proceed to explain how one can in principle distinguish excitonic coherences and their vibrational counterparts in nonlinear spectroscopy (4,5).

In the last part of my talk, I will describe current work (6) designing topologically nontrivial phases that robustly and selectively move excitons in particular spatial directions of a molecular crystal, simulating solid state “topologically protected” phenomena like the Quantum Hall Effect, which are robust against material imperfections and static disorder. I will end by presenting our most recent work on creating one-way waveguides of plexcitons (strongly coupled excitons and surface-plasmon polaritons).

(1) J. Yuen-Zhou, Jacob J. Krich, Masoud Mohseni, and A. Aspuru-Guzik, Quantum state and process tomography of energy transfer systems via ultrafast spectroscopy, Proc. Nat. Acad. Sci. USA. 108, 43, 17615 (2011).
(2) J. Yuen-Zhou, D. Arias, D. Eisele, J. J. Krich, C. Steiner, K. A. Nelson, and A. Aspuru. Guzik, Coherent exciton dynamics in supramolecular light-harvesting nanotubes revealed by ultrafast quantum process tomography, ACS Nano 8 (6) 5527 (2014).
(3) I. Kassal, J. Yuen-Zhou, and Saleh Rahim-Keshari, Does coherence enhance transport in photosynthesis, J. Phys. Chem. Lett. 4 (3), 362 (2012).
(4) J. Yuen-Zhou, Jacob J. Krich, and A. Aspuru-Guzik, A witness for coherent electronic vs vibronic-only oscillations in ultrafast spectroscopy, J. Chem. Phys. 136, 234501 (2012).
(5) A. Johnson, J. Yuen-Zhou, A. Aspuru-Guzik, and J. Krich, Practical witness for electronic coherences, J. Chem. Phys. 141, 244109 (2014).
(6) J. Yuen-Zhou, S. Saikin, N. Yao, and A. Aspuru-Guzik, Topologically protected excitons in porphyrin thin films, in press, Nature Materials 13, 1026 (2014).

bio
Joel Yuen-Zhou got his BSc in 2007 from the MIT, where he worked as a UROP with the late Robert J. Silbey, simulating the absorption spectra of light-harvesting antennae in purple bacteria. He then obtained his PhD in 2012 from Harvard under the supervision of Alán Aspuru-Guzik working on various aspects of time-dependent density functional theory, quantum information, and nonlinear spectroscopy. In 2013, he came back to MIT to work at the Center for Excitonics as the Robert J. Silbey postdoctoral fellow, where he got interested in the connections between excitonic systems and topologically nontrivial phases in condensed matter. He will start his own research group in theoretical chemical physics as an assistant professor at the University of California San Diego in the Department of Chemistry and Biochemistry as of July 2015.