Localized phonon polariton resonances in CaF2 nanoparticles

Confining infrared light at the nanoscale with low losses is a central challenge in nanophotonics. Surface phonon polaritons in polar dielectrics offer a compelling solution, yet the available materials palette and fabrication routes remain largely limited to top-down lithography. Here, we demonstrate localized phonon-polariton resonances in colloidal calcium fluoride nanoparticles—to our knowledge, the first such observation in a chemically synthesized alkaline-earth fluoride—in the 8–14 THz range. We introduce an analytical model that quantitatively reproduces the far-infrared absorption spectra without requiring full electrodynamic simulations and use it to extract nanoparticle densities in good agreement with those derived from the nanoparticle geometry. Our results establish colloidal synthesis as a viable, scalable route to phonon-polaritonic resonators, broadening both the materials toolbox and the fabrication strategies available for far-infrared nanophotonics.