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Dark Exciton Dynamics in Atomically Thin 2D Materials

Speaker:Prof. Ermin Malic
Affiliation:Department of Physics, Chalmers University of Technology
Date:July 12, 2018 (Thursday)
Time:4:00 p.m.
Venue:Room 522, 5/F, Chong Yuet Ming Physics Building, HKU


Monolayers of semiconducting transition metal dichalcogenides (TMDs) build a new class of atomically thin two-dimensional materials. They exhibit a remarkably strong Coulomb interaction giving rise to the formation of tightly bound excitons. In addition to regular bright excitonic states, there is also a variety of dark states that cannot be accessed by light due to the required momentum-transfer or spin-flip.

To model the exciton physics in TMD monolayers, we apply a microscopic approach combining semiconductor Bloch equations with the Wannier equation providing access to time- and momentum-resolved optical response and the non-equilibrium dynamics in TMDs. In this talk, we review our recent work focusing on:
(i) Uncovering the full exciton landscape including bright as well as momentum- and/or spin-forbidden dark excitonic states (Fig. 1a) [1].
(ii) Demonstrating signatures of dark excitonic states by probing the intra-excitonic 1s-2p transition directly after optical excitation and after exciton thermalization [2].
(iii) Activating momentum-forbidden dark excitonic states via efficient coupling with molecules suggesting a dark-exciton-based sensing mechanism for molecules (Fig. 1b,c) [3].
(iv) Providing a fully quantum mechanical description of formation, thermalization, and photoemission of interlayer excitons in TMD-based heterostructures [4,5].

Fig.1 (a) Exciton landscape including bright and dark states. (b) An optical pulse can only excite bright excitons resulting in a pronounced peak in optical spectra (orange). (b) In presence of molecules, dark excitons can be also excited via an efficient exciton-molecule coupling. The resulting additional peak (blue) can be exploited for sensing of molecules.

[1] E. Malic, M. Selig, M. Feierabend, S. Brem, D. Christiansen et al., Phys. Rev. Materials 2, 014002 (2018).
[2] G. Berghaeuser, P. Steinleitner, P. Merkl, R. Huber, A. Knorr, and E. Malic, arXiv: 1708.07725 (2017).
[3] M. Feierabend, G. Berghäuser, A. Knorr, E. Malic, Nature Commun. 8, 14776 (2017).
[4] M. Selig G. Berghaeuser, M. Richter, R. Bratschitsch, A. Knorr, E. Malic, 2D Mater. 5, 035017 (2018).
[5]S. Brem, G. Berghaeuser, M. Selig, E. Malic, Sci. Rep. 8, 8238 (2018).

Coffee and tea will be served 20 minutes prior to the seminar.

Anyone interested is welcome to attend.