Thermal Dynamics Induced by Surface Acoustic Waves in Lanthanide-Doped Core@Multishell Nanoparticles

Abstract

Accurate local temperature readout in integrated nanoscaled devices driven by high-frequency surface acoustic waves (SAWs) has a strong technological interest. The optical response of lanthanide (Ln)-doped NaLnF₄ core@multishell nanoparticles inserted into SAW delay lines is reported. The recombination dynamics between the ²H_11/2 and ⁴S_3/2 to the ⁴I_15/2 states of Er³⁺ is probed under SAWs modulation at 5, 225, and 290 K. It is shown that the spectral weight of the upconverted individual Stark levels of the ⁴S_3/2 band changes with the SAW and that their recombination lifetimes decrease as the SAW power is increased. By monitoring the luminescence intensity ratio (LIR), it is shown that such dynamics is consistent with a temperature increase and a carrier repopulation process across the ⁴S_3/2 Stark levels induced by Joule heating, which can reach up to 47 K at cryogenic temperatures. These effects, however, can be strongly suppressed by reducing the SAW duty cycle during the optical measurements. The results demonstrate that heating of nanoscopic systems integrated into SAW devices can be quite pronounced and that, since they do not interact with the SAW strain or piezoelectric field, these NaLnF₄ nanoparticles can be effectively employed as nanoscale in-situ and in-operando thermometers in such kind of application.