Nanoparticle

Optical tweezers

Materials science

Refractive index

Optical force

Molecule

Nanophotonics

Physics

Nanotechnology

Organic chemistry

Dielectric

Optics

Nanomaterials

Chemistry

Lanthanide

Polarizability

Optoelectronics

Tweezers

Permittivity

Nature Nanotechnology

Optical tweezers are widely used in materials assembly1, characterization2, biomechanical force sensing3,4 and the in vivo manipulation of cells5 and organs6. 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 μm–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 cooling7 capabilities. The resonance of highly doping lanthanide ions in NaYF4 nanocrystals enhances the permittivity and polarizability of nanocrystals, leading to enhanced optical trapping forces by orders of magnitude, bypassing the trapping requirement of refractive index mismatch.

Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles