420 research outputs found
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Origin of a-plane (Al,Ga)N formation on patterned c-plane AIN/sapphire templates
a-plane (Al,Ga)N layers can be grown on patterned c-plane AlN/sapphire templates with a ridge direction along [1bar 100]Al2O3. Scanning nanobeam diffraction reveals that the formation of a-plane layers can be explained by nucleation of c-plane (Al,Ga)N with [11bar 20](Al,Ga)N[0001]Al2O3 at the ridge sidewalls. Faster growth of the top (11bar 20)(Al,Ga)N facet in the vertical direction leads to the overgrowth of c-plane (Al,Ga)N nucleated on the horizontal ridge and trench surfaces. Phase separation into binary GaN and AlN takes place during the first growth stages. However, this fades out and does not influence the composition of the final thick a-plane (Al,Ga)N layer
Remote frequency measurement of the 1S0-3P1 transition in laser cooled Mg-24
We perform Ramsey-Bord\'e spectroscopy on laser-cooled magnesium atoms in
free fall to measure the 1S0 \rightarrow 3P1 intercombination transition
frequency. The measured value of 655 659 923 839 730 (48) Hz is consistent with
our former atomic beam measurement (Friebe et al 2008 Phys. Rev. A 78 033830).
We improve upon the fractional accuracy of the previous measurement by more
than an order of magnitude to 7e-14. The magnesium frequency standard was
referenced to a fountain clock of the Physikalisch-Technische Bundesanstalt
(PTB) via a phase-stabilized telecom fiber link and its stability was
characterized for interrogation times up to 8000 s. The high temperature of the
atomic ensemble leads to a systematic shift due to the motion of atoms across
the spectroscopy beams. In our regime, this leads to a counterintuitive
reduction of residual Doppler shift with increasing resolution. Our theoretical
model of the atom-light interaction is in agreement with the observed effect
and allows us to quantify its contribution in the uncertainty budget.Comment: 16 pages, 8 figures. Accepted in New Journal of Physic
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High-power diode lasers with in-situ-structured lateral current blocking for improved threshold, efficiency and brightness
We present high-power GaAs-based broad-area diode lasers with a novel variant of the enhanced self-aligned lateral structure ‘eSAS’, having a strongly reduced lasing threshold and improved peak conversion efficiency and beam quality in comparison to their standard gain-guided counterparts. To realize this new variant (eSAS-V2), a two-step epitaxial growth process involving in situ etching is used to integrate current-blocking layers, optimized for tunnel current suppression, within the p-Al0.8GaAs cladding layer of an extreme-triple-asymmetric epitaxial structure with a thin p-side waveguide. The blocking layers are thus in close proximity to the active zone, resulting in strong suppression of current spreading and lateral carrier accumulation. eSAS-V2 devices with 4 mm resonator length and varying stripe widths are characterized and compared to previous eSAS variant (eSAS-V1) as well as gain-guided reference devices, all having the same dimensions and epitaxial structure. Measurement results show that the new eSAS-V2 variant eliminates an estimated 89% of lateral current spreading, resulting in a strong threshold current reduction of 29% at 90 μm stripe width, while slope and series resistance are broadly unchanged. The novel eSAS-V2 devices also maintain high conversion efficiency up to high continuous-wave optical power, with an exemplary 90 μm device having 51.5% at 20 W. Near-field width is significantly narrowed in both eSAS variants, but eSAS-V2 exhibits a wider far-field angle, consistent with the presence of index guiding. Nonetheless, eSAS-V2 achieves higher beam quality and lateral brightness than gain-guided reference devices, but the index guiding in this realization prevents it from surpassing eSAS-V1. Overall, the different performance benefits of the eSAS approach are clearly demonstrated
Precision determination of band offsets in strained InGaAs/GaAs quantum wells by C-V-profiling and Schroedinger-Poisson self-consistent simulation
The results of measurements and numerical simulation of charge carrier
distribution and energy states in strained quantum wells In_xGa_{1-x}As/GaAs
(0.06 < x < 0.29) by C-V-profiling are presented. Precise values of conduction
band offsets for these pseudomorphic QWs have been obtained by means of
self-consistent solution of Schroedinger and Poisson equations and following
fitting to experimental data. For the conduction band offsets in strained
In_xGa_{1-x}As/GaAs - QWs the expression DE_C(x) = 0.814x - 0.21x^2 has been
obtained.Comment: 9 pages, 12 figures, RevTeX
Clean and As-covered zinc-blende GaN (001) surfaces: Novel surface structures and surfactant behavior
We have investigated clean and As-covered zinc-blende GaN (001) surfaces,
employing first-principles total-energy calculations. For clean GaN surfaces
our results reveal a novel surface structure very different from the
well-established dimer structures commonly observed on polar III-V (001)
surfaces: The energetically most stable surface is achieved by a Peierls
distortion of the truncated (1x1) surface rather than through addition or
removal of atoms. This surface exhibits a (1x4) reconstruction consisting of
linear Ga tetramers. Furthermore, we find that a submonolayer of arsenic
significantly lowers the surface energy indicating that As may be a good
surfactant. Analyzing surface energies and band structures we identify the
mechanisms which govern these unusual structures and discuss how they might
affect growth properties.Comment: 4 pages, 3 figures, to be published in Appears in Phys. Rev. Lett.
(in print). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
High-resolution microwave frequency dissemination on an 86-km urban optical link
We report the first demonstration of a long-distance ultra stable frequency
dissemination in the microwave range. A 9.15 GHz signal is transferred through
a 86-km urban optical link with a fractional frequency stability of 1.3x10-15
at 1 s integration time and below 10-18 at one day. The optical link phase
noise compensation is performed with a round-trip method. To achieve such a
result we implement light polarisation scrambling and dispersion compensation.
This link outperforms all the previous radiofrequency links and compares well
with recently demonstrated full optical links.Comment: 11 pages, 5 figure
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