Femtosecond ellipsometric study of nonequilibrium carrier dynamics in Ge and epitaxial Si_1-xGe_x

The time-resolved, above-gap optical response of optically thick Si 1-xGex alloys to carrier injection by a femtosecond pump pulse is measured across the entire compositional range (0≤x≤1) using a novel femtosecond ellipsometric technique which clearly distinguishes the real and imaginary parts of the time-varying dielectric function ε 1(t)+iε2(t). The results are modeled microscopically in terms of the Drude contribution from a diffusing hot electron-hole plasma, augmented by transient-induced absorption from hot-carrier-induced band renormalization. Further corrections from thermal band-gap shrinkage, intervalley scattering, and transient interband absorption saturation are also discussed.