Pressure dependence of the direct optical gap and refractive index of Ge and GaAs

The effect of pressure on the direct optical gap (E 0 ) and its spin-orbit split counterpart (E 0 + Delta 0 ) of Ge and GaAs has been investigated by optical absorption measurements of thin (2-10 mu m) samples at pressures up to 10 and 17 GPa, respectively. The refractive index n( omega ) of both materials has been determined at energies below the E 0 gap from the interface pattern of the transmitted light intensity. Extrapolation of the frequency dependence of n( omega ) to zero energy yields the static electronic dielectric constant in infinity as a function of pressure. The present results are discussed in the light of related experimental and theoretical studies.