MoWSe_Spin and pseudospin properties of energetically controllable dark and bright excitons in tailored Mo1-xWxSe2 alloys
Final Report Abstract
We demonstrate a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe2 monolayers. The upconversion is accompanied by the cooling of resident electrons and depends on the relation between the conduction-band spin splitting and the exchange splitting of the exciton. This interaction process observed between the inter- and intravalley excitons underlines the importance of dark excitons for the optics of 2D quantum materials. For the same hBN/WSe2/hBN heterostructures, we moreover studied the interlayer electronphonon coupling. The intensity of the emission from the WSe2 monolayer is strongly increased due to a double resonance, where the laser excitation energy resonates with the 2s A exciton, and the energy of the combined phonon mode ZO (hBN) + A’1(WSe2) is equal to the energy difference between the 2s and 1s exciton states. We also found a remarkable impact of the interlayer electron-phonon coupling on the preservation of the laser light helicity in the emission of the exciton, trions and biexcitons. These results highlight a further detail in the carrierphonon coupling in TMDCs and represent a way of enhancing the circular polarization degree of excitonic emissions. The latter point may be attractive for spintronic scenarios in which information is optically imprinted into quantum materials with high fidelity. For WSe2 monolayers placed on hBN, driving a phonon mode into resonance with an exciton or trion transition leads to spectral line shifts which are characteristic for a lower and upper polariton branch. The phonon-polaritons in photoluminescence excitation spectra of WSe2 are created through the optical excitation of an exciton in the WSe2 monolayer and an out-of-plane phonon in hBN. The splitting between the upper and lower polariton branch amounts to about 7 meV. These results may pave the way towards a further study of polaritonic coupling for TMDC heterostructures, along with potential applications in quantum information processing and nonlinear optical materials. A method of controlling their coupling strength in these systems may provide an attractive platform to explore light-matter interaction. In the emission spectra of a MoS2/hBN heterostructure without an hBN cap, the existence of two trion features suggests that the MoS2 monolayer has a dark excitonic ground state, despite having a ‘bright’ single-particle arrangement of spin-polarized conduction bands. In addition, we show that the effective excitonic g-factor significantly depends on the electron concentration and reaches the lowest value of -2.47 for hBN-encapsulated structures which reveals a nearly neutral doping regime. These results highlight a novel mechanism for the configuration and energetical ordering of the T1 and T2 trions. They present a further step towards understanding the fine structures and interactions of excitonic complexes in TMDCs. For Mo1-xWxSe2 monolayers, we found a hitherto unknown intervalley scattering and recombination process and a versatile tool to reveal the spin and valley states of electrons in 2D TMDCs using high-resolution inelastic light scattering. The scattering process is realized by an exchange-based valley-cross scattering, in which the hole stays at its initial valley while the electron is scattered to the opposite K valley under spin conservation. Besides that, time-resolved analyses reveal the contribution of the dark intervalley exciton to the picosecond decay dynamics of the bright intra-valley exciton. Depending on the temperature, disorder-induced scattering between different spin-split states and valleys affects the exciton and trion dynamics as well as the temporal evolution of the optical orientation degree. Changes in the optical orientation are also caused by the interaction with chiral phonons providing an additional scattering channel for electrons and holes. These timeresolved results for the tunable Mo1-xWxSe2 2D structures underline the importance of dark and bright excitonic states both for nanophotonics as well as spin-based research.
Publications
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Investigations of Electron-Electron and Interlayer Electron-Phonon Coupling in van der Waals hBN/WSe2/hBN Heterostructures by Photoluminescence Excitation Experiments, Materials 14, 399 (2021)
Joanna Jadczak, Joanna Kutrowska-Girzycka, Janina J. Schindler, Joerg Debus, Kenji Watanabe, Takashi Taniguchi, Ching-Hwa Ho, and Leszek Bryja
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Probing negatively charged and neutral excitons in MoS2/hBN and hBN/MoS2/hBN van der Waals heterostructures, Nanotechnology 32, 145717 (2021)
J. Jadczak, J. Kutrowska-Girzycka, M. Bieniek, T. Kazimierczuk, P. Kossacki, J. J. Schindler, J. Debus, K. Watanabe, T. Taniguchi, C. H. Ho, A. Wójs, P. Hawrylak, and L. Bryja
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Upconversion of Light into Bright Intravalley Excitons via Dark Intervalley Excitons in hBN-Encapsulated WSe2 Monolayers, ACS Nano 15, 19165 (2021)
Joanna Jadczak, Mikhail Glazov, Joanna Kutrowska-Girzycka, Janina J. Schindler, Joerg Debus, Ching-Hwa Ho, Kenji Watanabe, Takashi Taniguchi, Manfred Bayer, and Leszek Bryja
