39849 2017 eng 4283 4294 12 9 article The Royal Society of Chemistry 1 -- -- -- The rational design of brighter upconversion nanoparticles (UCNPs) requires a better understanding of the radiationless deactivation pathways in these materials. Here, we demonstrate the potential of excitation power density (P)-dependent studies of upconversion (UC) luminescence intensities, slope factors, and absolute quantum yields (ΦUC) of popular β-NaYF4:20% Yb3+,2% Er3+ UCNPs of different surface chemistries in organic solvents, D2O, and water as a tool to gain deeper insight into the UC mechanism including population and deactivation pathways particularly of the red emission. Our measurements, covering a P regime of three orders of magnitude, reveal a strong difference of the P-dependence of the ratio of the green and red luminescence bands (Ig/r) in water and organic solvents and P-dependent population pathways of the different emissive energy levels of Er3+. In summary, we provide experimental evidence for three photon processes in UCNPs, particularly for the red emission. Moreover, we demonstrate changes in the excited population dynamics via bi- and triphotonic processes dependent on the environment, surface chemistry, and P, and validate our findings theoretically Nanoscale 10.1039/c7nr00092h http://pubs.rsc.org/en/content/articlepdf/2017/nr/c7nr00092h 2040-3364 2040-3372 18.04.2017 Christian Würth Martin Kaiser Stefan Wilhelm Bettina Grauel Th. Hirsch Ute Resch-Genger eng uncontrolled Upconverion eng uncontrolled Quantum Yield eng uncontrolled Photo physics Ingenieurwissenschaften und zugeordnete Tätigkeiten Verlagsliteratur Datei im Netzwerk der BAM verfügbar ("Closed Access")