Measurement of optical functions of highly oriented pyrolytic graphite in the visible

The spectroscopic dielectric functions of highly oriented pyrolytic graphite (HOPG) are determined at nine different wavelengths from 405 to 750 nm (3.06-1.65 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.