(135d) Excitonic Fine Structure in Two-Dimensional Transition Metal Dichalcogenides

Atomically thin transition metal dichalcogenides (TMDCs) have extensively been investigated for potential applications in valleytronics, field-effect transistors, photodetectors, and solar water disinfection devices. Understanding the electron-electron and electron-phonon interaction in TMDCs is crucial for both fundamental material studies and potential applications in the future. With the reduced dimensions, monolayer TMDCs show strong Coulomb interactions compared to bulk semiconductors, enabling a new platform to study excitonic fine structures of different types of quasi-particles such as trions, biexcitons, and polarons. Besides, monolayer TMDCs with a strong spin-orbital coupling (SOC) and the lack of spatial inversion symmetry lead to unique valley physics such as valley-polarized excitons and chiral phonons. The exciton-phonon interaction can be investigated by helicity-resolved optical method, offering us an in-depth view of valley physics in TMDCs, which is critical in quantum computing.

We used helicity-resolved photoluminescence (PL), reflectance and photocurrent spectroscopy under a high magnetic field and low temperature to study the excitonic fine structures of hexagonal boron nitride (hBN) encapsulated monolayer tungsten diselenide (WSe₂). Considering the electron-electron interaction, we identified the biexciton state for the first time by controlling doping density electrostatically [1]. We also observed the dark trions through the back focal plane imaging of the radiation pattern and magneto-PL [2], from which the electron-hole interaction strength and dark trion g-factor can be extracted. By investigating electron-phonon interaction, we identified a circularly polarized exciton-phonon replica that inherits large magneto-tunability and long lifetime from dark exciton [3]. The replica shares an efficient radiative recombination channel and strong valley polarization with the bright exciton, providing a chirality dictated emission channel for both phonons and photons. Another momentum-dark, intervalley exciton [4] was also detected directly in free space in monolayer WSe₂, which involves a chiral phonon to brighten the intervalley exciton, shedding light on promising new routes of realizing excitonic valleytronics. Besides, exciton fine futures is observed in a high magnetic field, with the coexistence of Landau levels and strongly bound excitons [5]. The quantized excitonic resonance from both the exciton and exciton-polaron branches is a manifestation of strong many-body interactions. Beyond the monolayer WSe₂, we also investigated the stacked WSe₂/MoSe₂ heterostrucutes with magneto-PL spectroscopy [6]. A nearly unity valley polarization of the interlayer exciton was observed, inspiring future exploration of applications in valleytronics and spintronics.

In summary, many excitonic fine features in TMDCs have been revealed through magneto PL, reflectance and photocurrent spectroscopy. We observed plenty of novel quasiparticles such as biexciton, dark trion, exciton-replica, and intervalley exciton in 2D materials. We also investigated the effect of coexistence of Landau levels and strongly bound excitons from quantized excitonic resonance, revealing the strong many-body interactions. The study of excitonic fine structure gives us a better understanding of fundamental material properties and unique particle interactions in 2D material, paving the road to real applications in the future.

[1] Z. Li, T. Wang, Z. Lu, C. Jin, Y. Chen, Y. Meng, Z. Lian, T. Taniguchi, K. Watanabe, S. Zhang, D. Smirnov, and S.-F. Shi, Revealing the Biexciton and Trion-Exciton Complexes in BN Encapsulated WSe₂, Nat. Commun. 9, 3719 (2018).

[2] Z. Li, T. Wang, Z. Lu, M. Khatoniar, Z. Lian, Y. Meng, M. Blei, T. Taniguchi, K. Watanabe, S. A. McGill, S. Tongay, V. M. Menon, D. Smirnov, and S.-F. Shi, Direct Observation of Gate-Tunable Dark Trions in Monolayer WSe₂, Nano Lett. 19, 6886 (2019).

[3] Z. Li, T. Wang, C. Jin, Z. Lu, Z. Lian, Y. Meng, M. Blei, S. Gao, T. Taniguchi, K. Watanabe, T. Ren, S. Tongay, L. Yang, D. Smirnov, T. Cao, and S. F. Shi, Emerging Photoluminescence from the Dark-Exciton Phonon Replica in Monolayer WSe₂, Nat. Commun. 10, 2469 (2019).

[4] Z. Li, T. Wang, C. Jin, Z. Lu, Z. Lian, Y. Meng, M. Blei, M. Gao, T. Taniguchi, K. Watanabe, T. Ren, T. Cao, S. Tongay, D. Smirnov, L. Zhang, and S.-F. Shi, Momentum-Dark Intervalley Exciton in Monolayer Tungsten Diselenide Brightened via Chiral Phonon, ACS Nano 13, 14107 (2019).

[5] T. Wang, Z. Li, Z. Lu, Y. Li, S. Miao, Z. Lian, Y. Meng, M. Blei, T. Taniguchi, K. Watanabe, S. Tongay, W. Yao, D. Smirnov, C. Zhang, and S.-F. Shi, Observation of Quantized Exciton Energies in Monolayer WSe₂ under a Strong Magnetic Field, Phys. Rev. X 10, 21024 (2020).

[6] T. Wang, S. Miao, Z. Li, Y. Meng, Z. Lu, Z. Lian, M. Blei, T. Taniguchi, K. Watanabe, S. Tongay, D. Smirnov, and S.-F. Shi, Giant Valley-Zeeman Splitting from Spin-Singlet and Spin-Triplet Interlayer Excitons in WSe2/MoSe2 Heterostructure, Nano Lett. 20, 694 (2020).