Normal State Optical Properties of CuO-Based Superconductors

We have studied the reflectivity of crystals and ceramics of La sub 2 CuO sub 4 as a function of frequency from ~3meV to ~3eV and as a function of Sr doping. In the insulating state we find peaks in sigma(omega) of electronic origin near .5eV and 2eV. Doping transfers oscillator strength from above 2eV to below 1eV and suggests that La sub 2 CuO sub 4 is a semiconductor with a 2 eV gap for charge excitations. We explain the discrepancy between results in metallic polycrystals and crystals in terms of the anisotropic optical propagation in these uniaxial materials.