Our group (Information shared by  Hadi Maghsoudiganjeh) discovered photon upconversion in nanoparticles for the first time. Here is the link of two of our papers:

• Hybrid Molecule–Nanocrystal Photon Upconversion Across the Visible and Near-Infrared

• Triplet Energy Transfer from PbS(Se) Nanocrystals to Rubrene: the Relationship between the Upconversion Quantum Yield and Size

Advanced Functional Materials 2016