Estimation of motional parameters using emission from upconverting particles optically trapped at the pump wavelength

Lanthanide-doped upconverting particles of materials like NaYF4:Yb,Er have the ability to generate visible photons upon absorption of infra-red light. We trap a single such hexagonal shaped upconverting particles on the absorption resonance pump wavelength at 975 nm with linearly polarized light and find that these align with their long side along the tweezers beam. We then place a polarizer orthogonal to the input polarization in the backscattered direction and find that the spectra varies upon the orientation of the polarizer with respect to the side axis of the particle. We use this to ascertain the rotational Brownian motion in the yaw sense. We also use the visible emission to ascertain the axial Brownian motion of the particle without actually accessing the infra-red trapping light by the usage of a correctly sized aperture along the path of the backscatter. This is particularly useful in situations where the forward scattered light is inaccessible and only the fluorescence from the particle in the backscattered direction is accessible.