543 research outputs found
Observation of transient band-gap renormalization in quantum wells.
Mesure directe de la variation de la renormalisation de la bande interdite en fonction de la densite en porteurs de charge ainsi que son evolution temporelle dans un puits quantique GaAs/Al x Ga 1−x As. Le comportement transitoire de la renormalisation de la bande interdite est gouverne par la duree de vie du porteur de charg
Split-disk micro-lasers: Tunable whispering gallery mode cavities
Optical micro-cavities of various types have emerged as promising photonic structures, for both the investigation of fundamental science in cavity quantum electrodynamics and simultaneously for various applications, e.g., lasers, filters, or modulators. In either branch a demand for adjustable and tunable photonic devices becomes apparent, which has been mainly based on the modification of the refractive index of the micro-resonators so far. In this paper, we report on a novel type of whispering gallery mode resonator where resonance tuning is achieved by modification of the configuration. This is realized by polymeric split-disks consisting of opposing half-disks with an intermediate air gap. Functionality of the split-disk concept and its figures of merit like low-threshold lasing are demonstrated for laser dye-doped split-disks fabricated by electron beam lithography on Si substrates. Reversible resonance tuning is achieved for split-disks structured onto elastomeric substrates by direct laser writing. The gap width and hence the resonance wavelength can be well-controlled by mechanically stretching the elastomer and exploiting the lateral shrinkage of the substrate. We demonstrate a broad spectral tunability of laser modes by more than three times the free spectral range. These cavities have the potential to form a key element of flexible and tunable photonic circuits based on polymers
Optical spin pumping of modulation doped electrons probed by a two-color Kerr rotation technique
We report on optical spin pumping of modulation electrons in CdTe-based
quantum wells with low intrinsic electron density (by 10^10 cm^{-2}). Under
continuous wave excitation, we reach a steady state accumulated spin density of
about 10^8 cm^{-2}. Using a two-color Hanle-MOKE technique, we find a spin
relaxation time of 34 ns for the localized electrons in the nearly unperturbed
electron gas. Independent variation of the pump and probe energies demonstrates
the presence of additional non-localized electrons in the quantum well, whose
spin relaxation time is substantially shorter
Similarity and contrasts between thermodynamic properties at the critical point of liquid alkali metals and of electron-hole droplets
The recent experimental study by means of time-resolved luminescence
measurements of an electron-hole liquid (EHL) in diamond by Shimano et al.
[Phys. Rev. Lett. 88 (2002) 057404] prompts us to compare and contrast critical
temperature T_c and critical density n_c relations in liquid alkali metals with
those in electron-hole liquids. The conclusion drawn is that these systems have
similarities with regard to critical properties. In both cases the critical
temperature is related to the cube root of the critical density. The existence
of this relation is traced to Coulomb interactions and to systematic trends in
the dielectric constant of the electron-hole systems. Finally a brief
comparison between the alkalis and EHLs of the critical values for the
compressibility ratio Z_c is also given
A solar disinfection water treatment system for remote communities
Abstract not availablePeter Kalt, Cristian Birzer, Harrison Evans, Anthony Liew, Mark Padovan, Michael Watchma
Coherence Length of Excitons in a Semiconductor Quantum Well
We report on the first experimental determination of the coherence length of
excitons in semiconductors using the combination of spatially resolved
photoluminescence with phonon sideband spectroscopy. The coherence length of
excitons in ZnSe quantum wells is determined to be 300 ~ 400 nm, about 25 ~ 30
times the exciton de Broglie wavelength. With increasing exciton kinetic
energy, the coherence length decreases slowly. The discrepancy between the
coherence lengths measured and calculated by only considering the acoustic
phonon scattering suggests an important influence of static disorder.Comment: 4 Pages, 4 figure
Ab initio and nuclear inelastic scattering studies of FeSi/GaAs heterostructures
The structure and dynamical properties of the FeSi/GaAs(001) interface
are investigated by density functional theory and nuclear inelastic scattering
measurements. The stability of four different atomic configurations of the
FeSi/GaAs multilayers is analyzed by calculating the formation energies and
phonon dispersion curves. The differences in charge density, magnetization, and
electronic density of states between the configurations are examined. Our
calculations unveil that magnetic moments of the Fe atoms tend to align in a
plane parallel to the interface, along the [110] direction of the FeSi
crystallographic unit cell. In some configurations, the spin polarization of
interface layers is larger than that of bulk FeSi. The effect of the
interface on element-specific and layer-resolved phonon density of states is
discussed. The Fe-partial phonon density of states measured for the FeSi
layer thickness of three monolayers is compared with theoretical results
obtained for each interface atomic configuration. The best agreement is found
for one of the configurations with a mixed Fe-Si interface layer, which
reproduces the anomalous enhancement of the phonon density of states below 10
meVComment: 14 pages, 9 figures, 4 table
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