Ion resonance optical tweezers and its applications

Optical tweezers face challenges in trapping low-refractive-index nanoparticles and detecting weak forces. Here, we introduce lanthanide-doped upconversion nanoparticles (UCNPs) that leverage nonlinear photoresponse and ion resonance to amplify optical forces, bypassing refractive-index limitations. Resonance-enhanced trapping of NaYF4 nanocrystals yields a record stiffness of 0.086 pN μm⁻¹ mW⁻¹—30× stronger than gold nanoparticles of similar size—enabling nanoscale manipulation and integration with temperature sensing. For ultrasensitive force detection, we developed a super-resolved photonic force microscope combining UCNPs with neural-network-enhanced 3D tracking. This achieves thermal-limit sensitivity (1.8 fN Hz⁻¹/₂) and aN resolution, probing electrophoresis and surface forces at the single-particle level. Additionally, a multilayer microprobe attains 14.24 pN μm⁻¹ mW⁻¹ trap stiffness, tripling prior benchmarks. It measures mitotic HeLa cell elasticity, demonstrating nanonewton biomechanical sensing. These advances bridge material innovation and machine learning, enabling unprecedented studies of nanoscale forces, cellular mechanics, and sub-fN interactions.