Infrared permittivity of the biaxial van der Waals semiconductor α‐MoO3 from near‐ and far‐field correlative studies

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—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, the accurate IR dielectric function of α‐MoO3 is reported by modeling far‐field polarized IR reflectance spectra acquired on a single thick flake of this material. Unique to this 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 this model, while offering a novel approach to extracting dielectric functions of nanomaterials. In addition, by employing density functional theory (DFT), insights into the various vibrational states dictating the dielectric function model and the intriguing optical properties of α‐MoO3 are provided.G.Á-P. and J.T.-G. acknowledge support through the Severo Ochoa Program from the I of the Principality of Asturias (grants No. PA-20-PF-BP19-053 and PA-18-PF-BP17-126, respectively). I.E. acknowledges support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P). J.M.-S. acknowledges support through a Clarín Marie Curie-COFUND grant from the Government of the Principality of Asturias and the EU (PA18-ACB17-29), and the Ramón y Cajal Program (RYC2018-026196-I) from the Government of Spain. Q. B. acknowledges support from Australian Research Council (ARC, FT150100450, IH150100006 and CE170100039). J.D.C. was supported by the National Science Foundation (U.S.A.) under grant number U0048926. A.Y.N. acknowledges the Spanish Ministry of Science, Innovation and Universities (national project MAT2017-88358-C3-3-R) and Basque Government (grant No. IT1164-19). P.A-G. acknowledges support from the European Research Council under Starting Grant 715496, 2DNANOPTICA.Peer reviewe