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We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ∼37% and ∼85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ∼0.5ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons

We have studied experimentally and theoretically the effects of series and parallel resistances on the current-voltage (I-V) characteristics of the resonant tunneling diode (RTD) in the format of a small-area freestanding air bridge. It has been shown by standard quantum mechanical considerations that the I-V characteristics consists of a simple resonant current peak (without bistability) and the voltage at the current peak is quite low, whereas experiments show not only a much high current-peak voltage, but also the bistability. Furthermore, experimental peak-to-valley ratio is in general quite low. By analyzing material and device structures, it has been demonstrated that the series resistance in the RTD contacts shifts the peak in current to higher voltage, it also creates the experimentally observed bistability effect in the region of the tunneling resonance. The parallel resistance induces a leakage current and reduces the peak-to-valley ratio. We have further performed similar analyses to many RTD structures reported in the literature and the conclusions remain the same., QC 20100525

QC 20100525

The propagation in two dimensions of "optical" pulses in electromagnetically induced transparency media is analyzed. Results are presented for coupled Maxwell-Bloch equations with slowly varying envelope approximation, for both adiabatic and nonadiabatic situations. The possibility of changing the direction of the pulse by a switch of control beam direction is investigated in detail., QC 20100903

©2003 The American Physical Society. We are grateful to W. I. Wang, who provided some of the samples used in this study, and to B. Z. Nosho, L. J. Whitman, and B. V. Shanabrook for their assistance in developing MBE techniques applied to the growth of samples C and D discussed here. This work was supported by the US Army Research Office (Grant No. DAAD- 19-99-1-0270) and the Office of Naval Research., We have observed that the tunneling magnetoconductance between two-dimensional (2D) electron gases, formed at nominally identical InAs-AlSb interfaces, most often exhibits two sets of Shubnikov-de Haas oscillations with almost the same frequency. This result is explained quantitatively with a model of the conductance in which the 2D gases have different densities and can tunnel between Landau levels with different quantum indices. When the epitaxial growth conditions of the interfaces are optimized, the zero-bias magnetoconductance shows a single set of oscillations, thus proving that the asymmetry between the two electron gases can be eliminated., Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

We describe the principle and analyze the operation of an integrated optics directional coupler filter based on coupling between a regular waveguide and one, that exhibits electromagnetically induced transparency. Bandwidth length products on the order of 2 pm x mm are obtainable, as an example, using this approach., QC 20100525

We present an experimental and theoretical investigation of ultrafast transient four-wave mixing of GaAs/AlxGa1-xAs quantum wells for coherent excitation of exciton and continuum states. The signal appears at the exciton resonance and is shown to consist of two contributions: an intense spectrally narrow part only present for short time delays and a weak exponentially decaying part reflecting the intrinsic coherence properties of the exciton states. The intense part of the signal is due to the excitation of the continuum states mediated by interaction-induced nonlinearities.