Keshav M. Dani
Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology, 1919-1 Tancha, Onna-son, Okinawa, Japan

“Imaging the distribution of the electron around the hole in an exciton “

An exciton – a fundamental optical excitation in a semiconductor, consist of a photoexcited electron bound to its partner the hole. Analogous to the hydrogen atom, the wavefunction of this two-particle bound state is described in terms of the relative co-ordinate between the electron and hole. While such theoretical descriptions of the exciton have been understood for decades, direct experimental measurements of the wavefunction have not been possible to date. Challenges include measuring the relative co-ordinates between the delocalized electron and hole, as well as the very short few-picosecond lifetime of the exciton.
In this talk, I will briefly describe the time-, space-, angle- and energy-resolved photoemission spectroscopy instrumentation we have developed at OIST over the past decade [1, 2]. Then, in analogy to collider experiments of high-energy physics, we use this novel instrumentation to split open the exciton in monolayer WSe2, photoemit the constituent electron and measure its momentum co-ordinate. Under the right experimental conditions, we directly obtain the excitonic wavefunction in momentum-space. By Fourier transforming, we image the distribution of the electron around the hole in real space [3]. Finally, we will also use our experimental capability to visualize the momentum-forbidden dark excitons, their properties and their formation dynamics in monolayer WSe2 [2].

[1] T. Doherty*, A. Winchester*, et al. Nature 580 360 (2020)
[2] J. Madeo*, M. K. L. Man*, et al. Science 370 1199 (2020).
[3] M. K. L. Man*, J. Madeo*, et al. arXiv:2011.13104 (2020)

* equal authors