Ultrafast spectroscopy of two-dimensional crystals in external magnetic fields
Final Report Abstract
In this project, the temporal dynamics of excitons in mono- and multilayer transition-metal dichalcogenides (TMDCs) has been investigated via transient differential transmission (DT), time-resolved Faraday ellipticity (TRFE) and time-integrated four-wave mixing (TI-FWM) experiments. The project included also sample fabrication, as well as their characterization via photoluminescence and white-light reflectance. Based on the findings of DT and TRFE experiments, a comprehensive picture of the intricate valley dynamics of mono- and multilayer MoSe2 and WSe2 can be drawn. Moreover, the influence of in- and out-of-plane magnetic fields on the valley dynamics have been studied. In monolayer MoSe2 and WSe2, influences of the valley Zeeman shift on the spin/valley relaxation dynamics are observed, which manifest in a soft prolongation of the valley relaxation times. The most important result of this project is the discovery of ultrafast pseudospin quantum beats for H-type multilayer TMDCs, exposed to in-plane magnetic fields. The existence of the beating implicates a valley Zeeman splitting, which has neither been predicted nor been observed for bulk TMDCs in in-plane magnetic fields before. From the beating frequency, the effective excitonic g factor can be deduced with very high accuracy without the need of high-magnetic field facilities. Moreover, it has been shown that the excitonic g factor is isotropic. In degenerate, TI-FWM experiments, the influence of the exciton density and external magnetic fields on the coherent dynamics of excitons in TMDC monolayers has been investigated. The analysis of the T2 time does not reveal a significant change of the exciton dynamics in magnetic fields. For both, monolayer MoSe2 and WSe2, a coherent exciton-trion coupling, manifesting in a quantum beating, has been detected. For both cases, the polarisation of excitation and detection turns out to be decisive for the emergence of the quantum beating.
Publications
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Ultrafast pseudospin quantum beats in multilayer WSe2 and MoSe2. Nature Communications, 13(1).
Raiber, Simon; Faria, Junior Paulo E.; Falter, Dennis; Feldl, Simon; Marzena, Petter; Watanabe, Kenji; Taniguchi, Takashi; Fabian, Jaroslav & Schüller, Christian
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Ultrafast dynamics of excitons in bulk and monolayer selenide transition metal dichalcogenides, PhD thesis, Universität Regensburg
Simon Raiber
