Study of absorption and emission dipoles of upconverting nanoparticles optically trapped at the absorption resonance

Rare-earth doped upconverting nanoparticles like NaYF4:Yb,Er absorb infra-red light and emit visible radiation. We use light at the absorption resonance, namely 975 nm, to optically trap the nanoparticles and subsequently study the effect on the trapping parameters like the trap stiffness. We trap hexagonal shaped upconverting nanoparticles and find that these align with their side along the tweezers beam. We also place a polarizers to study the backscatter emission features and find that the emission spectra vary depending upon the orientation of the polarizer with respect to the side axis of the particle. We use this to find the rotational Brownian motion along the yaw sense. We also use the fluorescent emission to ascertain the axial Brownian motion of the particle. This can be used to ascertain the motion of the particle while trapped on resonance in tweezers without actually accessing the infra-red trapping light.