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The spectroscopic dielectric functions of highly oriented pyrolytic graphite (HOPG) are determined at nine different wavelengths from 4050. 3em{0ex}to0. 3em{0ex}7500. 3em{0ex}nm (3. 06--1. 650. 3em{0ex}eV). This determination is made on the basis of two ellipsometry measurements: (1) Standard ellipsometry measurements are performed on HOPG with the c axis perpendicular to the sample surface, and (2) two-modulator generalized ellipsometry microscope (2-MGEM) measurements are performed on HOPG cut and polished such that the c axis is parallel to the sample surface. Both the ordinary and extraordinary complex dielectric functions show nonzero absorption throughout the observed spectral range, while the ordinary dielectric function shows Drude-like behavior at longer wavelengths. From this, it can be concluded that graphite is metallic for visible light polarized parallel to the graphene planes, but acts more as a semiconductor or semimetal for visible light polarized perpendicular to the graphene planes. The 2-MGEM technique can also be used to generate images of the diattenuation, retardation, and direction of the principal axis.
Jellison et al. (2007) studied this question.