The biaxial van der Waals semiconductor α-phase molybdenum trioxide
(α-MoO3) has recently received significant attention due to its
ability to support highly anisotropic phonon polaritons (PhPs) -infrared (IR)
light coupled to lattice vibrations in polar materials-, offering an
unprecedented platform for controlling the flow of energy at the nanoscale.
However, to fully exploit the extraordinary IR response of this material, an
accurate dielectric function is required. Here, we report the accurate IR
dielectric function of α-MoO3 by modelling far-field, polarized IR
reflectance spectra acquired on a single thick flake of this material. Unique
to our work, the far-field model is refined by contrasting the experimental
dispersion and damping of PhPs, revealed by polariton interferometry using
scattering-type scanning near-field optical microscopy (s-SNOM) on thin flakes
of α-MoO3, with analytical and transfer-matrix calculations, as well
as full-wave simulations. Through these correlative efforts, exceptional
quantitative agreement is attained to both far- and near-field properties for
multiple flakes, thus providing strong verification of the accuracy of our
model, while offering a novel approach to extracting dielectric functions of
nanomaterials, usually too small or inhomogeneous for establishing accurate
models only from standard far-field methods. In addition, by employing density
functional theory (DFT), we provide insights into the various vibrational
states dictating our dielectric function model and the intriguing optical
properties of α-MoO3