We present spectral- and time-resolved reflectivity and transmission measurements on hydrogenated amorphous silicon, a-Si:H, with 50 fs time resolution. Electron-hole pairs are photoexcited into the extended states of a-Si:H by an ultrashort pump pulse at a photon energy of 2 eV. The optical response of photogenerated carriers is studied on an ultrashort time scale, Δt∼100 fs, after the excitation process where recombination and trapping can be neglected. The spectral dependence of the optical response is analyzed in terms of a hopping model for the transport of the photogenerated carriers in the extended states of a-Si:H.

• Received 1 June 1992

DOI:https://doi.org/10.1103/PhysRevB.47.3593