Upconversion photoluminescence by charge transfer in a van der Waals trilayer

As an elementary process of light-matter interaction in solids, upconversion photoluminescence has been extensively studied in rare-earth-doped materials and found applications in biological imaging, infrared light detection, and laser cooling. More recently, it has been shown that upconversion photoluminescence can be achieved in two-dimensional semiconductors by utilizing the strong coupling between charge carriers. Here, we show that the interlayer charge transfer, which has been widely observed in van der Waals heterostructures, can be utilized for upconversion photoluminescence. Using a MoSe2/WS2/MoS2 trilayer as an example, we show that by exciting the MoSe2 and MoS2 layers with a low-energy 670-nm laser beam, photoluminescence of 620 nm can be obtained. The upconversion photoluminescence originates from the transfer of electrons and holes from MoSe2 and MoS2, respectively, to the middle WS2 layer, where they recombine. The results illustrate an unexplored physical mechanism for upconversion photoluminescence in solids and introduce van der Waals heterostructures as materials to achieve upconversion photoluminescence.As an elementary process of light-matter interaction in solids, upconversion photoluminescence has been extensively studied in rare-earth-doped materials and found applications in biological imaging, infrared light detection, and laser cooling. More recently, it has been shown that upconversion photoluminescence can be achieved in two-dimensional semiconductors by utilizing the strong coupling between charge carriers. Here, we show that the interlayer charge transfer, which has been widely observed in van der Waals heterostructures, can be utilized for upconversion photoluminescence. Using a MoSe2/WS2/MoS2 trilayer as an example, we show that by exciting the MoSe2 and MoS2 layers with a low-energy 670-nm laser beam, photoluminescence of 620 nm can be obtained. The upconversion photoluminescence originates from the transfer of electrons and holes from MoSe2 and MoS2, respectively, to the middle WS2 layer, where they recombine. The results illustrate an unexplored physical mechanism for upconversion photolu...