Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles

Optical tweezers are widely used in materials assembly(1), characterization(2), biomechanical force sensing(3,4) and the in vivo manipulation of cells(5) and organs(6). The trapping force has primarily been generated through the refractive index mismatch between a trapped object and its surrounding medium. This poses a fundamental challenge for the optical trapping of low-refractive-index nanoscale objects, including nanoparticles and intracellular organelles. Here, we report a technology that employs a resonance effect to enhance the permittivity and polarizability of nanocrystals, leading to enhanced optical trapping forces by orders of magnitude. This effectively bypasses the requirement of refractive index mismatch at the nanoscale. We show that under resonance conditions, highly doping lanthanide ions in NaYF4 nanocrystals makes the real part of the Clausius-Mossotti factor approach its asymptotic limit, thereby achieving a maximum optical trap stiffness of 0.086 pN mu m(-1) mW(-1) for 23.3-nm-radius low-refractive-index (1.46) nanoparticles, that is, more than 30 times stronger than the reported value for gold nanoparticles of the same size. Our results suggest a new potential of lanthanide doping for the optical control of the refractive index of nanomaterials, developing the optical force tag for the intracellular manipulation of organelles and integrating optical tweezers with temperature sensing and laser cooling(7) capabilities.

Automated summary

Optical tweezers are widely used in various fields, including materials assembly, characterization, biomechanical force sensing, and cell and organ manipulation. A new technology using a resonance effect to enhance the permittivity and polarizability of nanocrystals has been developed, allowing for significantly stronger optical trapping forces for low-refractive-index nanoparticles. The use of lanthanide doping shows great potential in controlling the refractive index of nanomaterials for intracellular manipulation of organelles and integration with other optical technologies.