Near-infrared emitting NaLuF$_4$: Yb$^{3+}$, Er$^{3+}$, Ce$^{3+}$ nanoparticles for compact waveguide amplifiers and bioimaging

Fluoride nanocrystals co-doped with lanthanide ions are well known due to the possibility of conversion near-infrared (NIR) radiation into photoluminescence with large anti-Stokes shift. Owing to upconversion effect, such nanomaterials have shown great potential in photonics and biomedicine. However, fluoride nanoparticles can be reconfigured to photoluminescence with a Stokes shift into the near-infrared region of the spectrum. In this work, we focused on the properties of NaRF$_4$: Yb$^{3+}$, Er$^{3+}$, Ce$^{3+}$ (R = Y, Lu) nanoparticles exhibiting intense stokes luminescence in the vicinity of 1530 nm at 975 nm excitation. Photoluminescence quantum efficiency of synthesized nanoparticles was evaluated as 28% at 0.6 W/cm$^2$ excitation intensity. Based on the photoluminescent properties of nanoparticles we designed compact waveguide amplifier for C-band telecommunication and developed time gated imaging system for NIR-to-NIR biovisualization.