About a decade ago, the discovery of monolayers of transition metal dichalcogenides opened a new frontier in the study of optically excited states in semiconductors. These materials exhibit a plethora of robust excitonic states – bound quasiparticle pairs of electrons and holes, such as bright excitons, momentum- and spin-forbidden dark excitons, and hot excitons. While optics-based experiments can access bright excitons, measuring dark excitons requires a momentum-resolving technique like ARPES – a decades-old grand challenge.

Accessing excitons with ARPES is not just conceptually unclear, but also technically challenging. One needs to study micron-scale samples of 2D materials, with ultrafast time-resolution, using XUV photons. Prof. Dani will discuss the progress in his lab towards these goals utilizing ultrafast photoemission techniques with spatial- and momentum-resolution [1, 2] that we have also used to learn valuable information in a variety of other materials systems [3, 4, 5]. 

Hot-Topic Literature on Ultrafast ARPES Microscopy from the Dani Group:
[1] J. Madeo, M. K. L. Man, et al. Science 370, 1199 (2020)
[2] M. K. L. Man, J. Madeo, et al. Science Advances 7, eabg0192 (2021)
[3] T. Doherty, A. Winchester, et al. Nature 580, 360 (2020)
[4] E. L. Wong, et al. Science Advances 4, eaat9722 (2018)
[5] M. K. L. Man, et al. Nature Nanotechnol. 12, 36 (2017)

About the Presenter: Keshav M. Dani, Okinawa Institute of Science and Technology

Keshav M. Dani is currently an Associate Professor at the Okinawa Institute of Science and Technology (OIST), Graduate University in Okinawa, Japan. He joined OIST in November 2011 as an Assistant Professor after completing a Director's Postdoctoral Fellowship at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory. Prof. Dani obtained a BS from Caltech in Mathematics in 2000 and graduated from the University of California at Berkeley in 2006 with a PhD in Physics. Additional information on the work of the Femtosecond Spectroscopy Unit at OIST can be found at https://groups.oist.jp/fsu.