Energy Transfer Phenomenon Leading To Photon Up-conversion With Application To Optical Thermometry

Temperature is a fundamental property of matter and it is related to the degree of warmth of a material. Temperature is measured by a thermometer, which is a device that must contain some temperature dependent element in order to estimate the temperature, as for example the volume of liquid mercury in a glass tube.

It may be used to assess the operation condition of electrical, mechanical, chemical, and biological systems. Among various types of thermometers or temperature sensors, the ones based on some optical response are particularly attractive because they allow remote, real-time, and large-scale reading of the temperature.

Fiber optics thermometers, for example, are advantageous because they have non-toxic, non-conductive, non-corrosive elements. One class of fiber optics thermometer is based on the change of fluorescence spectral profile with temperature and in this category, we find lanthanide-based optical thermometers. Lanthanides are particularly interesting because they may produce fluorescence with a broad spectral coverage (spanning from the UV to the near-infrared) when incorporated in convenient solid-state hosts.

This characteristic allowed lanthanide-doped materials (LDMATs) to become popular phosphors for use in lighting and display technology. Concerning application in optical thermometry, LDMATs in fiber optics form have been considered the top choice for environments where contact thermometers cannot be used such as electric power plants and refineries. More recently, optical thermometry using LDMATs in nanoparticle form has been successfully employed in studies using rats to demonstrate its potential use for photothermal intratumoral therapy where real-time temperature regulation is crucial.

Holmium is a lanthanide that when it is incorporated into a crystalline network may generate fluorescence at the visible spectral range due to transitions inside the 4f electronic shell. Fluorescence at the visible is generally produced in holmium doped materials by exposing the sample to near-infrared light, a process known as energy up-conversion. We investigated the up-conversion fluorescence properties of holmium doped in calcium fluoride powders and we observed that the apparent color of its fluorescence changed from green to yellow when the temperature of the sample was raised above room temperature.

The change in color occurs because the relative intensity between two fluorescence lines of holmium peaked at the green and red spectral regions changes with the temperature. We developed a rate equation model in which the color change of the fluorescence with temperature could be explained based on the dynamics of the electronic populations of holmium.

The temperature sensitivity of this kind of thermometer was estimated by analyzing the rate in which the fluorescence intensity ratio between the green and red emission lines changed with the temperature. This is a reliable way to perform real-time readout of temperature because it is independent of fluctuations of the excitation source and detection. This method is also inexpensive as a simple two-channel signal division electronic circuitry can be employed for signal processing.

These findings are described in the article entitled A study of energy transfer phenomenon leading to photon up-conversion in Ho³⁺:Yb³⁺:CaF₂ crystalline powders and its temperature sensing properties, recently published in the journal Current Applied Physics. This work was conducted by Glauco S. Maciel from Universidade Federal Fluminense.