The main goal of this study was creating multifunctional nanoparticles based on rare-earth doped LaF₃ nanocrystals, which can be used as fluorescence thermal sensors operating over the 80–320 K temperature range including physiological temperature range (10–50°C). The Pr³⁺:LaF₃ (CPr = 1%) microcrystalline powder and the Pr³⁺:LaF₃ (CPr = 12%, 20%) nanoparticles were studied. It was proved that all the samples were capable of thermal sensing into the temperature range from 80 to 320 K. It was revealed that the mechanisms of temperature sensitivity for the microcrystalline powder and the nanoparticles are different. In the powder, the ³P₁ and ³P₀ states of Pr³⁺ ion share their electronic populations according to the Boltzmann and thermalization of the ³P₁ state takes place. In the nanoparticles, two temperature dependent mechanisms were suggested: energy migration within ³P₀ state in the temperature range from 80 K to 200 K followed by quenching of ³P₀ state by OH groups at higher temperatures. The values of the relative sensitivities for the Pr³⁺:LaF₃ (CPr = 1%) microcrystalline powder and the Pr³⁺:LaF₃ (CPr = 12%, 20%) nanoparticles into the physiological temperature range (at 45°C) were 1, 0.5, and 0.3% °C⁻¹, respectively.

The Pr³⁺:LaF₃ (C_Pr = 3, 7, 12, 20, 30%) nanoparticles were characterized by means of high-resolution transmission electron microscopy, X-ray diffraction, optical spectroscopy, energy dispersive X-ray spectroscopy, dynamic light scattering, and MTT assay. It was revealed that the average diameter of all the NPs is around 14–18 nm. The hydrodynamic radius of the Pr³⁺:LaF₃ (C_Pr = 7%) nanoparticles strongly depends on the medium. It was revealed that hydrodynamic radii of the Pr³⁺:LaF₃ (C_Pr = 7%) nanoparticles in water, DMEM, and RPMI-1640 biological mediums were 18 ± 5, 41 ± 6, and 186 ± 8 nm, respectively. The Pr³⁺:LaF₃ (C_Pr = 7%) nanoparticles were nontoxic at micromolar concentrations toward COLO-320 cell line. The lifetime curves were fitted biexponentially, and for the Pr³⁺:LaF₃ (C_Pr = 7%) NPs, the luminescence lifetimes of Pr³⁺ ions were 480 ± 2 and 53 ± 5 nanosec.

We present optical vector network analysis (OVNA) of an isotopically purified ${{}^{166}\mathrm{Er}}^{3+}\text{:}{}^7{\mathrm{LiYF}}_4$ crystal. The OVNA method is based on generation and detection of a modulated optical sideband by using a radio-frequency vector network analyzer. This technique is widely used in the field of microwave photonics for the characterization of optical responses of optical devices such as filters and high-Q resonators. However, dense solid-state atomic ensembles induce a large phase shift on one of the optical sidebands that results in the appearance of extra features on the measured transmission response. We present a simple theoretical model that accurately describes the observed spectra and helps to reconstruct the absorption profile of a solid-state atomic ensemble as well as corresponding change of the refractive index in the vicinity of atomic resonances.