On the resonance-enhanced optical trapping of upconversion nanoparticles

Resonance-enhanced optical trapping amplifies optical forces at the nanoscale. However, the extent of the observed effect varies greatly, the reasons remaining unclear. Here, we investigated the resonance-enhancement by trapping ytterbium-doped NaYF₄ upconversion nanoparticles using a dual-wavelength optical tweezers setup operating at 976 nm (near-resonant) and 1064 nm (off-resonant). Combining power-spectral-density analysis and active drag-force calibration, we measured the trap stiffness and hydrodynamic particle size of each trapped particle at both wavelengths. Measured upconversion nanoparticle sizes ranged from the size of single particles to larger-sized clusters that we also observed by electron microscopy. Near-resonant trapping increased the trap stiffness about twofold for all particle sizes compared to off-resonant conditions. However, larger particles were trapped with a higher trap stiffness under both near- and off-resonant conditions. This increase with size suggests that trap stiffness measurements under near-resonant conditions without direct size measurements may overestimate the enhancement. Our results demonstrate that resonance effects enhance optical trapping, while emphasizing the critical roles of trapping power, particle size, and clustering in the quantitative evaluation of trapping enhancement.