The Role of Optical Centers in Neodymium‐Doped Calcium Fluoride: Optical Tagging and Nanothermometry in the Second Biological Window

Abstract Nd ³⁺ has been proposed for nanothermometry in biological media, as both its emission and excitation wavelengths fall within the regions of low attenuation in tissues. While most crystal hosts only allow for thermometry through the low‐intensity emission near 900 nm, CaF ₂ enables intraband thermometry at the highest intensity emission band, ≈1042 nm. The incorporation of Y ³⁺ ions into CaF ₂ :Nd ³⁺ ,Y ³⁺ nanoparticles enhances the emission intensity of Nd ³⁺ . However, this also induces significant modifications in the spectral lineshape, which are detrimental to thermometry, as the required peaks needed to define a reliable thermometric ratio are eliminated at [Y ³⁺ ] > 10 mol%. In this study, we demostrate that this limitation depends on the excitation wavelength, as three different optical centers can be selectively activated by tuning it within a narrow range (<6 nm), with profound implications for the emission spectra. The fact that optical centers can be differentiated in nanoparticles even at room temperature can be exploited to recover the thermometric ability even with high [Y ³⁺ ] concentrations, thus providing nanothermometers with potentially better signal‐to‐noise ratios.