Graphite is one of the most important materials in our industrialized economy, but many of its fundamental properties are poorly understood. In particular, the optical properties are not even known in the visible part of the spectrum, due primarily to the unavailability of large single crystals of graphite. The best samples are highly oriented pyrolytic graphite (HOPG), which consist of microcrystals of graphite roughly oriented with the c-axis oriented perpendicular to the surface. In this paper, we present a technique to determine the complex optical functions of graphite using a combination of standard ellipsometry and normal-incidence generalized ellipsometry. Two ellipsometry measurements are made on oriented HOPG: 1) standard ellipsometry measurements are performed on cleaved HOPG with the optic axis perpendicular to the sample surface and 2) two-modulator generalized ellipsometry microscope (2-MGEM) measurements are made on HOPG cut and polished such that the optic axis is parallel to the sample surface. The measurements are performed at 9 different wavelengths from 405 to 750 nm and determine the optical functions of graphite for light polarized perpendicular and parallel to the optic axis (ordinary and extraordinary). The measurements show that the optical functions for the two different polarizations are quite different: the ordinary optical functions show evidence of band-to-band transitions throughout the visible and a Drude-like behavior in the red part of the spectrum. In contrast, the extraordinary dielectric functions do not show any Drude-like behavior, but do show band-to-band transitions throughout the observed spectrum. The 2-MGEM measurements also generate images of optical polarization parameters such as diattenuation, retardation, and the direction of the principal axis. Research sponsored by Division of Materials Sciences and Engineering and the DOE Office of Nuclear Energy, Science and Technology’s Advanced Gas Reactor program.

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