Experimental data for the anisotropy of the dielectric tensor epsilon = epsilon 1+i epsilon 2 in YBa2Cu3O6 are presented and interpreted. The spectra in the range between 1.7 and 5.2 eV were obtained by rotating-analyzer ellipsometry using a single crystal. It is found that the component of epsilon 2 in the ab plane ( epsilon 2a) displays a sharp peak at 4.1 eV, while the component along the c axis ( epsilon 2c) has a less pronounced peak at 3.8 eV, a small peak at 4.2 eV, and a broad maximum near 5 eV. Similar structures were found in the interband spectra calculated using the local-density approximation. The 4-eV optical structure is caused by nearly parallel initial- and final-state bands whose wave functions are located, respectively in the O-Cu-O dumbbells and in the BaO-Cu-OBa triple layers. Intraionic 3d to 4p transitions in dumbbell Cu contribute 90\% of the optical matrix elements.
 

Physical Review B, 40 7368-71, 1989.