Giant sensitivity of an optical nanothermometer based on parametric and non-parametric emissions from Tm3+ doped NaNbO3 nanocrystals

In the present work, we exploited the capability of the trivalent thulium (Tm3+) ions doped sodium niobate (NaNbO3) nanocrystals as an optical nanothermometer with a giant relative thermal sensitivity (Sr) of up to 12%°C−1. Tm3+:NaNbO3 nanoparticles, under excitation at 1064 nm using a Q-switched and mode-locked Nd:YAG laser, provided emissions at multi-wavelengths from 460 nm to 840 nm through non-parametric transitions in Tm3+ ions, and parametric second-harmonic generation (SHG) at 532 nm (I532) of the excitation beam thanks to the large nonlinear response of the crystalline matrix. The proposed nanothermometer is based on the intensity ratio between the upconverted fluorescence at 695 nm (I695), associated with the 3F2,3 → 3H6 Tm3+ transitions, and I532. Due to the opposite behaviors of these two emissions with the temperature in the range of 20–80 °C, a super sensitive nanothermometer was demonstrated with a record of Sr ~12% oC−1 around 40–50 °C (the biological temperature regime). In addition, an efficient pump-induced self-heating system was reported and assigned to multiphonon relaxations in Tm3+ ions. These properties make this dual emitting system a nanoprobe for many potential applications, e.g., for studying dynamical biological and pump-induced self-monitored photothermal events. The combination of SHG in a wide spectral range (e.g., in complete first biological window) and emissions of lanthanide ions greatly increases the range of applications using this system, including as a multifunctional nanoprobe.