Absolute primary thermometry using individual Stark sublevels in rare-earth-doped nanocrystals

Absolute primary thermometry enables direct temperature measurement from first principles without calibration, but existing methods are bulky and unsuitable for microscale systems. Rare-earth-doped nanocrystals offer a promising alternative, as thermally coupled Stark sublevels follow Boltzmann statistics, enabling intrinsic temperature readout with possible nanoscale resolution. Here, we demonstrate two independent experimental strategies for absolute thermometry using Er³⁺-doped upconverting nanoparticles. Both methods rely on luminescence intensity ratios (LIR) between Stark-resolved transitions in the ⁴S_3/2 and ⁴I_13/2 manifolds. The first approach uses the high-temperature asymptotic regime to extract radiative prefactors, while the second identifies the inflection point of the LIR response, determined solely by the Stark energy splitting. Both strategies eliminate the need for external calibration. The results achieve sub-kelvin uncertainty, establishing a practical route toward nanoscale primary thermometry for probing heat transport in complex environments.