Abstract
High-order nonlinearity enables larger signal enhancements in two-photon spectroscopy compared to one-photon spectroscopy, which is important for conducting photoluminescence measurements at the single-molecule level. Herein, the two-photon-excited fluorescence (2pF) of monolayer MoS2 in various tip-enhanced spectroscopy configurations was theoretically investigated using the finite-difference time-domain method. Large enhancement factor for 2pF on the order of 105 was achieved by optimizing the radius and cone angle of the tip together with the distance between the tip and MoS2 film. The enhancement factor for 2pF is 40 times as much as that for one-photon-excited fluorescence because of the high-order nonlinear effect. Radiative and nonradiative decay rates were further calculated for determining quantum yields and elucidating the 2pF emission mechanism. These results provide a comprehensive understanding of 2pF behavior, which is highly useful for the design of MoS2-based optoelectronic devices.
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