Theory of optical absorption in diamond, Si, Ge, and GaAs

We report first-principles calculations of ${\epsilon }_2\left(\omega \right)$ for diamond, Si, Ge, and GaAs that include the electron-hole interaction in detail. The only essential experimental input is the crystal structure of the materials. Comparison with reflectivity and ellipsometry measurements allows us to assess the validity of the approximations made in the calculation. We conclude that the approximation of singly excited electronic states and a statically screened electron-hole interaction is sufficient to understand the main features of the absorption spectra of these materials in the visible to near ultraviolet.