Near-infrared (NIR) light sensitive lanthanide-doped NaYF4 upconversion particles (UCPs) are gaining increasing attention as local probes in biomedical applications. Here, we implemented a photonic force microscope (PFM) to manipulate and study the optical properties of trapped single UCPs, beta-NaYF4:Yb,Er. In particular, we focused on the mechanisms of the optical trapping of nonspherical UCPs of different sizes, in the range 0.5-2 mu m, as well as on their upconversion photoluminescence (UCL) properties under excitation with the strongly focused laser beam (lambda = 1064 nm) of the PFM, operating at power densities up to 14.7 MW cm(-2). A careful analysis of UCL under such conditions points to three emission peaks at 469, 503.6, and 616.1 nm, which were enhanced by the high laser power density. The analysis of Brownian motion was used to quantify the thermal fluctuations of the particle inside the optical trap as well as the particle sizes and optical forces acting in the two dimensions perpendicular to the optical axis. A steep dependence of UCL as a function of the particle diameter was found for UCPs having sizes smaller than the focal spot (similar to 900 nm) of the NIR laser.