Lanthanide (Ln)-doped nanoparticles have shown potential for applications in various fields. However, the weak and narrow absorption bands of the Ln ions (${\mathrm{Ln}}^3$), hamper efficient optical pumping and severely limit the emission intensity. Dye sensitization is a promising way to boost the near-infrared (NIR) emission of ${\mathrm{Er}}^3$, hence promoting possible application in optical amplification at 1.5 μm, a region that is much sought after for telecommunication technology. Herein, we introduce the fluorescein isothiocyanate (FITC) organic dye with large absorption cross section as energy donor of small-sized ($\sim 3.6\mathrm{nm}$) ${\mathrm{Er}}^3$-doped ${\mathrm{CaF}}_2$ nanoparticles. FITC molecules on the surface of ${\mathrm{CaF}}_2$ work as antennas to efficiently absorb light, and provide the indirect sensitization of ${\mathrm{Er}}^3$ boosting its emission. In this paper, we employ photoluminescence and transient absorption spectroscopy, as well as density functional theory calculations, to provide an in-depth investigation of the $\mathrm{FITC}\to {\mathrm{Er}}^3$ energy transfer process. We show that an energy transfer efficiency of over 89% is achieved in ${\mathrm{CaF}}_2\text{:}{\mathrm{Er}}^3@\mathrm{FITC}$ nanoparticles resulting in a 28 times enhancement of the ${\mathrm{Er}}^3$ NIR emission with respect to bare ${\mathrm{CaF}}_2\text{:}{\mathrm{Er}}^3$. Through the multidisciplinary approach used in our work, we are able to show that the reason for such high sensitization efficiency stems from the suitable size and geometry of the FITC dye with a localized transition dipole moment at a short distance from the surface of the nanoparticle.