Unusual Thermally Induced Blueshift and Emission Amplification of Mn2+ ions Enable Filter-Free Luminescent Thermal Imaging
The shift from point-based thermal sensing to filter-free thermal imaging requires luminescent thermometers that exhibit pronounced and thermally driven spectral changes within spectral regions matching the sensitivity profiles of the R, G, and B channels of a digital camera. In this work, we introduce such a system, enabled by the synergistic interplay between (i) thermal redistribution among the vibronic components of the 4T1 excited state of Mn2+ ions and (ii) thermally assisted population of this state via optical trap sites. These combined processes result in a simultaneous thermal enhancement and blueshift of the Mn2+ emission band associated with the 4T1 -> 6A1 electronic transition. Consequently, the emission intensity recorded in the G channel increases with temperature, while the luminescence signals detected in the B channel exhibit a corresponding decrease. As demonstrated, Ca19Zn2(PO4)14:Mn2+, Ce3+ supports not only sensitive filter-free thermal imaging, but also two additional ratiometric readout schemes: one based on the intensity ratio of Ce3+ and Mn2+ emissions, and another based on two distinct spectral regions of the Mn2+ emission band, yielding maximum relative sensitivities of 0.42% K^-1 and 2.7% K^-1, respectively. This approach introduces a unique thermometric strategy that enables simple, robust, and cost-effective two-dimensional thermal imaging without the need for optical filters or specialized instrumentation.