Infrared Permittivity of \(\alpha \) -MoO \({ }_{3}\) from Near- and Far-Field Correlative Studies

In this chapter, the accurate mid-infrared (mid-IR) dielectric function of alpha-phase molybdenum trioxide ( \(\alpha \text{-MoO}_{3}\) ) is reported by modeling far-field polarized mid-IR reflectance spectra acquired on a single thick flake of this material. The far-field model is refined by contrasting the experimental dispersion and damping of phonon polaritons (PhPs), revealed by polariton interferometry using scattering-type scanning near-field optical microscopy (s-SNOM) on thin flakes of \(\alpha \text{-MoO}_{3}\) , 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 \(\alpha \text{-MoO}_{3}\) are provided. The far-field experiments reported in this chapter were carried out in collaboration with Dr. Thomas G. Folland and Prof. Joshua D. Caldwell during a short stay of the candidate at Vanderbilt University, USA. The near-field experiments were performed in collaboration with Ana Isabel Fernández-Tresguerres Mata, while sample preparation was done by Javier Taboada Gutiérrez (University of Oviedo, Spain). DFT calculations were carried out by Prof. Ion Errea (University of the Basque Country, Spain). The work was published in “Infrared Permittivity of the Biaxial van der Waals Semiconductor \(\alpha \text{-MoO}_{3}\) from Near- and Far-Field Correlative Studies” by Gonzalo Álvarez-Pérez et al., in Advanced Materials 32 (29), 1908176 (2020). The final note on the far-IR dielectric function of \(\alpha \text{-MoO}_{3}\) was published in “Nanoscale-Confined Terahertz Polaritons in a van der Waals Crystal” by Thales V. A. G. de Oliveira et al., in Advanced Materials 33 (2), 2005777 (2021).