Directly visualizing the momentum forbidden dark excitons and their dynamics in atomically thin semiconductors (2005.00241v1)

Abstract: Resolving the momentum degree of freedom of excitons - electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained a largely elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum forbidden dark excitons, which critically impact proposed opto-electronic technologies, but are not directly accessible via optical techniques. Here, we probe the momentum-state of excitons in a WSe2 monolayer by photoemitting their constituent electrons, and resolving them in time, momentum and energy. We obtain a direct visual of the momentum forbidden dark excitons, and study their properties, including their near-degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominate the excited state distribution - a surprising finding that highlights their importance in atomically thin semiconductors.

Insight into Momentum-Resolved Dark Excitons in Atomically Thin WSe2

The paper "Directly visualizing the momentum forbidden dark excitons and their dynamics in atomically thin semiconductors" provides a detailed examination of dark excitons in monolayer transition metal dichalcogenide (TMD), specifically tungsten diselenide (WSe2). Through a novel experimental setup using time-resolved extreme ultraviolet micro-angle resolved photoemission spectroscopy (TR-XUV-µ-ARPES), the authors achieve direct visualization of momentum-forbidden dark excitons, a significant advancement given their elusive nature in prior excitonic studies.

Experimental Method and Observations

The authors successfully leveraged a custom-built platform combining a tabletop XUV source and a photoemission electron microscope (PEEM) to overcome traditional barriers in capturing excitonic states. This technique allowed the researchers to visualize both bright and dark excitons in the entire Brillouin zone of WSe2 under different excitation conditions. The experiment was conducted at 90 K, ensuring high energy resolution, crucial for resolving the spectral features associated with dark excitons.

Significantly, the paper found dark excitons, typically inaccessible through optical absorption, to dominate the excitonic landscape in WSe2. This was evidenced by the convergence within 500 fs towards a quasi-steady state, with a prominent presence of Σ-K excitons, under both resonant and above-bandgap excitations. The Σ-K excitons were confirmed to act as a long-lived reservoir, maintaining their dominance over time.

Numerical and Theoretical Insights

The experimental results were consistent with theoretical predictions, providing a pivotal verification of models that hypothesize about the exciton formation and their energy-momentum dispersion. Notably, the K-K and Σ-K excitons exhibited near-degeneracy, reinforcing the importance of inter-valley scattering events in the exciton dynamics. Furthermore, the calculated binding energies of approximately 390 meV for K-K excitons and 480 meV for Σ-K excitons align with theoretical expectations and extend our comprehension of the electronic properties of monolayer TMDs.

Implications and Future Directions

The research impacts several domains within semiconductor physics and materials science. In particular, the direct visualization of dark excitons opens pathways for utilizing these states in quantum information processing, valleytronics, and spintronics. The ability to control and manipulate dark excitons would pave the way for innovative optoelectronic devices, potentially leading to practical applications such as low-power electronics and next-generation solar technologies.

Future studies could expand upon this work by focusing on the real-time mapping of excitonic wavefunctions and examining few-particle complexes like trions and biexcitons in heterostructures. Additionally, improvements in experimental resolution could illuminate early-time carrier dynamics, further enhancing our understanding of above-bandgap excitation and the role of free carriers.

Conclusion

This paper marks a substantial contribution to the field of excitonic research in 2D materials, laying groundwork for further experimental and theoretical exploration of excitons in TMDs. By achieving direct access to dark excitons, the paper not only validates existing hypotheses but also sets a precedent for future endeavors aiming to explore exotic states of matter in atomically thin semiconductors.