The high thermal sensitivity of upconversion nanoparticles (UCNPs) luminescence at the nanoscale enables precise and rapid temperature detection, advancing nano-thermometry applications in quantum optics, catalytic reactions, and nanophotonics. Here, the phonon-assisted upconversion luminescence with a high thermal sensitivity is achieved by rare-earth doped UCNPs (${\mathrm{NaYF}}_4:{\mathrm{Yb}}^{3+}/{\mathrm{Er}}^{3+}$) in vacuum optical tweezers. We found that an essential contribution of phonon-assisted luminescence processes leading to high thermal sensitivity is not only from increasing the absorption cross section of ${\mathrm{Yb}}^{3+}$ but also from facilitating efficient energy transfer from ${\mathrm{Yb}}^{3+}$ to ${\mathrm{Er}}^{3+}$ ions. The increase in thermal sensitivity with the number of phonons involved in the progress of phonon-assisted luminescence can be well described by the temperature-dependent phonon population model. The study reveals how phonon-assisted processes influence thermal sensitivity in UCNPs, contributing to a deeper understanding of nanoscale temperature-dependent luminescence.

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• Light-matter interaction
• Crystal structures
• Nanoparticles
• Boltzmann theory
• Luminescence
• Optical levitation
• Optical spectroscopy
• Photoluminescence
• Thermometry