Infrared Dielectric Anisotropy in Rhenium Diselenide (ReSe₂), an Excitonic van der Waals Material

Polaritons, hybrid light-matter excitations that arise from the strong coupling between photons and various excitations in materials, such as excitons, plasmons, and phonons, have garnered significant attention in the field of nanophotonics. Recent studies have demonstrated that van der Waals materials with monoclinic and triclinic crystal structures, such as β-Ga₂O₃ (bGO), can support a new type of polariton called hyperbolic shear polariton with anisotropic propagation along the material surface [1]. These low-symmetry materials show promise for applications in directional light propagation, polarization and phase control of light, and polarization-sensitive photodetectors.<br/><br/>In this study, we investigate Rhenium diselenide (ReSe₂), a two-dimensional (2D) van der Waals material with reduced in-plane symmetry, using infrared spectroscopy to determine its optical constants. ReSe₂, a member of the group VII transition metal dichalcogenides (TMDCs), exhibits distinct anisotropic characteristics that set it apart from more commonly studied hexagonal TMDCs like MoS₂ or WS₂. Previous literature has reported polarization-sensitive photoluminescence [2] of ReSe₂ in the infrared, arising from excitonic oscillators, which aligns with our findings.<br/><br/>Our research employs temperature dependent polarized FTIR (Fourier Transform Infrared) microscopy and numerical analysis to probe the birefringence and excitonic behavior in ReSe₂. This technique, like spectroscopic ellipsometry, allows us to extract in-plane anisotropy in the dielectric function due to the excitonic oscillators observed in the near-infrared region (850nm-950nm). We utilize the transfer matrix method [3] to computationally fit the reflectivity calculated from a dielectric function model comprising directional Lorentzian oscillators to the experimental data, enabling us to optimize the optical constants. Our results show that the non-orthogonal oscillators give rise to a non-diagonalizable dielectric tensor, influencing the optical response in the infrared spectrum. Understanding these optical parameters is crucial for modeling polaritons in ReSe₂ and exploring its potential for hosting novel types of polaritons. Our work contributes to the growing knowledge of anisotropic 2D materials and their prospective applications in next-generation optoelectronic devices.<br/><br/>1. Passler, N.C., et al., <i>Hyperbolic shear polaritons in low-symmetry crystals.</i> Nature, 2022. <b>602</b>(7898): p. 595-600.<br/>2. Arora, A., et al., <i>Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T′-ReSe2</i>. 2017, American Chemical Society. p. 3202-3207.<br/>3. Passler, N.C. and A. Paarmann, <i>Generalized 4 × 4 matrix formalism for light propagation in anisotropic stratified media: study of surface phonon polaritons in polar dielectric heterostructures.</i> Journal of the Optical Society of America B, 2017. <b>34</b>(10): p. 2128.