Thermalization, intervalley scattering, and cooling of photoexcited carriers are observed in the indirect-band-gap semiconductor Ge through time-resolved direct-band-gap transmission and luminescence with 100-fs resolution. The transmission experiment, which uses k= 1.53 pm, 120-fs pulses from a 76-MHz optical parametric oscillator takes advantage of the indirect-band-gap character of Ge to unambiguously determine the I~L intervalley scattering time at 295 K. The measured I~L transfer time for electrons at the I valley edge is 230+25 fs yielding 4.2+0.2X 10' eV/cm for the D~L intervalley deformation potential. The luminescence experiments were performed at 10 K using a standard time and spectrally gated up-conversion technique employing an 82-MHz, 80-fs pulse width Ti:sapphire laser operating at A, =750 nm. From the luminescence experiments conducted with peak carrier densities of
We report the observation of optical second harmonic generation (SHG) from SimGen short-period superlattices grown on different substrates. For a 775 nm fundamental beam, the SHG signals from odd-odd, even-even, and even-odd specimens are comparable, apart from substrate-dependent strain enhancement. It is concluded that the lack of a large bulk dipole-allowed signal in odd-odd superlattices predicted by Ghahramani, Moss, and Sipe [Phys. Rev. Lett. 64, 2815 (1990)] is due to the inevitable presence of monatomic steps on the substrate which lead to antiphase domains; a possible circumvention of this limitation is outlined based on the use of a single domain vicinal Si(001) substrate.
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