A detailed analysis based on first-principles calculations with self-energy corrections is combined with photoemission spectroscopy to determine the origin of features observed in reflectance anisotropy spectroscopy ͑RAS͒ at semiconductor surfaces. Using the InP(001)(2ϫ4) surface as a model case we obtain quantitative agreement between slab calculations and low-temperature RAS measurements. We find the contributions to the anisotropy signal related either directly to surface states or to transitions between surface perturbed bulk wave functions. Our results demonstrate the high sensitivity of RAS to the surface structure and chemistry and show that the absorption processes causing the anisotropy signal take place in the uppermost few atomic layers of the substrate. RAPID COMMUNICATIONS R16 336PRB 61 W. G. SCHMIDT et al. RAPID COMMUNICATIONS R16 338PRB 61 W. G. SCHMIDT et al.

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A semiconductor disk laser based on an InGaAs/AlGaAs quantum-well gain medium was mode-locked by a fast semiconductor saturable absorber mirror. By high-order harmonic mode-locking a 92 GHz pulse train was obtained with a pulse duration of <200 fs. In order to achieve fundamental mode-locking, too strong saturation of the semiconductor elements had to be avoided. In a single-pulse regime, pulses shorter than 110 fs were generated at a wavelength of 1030 nm.

CuPt B -type ordering of InGaP grown lattice matched to GaAs was investigated by in situ reflectance anisotropy spectroscopy and reflection high-energy electron diffraction. The experiments have been performed during InGaP growth both via metal-organic vapor phase epitaxy and chemical beam epitaxy. Additionally, total energy calculations ͑TE͒ have been performed for differently ordered InGaP slabs. From both experiment and TE calculations we conclude that bulk ordering only occurs when InGaP growth is performed under phosphorus-rich (2ϫ1)-like surface conditions. Bulk ordering completely disappears under growth conditions which cause a less-phosphorus-rich (2ϫ4)-like surface dimer configuration. P dimers in the (2ϫ1) symmetry at the growth surface trigger the bulk ordering driven by an energy gain of 0.26 eV per surface atom according to our TE calculations. ͓S0163-1829͑99͒05635-0͔

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