First-Principles Insights into Phonon Polaritons and Dielectric Response in Polar Materials

Phonon polaritons in polar crystals enable deep-subwavelength light confinement and are a key component within IR nanophotonics. The pertinent theory is derived from Maxwell’s equations, with the IR dielectric tensor ε(ω) being the only component that can be informed by quantum theory. Quantitative analysis of experimental data has typically relied on ε(ω) “TOLO” models fitted to optical data. Here, we use DFT calculations of phonons, resulting ε(ω) tensors, and Reststrahlen bands (RBs) to gain insights into the TOLO form of ε(ω) and explore the relation of rotated TOLO ε(ω) tensors for polar-angle probes and the corresponding RBs to phonon-derived properties. We present results for the cubic material NaCl as a reference point and the anisotropic layered materials h-BN, HfS₂ HfSe₂. In the case of HfS₂ and HfSe₂, DFT-calculated out-of-plane dielectric tensors and RBs are at variance with dielectric tensors extracted from IR-reflectance data, while the G-point TO, LO, and Raman-active phonon frequencies are in good agreement with experimental data obtained by vibrational electron-energy-loss and Raman spectroscopies, respectively. Recent experimental phonon-polariton data that are in accord with the empirical dielectric tensors are an open issue for further investigations.