Optical thermometry and non-radiative transitions

Luminescence is a temperature-dependent phenomenon. With the right choice of signal and appropriate calibration, the luminescence of a phosphor can therefore also be used indirectly as a measure of temperature. This possibility of non-destructive temperature measurement from a distance now allows catalytic or electronic processes to be analyzed with lateral resolution on the micrometer scale. Our group has developed the first theoretical models for the operation and design of such luminescent thermometers and is constantly testing the limits of this model. For this purpose, we frequently use trivalent lanthanides as activators, since their intrinsic narrow-band luminescence allows the resolution of energy gaps ΔE in the order of desired thermal energies kBT due to the intraconfigurational 4fⁿ-4fⁿ transitions. However, we also investigate rare-earth-free alternatives and systematically work out advantages and disadvantages of different classes of emitters and luminescent thermometers.

The dynamic operating range of a luminescent thermometer depends largely on the ratio of the radiative and non-radiative transition rates. Therefore, the limit of luminescent thermometers at sufficiently low temperatures contains very important information about the magnitude of non-radiative coupling constants. We exploit this property to gain a more fundamental understanding of non-radiative transitions. While radiative transitions under photon emission have been widely studied and are well understood through photonic effects and quantum electrodynamics, nonradiative transitions have long been considered perturbative. For this reason, far fewer design possibilities for non-radiative transitions are known besides the historically established energy gap law. Such an understanding could open up new possibilities for e.g. coherent THz sources, but is also very interesting from a theoretical point of view and may allow to consider radiative and non-radiative transitions as different realizations of a common quantum field theory.