Influence of Pumping Regime on Temperature Resolution in Nanothermometry.

Jonas Thiem,Axel Ruehl,Detlev Ristau

doi:10.3390/nano11071782

Abstract

In recent years, optical nanothermometers have seen huge improvements in terms of precision as well as versatility, and several research efforts have been directed at adapting novel active materials or further optimizing the temperature sensitivity. The signal-to-noise ratio of the emission lines is commonly seen as the only limitation regarding high precision measurements. The role of re-absorption caused by a population of lower energy levels, however, has so far been neglected as a potential bottleneck for both high resolution and material selection. In this work, we conduct a study of the time dependent evolution of population densities in different luminescence nanothermometer classes under the commonly used pulsed excitation scheme. It is shown that the population of lower energy levels varies when the pump source fluctuates in terms of power and pulse duration. This leads to a significant degradation in temperature resolution, with limiting values of 0.5 K for common systems. Our study on the error margin indicates that either short pulsed or continuous excitation should be preferred for high precision measurements. Additionally, we derive conversion factors, enabling the re-calibration of currently available intensity ratio measurements to the steady state regime, thus facilitating the transition from pulse regimes to continuous excitation.

Highlights

Luminescence based nanothermometry has established itself as a versatile approach to measuring temperature with high resolution in both thermal and spatial dimensions
The fundamental quantity to measure in nanothermometry is the luminescence intensity ratio (FIR) of two emission lines
Temperature dependence of the FIR is ensured by choosing emission lines originating from thermally coupled energy levels, separated by

Summary

Introduction

Luminescence based nanothermometry has established itself as a versatile approach to measuring temperature with high resolution in both thermal and spatial dimensions. The pumping regime is mostly overlooked as a potential source of temperature errors it strongly affects the population densities of the lower energy levels during the measurement. A population of lower energy levels has to be considered as the origin of FIR deviations, even in the absence of temperature variations, causing measurement errors. We addressed this issue by utilizing time dependent laser rate equations. Since the two main factors determining the population density in luminescence applications are the pumping rate and the emission lifetime, we derived the corresponding relationships for a population of lower energy levels. We developed a formalism to unify pumping schemes and to maintain baseline measurements

Population Densities

Influence of Pump Fluctuations in Pulsed Regime

Calibration for Different Pump Regimes

Findings

Conclusions and Outlook