We report on femtosecond time-resolved transmission and reflectivity measurements on bulk GaAs. Spectral-hole burning is observed, to our knowledge, for the first time in GaAs at room temperature. Carrier thermalization occurs within 200 fs and shows no significant dependence on excitation density or excess energy in the range from 2 x 10 cm to 2 x 10 cm and 35 meV to 90 meV, respectively. Calculations of the carrier dynamics are performed and include full dynamic screening of the carrier-carrier and the carrier LO-phonon interaction. The calculated thermalization times agree well with the experimental results. Negative transmission changes above the spectral hole are mainly caused by a complementary increase of reflectivity, but not by an increase of absorption.

The transient development of electric-field distributions in a biased GaAs film after low density optical excitation is determined by measurements of Franz-Keldysh modulations with a time resolution of 100 fs. The experimental results are compared with theoretical calculations. The transient field is calculated with a drift-diffusion model. Our calculation of the dielectric function of GaAs includes the Coulomb coupling and the electric field. The resulting optical transmission changes are calculated with a transfer-matrix method. The theory predicts a modification of the FranzKeldysh modulation due to the nonuniform field, in quantitative agreement with the experimental observations. lation. Calculations of the optical spectra demonstrate that this nonuniformity is essential to explain the experimental data.We perform pump-probe experiments using amplified 2-eV pump pulses of 50-fs duration and a white-light probe continuum in the range 1.3-1.9 eV and measure the transmission spectra TE and T o of the probe beam with and without applied eleetrie field at various time delays between pump and probe beam. The spectra are recorded with an optieal multiehannel analyzer (OMA 111). The differential transmission spectra (DTS) are caleulated as the normalized transmission change of the probe beam:The DTS are numerically correeted for the chirp of the probe eontinuum as previously described. 15 By this chirp correction the time resolution obtained is mainly determined by the duration of the pump pulse without any eompression of the continuum probe pulses.The experiments are performed on a GaAs/AlxGa1-xAs double heterostructure grown by molecular-beam epitaxy on a (lOO)-oriented n-type GaAs substrate. The sampie eonsists of a 0.2-j.tm-thick iAlo.sGao.sAs layer, a 0.22-j.tm-thick i-GaAs layer, a 0.5-j.tm-thick i-Alo.sGao.sAs layer, and a 0.5-j.tm-thick n-type Alo.5Gao.5As layer doped to n = 1 x 10 18 cm-3 . A Schottky contact is formed by al-nm Cr/5-nm Au film on the frontside and a standard Ohmic contact to the substrate. The substrate is removed over an area of 2 mm 2 by wet chemieal etching to perform transmission measurements. The breakdown voltage of the diode is about 9 V corresponding to a maximal internal electrie field of 90 kV / em. The actual sampie strueture is eonfirmed by measurements of statie transmission and reflection spectra, which agree weIl with calculated ones. The Fabry-Perot oscillations permit apreeise determination of the aetual layer thieknesses. This sampie design prevents unwanted surface potentials as weH as unintentional dopant diffusion out of the highly doped layer. We therefore assume a Electromodulation spectroscopyl is a well-established technique to obtain information about internal field strength, band gap, and composition of bulk semiconductors as weH as of heterostructures.In particular, the screening of internal or external electric fields by optically injected carriers has been investigated by photoreflectance,2-4 electrorefiectance, and electroabsorption measurements.S,6 In all th...

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