Imaging photoexcited phenomenon in real and momentum space

Abstract:  Photoemission spectroscopy techniques – wherein one photoemits an electron from a material using a high-energy photon to study its properties – have provided unparalleled insight into materials and condensed matter systems over the past several decades. Among these, there are two particularly powerful and complementary techniques: angle-resolved photoemission spectroscopy (ARPES), which resolves the momentum of the photoemitted electron in the material; and photoemission electron microscopy (PEEM), which resolves its spatial coordinate. Recently, the merger of these techniques into multi-dimensional platforms of photoemission spectroscopy, along with access to the temporal dimension by further incorporating ultrafast spectroscopy techniques, have enabled powerful visuals of the dynamics of photoexcited systems in real and momentum space. In the first part of the talk, I will discuss some recent work in my lab in visualizing photoexcited carriers in space, time and energy [1]. Applying these techniques to state-of-the-art perovskite photovoltaic films, we will image the performance limiting nanoscale defect clusters [2] in these next-gen solar materials, and understand their role in charge trapping [3, 4]. In the second part of the talk, we will turn our attention to imaging momentum space in photoexcited 2D semiconductors and heterostructures. Thereby, we will directly image the distribution of an electron around a hole in an exciton [5] – a hydrogen-like state that forms when a semiconductor absorbs light; visualize dark excitonic states that have largely remained hidden to optical experiments [6], and observe the structure of a moiré trapped interlayer exciton [7].

References

[1] Nature Nanotech. 12, 36 (2017); [2] Nature 580, 360 (2020); [3] Energy & Environ. Science 14, 6320 (2021); [4] arXiv:2107.09549 (2021); [5] Science Advances 7, eabg0192 (2021); [6] Science 370, 1199 (2020); [7] Nature 603, 247 (2022)

Bio: Keshav Dani is currently an Associate Professor at the Okinawa Institute of Science and Technology (OIST), Graduate University in Okinawa, Japan. He joined OIST in Nov. 2011 as a tenure-track Assistant Professor after completing a Director’s Postdoctoral Fellowship at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory. Keshav graduated from UC Berkeley in 2006 with a PhD in Physics, where he explored the nonlinear optical response of the quantum Hall system under the supervision of Daniel Chemla at LBNL. Prior to his PhD, he obtained a BS from Caltech in Mathematics with a senior thesis in Quantum Information Theory under John Preskill and Hideo Mabuchi. His current research interests lie in the use of time-resolved multi-dimensional photoemission techniques to study electron dynamics of two-dimensional materials and energy materials.

Zoom Link: https://stanford.zoom.us/j/92153920201?pwd=YW5PV1kxek9Cd2xuY0xwWU9zNWdWUT09

Zoom Password: 257509