| Time：1140402 (Wed.) 14:20~16:20 Speaker：Prof.羅炳蒝(Ping-Yuan Lo) 國立陽明交通大學電子物理學系/Department of Electrophysics, National Yang Ming Chiao Tung University Title： Efficient light upconversion via resonant exciton-exciton annihilation of dark excitons in few-layer transition metal dichalcogenides Abstract： Transition-metal dichalcogenides (TMDs) have attracted broad interest due to their exceptional electronic, optical and excitonic properties. When thin down to monolayer, the crossover from indirect to direct band gap together with the optical accessibility of the intrinsic valley degree of freedom making TMD monolayers promising candidate for photonic nanodevices. However, it is known that, due to the phonon scatterings, dark exciton states are highly involved in the exciton dynamics and play important roles in various optical phenomena. The prolonged lifetime due to the coupling with dark exciton states significantly enhances the effects of many-body interactions, including the exciton-exciton annihilation (EEA) which is a non-radiative process widely recognized as the main cause of low photoluminescence quantum yield. On the other hand, despite the large exciton binding energy and rich exciton fine structure, few-layer TMDs are often overlooked because of the indirect band gap and weaker light-matter interaction. Here, in this work, we report our recent findings of efficient light upconversion in few-layer TMDs. Our joint theory-experiment study attributes the underlying mechanism of the upconversion photoluminescence (UPL) to resonant EEA, which upconverts a pair of dark excitons carrying opposite momenta into a high-lying bright exciton, followed by the spontaneous emission from the upconverted bright exciton. In addition, the UPL via resonant EEA is generic in MoS2, MoSe2, WS2 and WSe2, showing a high tuneability from green to ultraviolet light (2.34-3.1eV). This finding repositions the EEA as a beneficial mechanism in few-layer TMDs, pave the way for further exploration of light upconversion regarding fundamental properties and device applications in two-dimensional semiconductors. Place：S101, Gongguan Campus, NTNU |
