Anisotropic Dielectric Function of Graphite Probed by Far- and Near-Field Spectroscopies

Graphite is a cornerstone material for revolutionary technologies, from energy storage to the entire field of two-dimensional materials. Despite its foundational role, the predictive power required for engineering emergent optical behavior in van der Waals heterostructures is severely constrained by persistent discrepancies in reported optical constants. We resolve this long-standing ambiguity by deploying a multi-modal approach that synergizes far-field spectroscopic ellipsometry with nanoscale near-field optical probing (s-SNOM) and micro-reflectance spectroscopy. We have established a new, self-consistent set of optical constants (n and k) for both in-plane and out-of-plane crystallographic directions across the ultraviolet-to-near-infrared spectrum. This work presents a unified set of optical constants that addresses inconsistencies in the existing literature. By establishing this definitive reference, we provide the essential foundation for the quantitative modeling and engineering of light-matter interactions in the evolving landscape of carbon-based nanophotonics.