294 research outputs found
Buried dislocation networks designed to organize the growth of III-V semiconductor nanostructures
We first report a detailed transmission electron microscopy study of
dislocation networks (DNs) formed at shallowly buried interfaces obtained by
bonding two GaAs crystals between which we establish in a controlled manner a
twist and a tilt around a k110l direction. For large enough twists, the DN
consists of a twodimensional network of screw dislocations accommodating mainly
the twist and of a one-dimensional network of mixed dislocations accommodating
mainly the tilt. We show that in addition the mixed dislocations accommodate
part of the twist and we observe and explain slight unexpected disorientations
of the screw dislocations with respect to the k110l directions. By performing a
quantitative analysis of the whole DN, we propose a coherent interpretation of
these observations which also provides data inaccessible by direct experiments.
When the twist is small enough, one screw subnetwork vanishes. The surface
strain field induced by such DNs has been used to pilot the lateral ordering of
GaAs and InGaAs nanostructures during metal-organic vapor phase epitaxy. We
prove that the dimensions and orientations of the nanostructures are correlated
with those of the cells of the underlying DN and explain how the interface
dislocation structure governs the formation of the nanostructures
Exciton polaritons in two-dimensional photonic crystals
Experimental evidence of strong coupling between excitons confined in a
quantum well and the photonic modes of a two-dimensional dielectric lattice is
reported. Both resonant scattering and photoluminescence spectra at low
temperature show the anticrossing of the polariton branches, fingerprint of
strong coupling regime. The experiments are successfully interpreted in terms
of a quantum theory of exciton-photon coupling in the investigated structure.
These results show that the polariton dispersion can be tailored by properly
varying the photonic crystal lattice parameter, which opens the possibility to
obtain the generation of entangled photon pairs through polariton stimulated
scattering.Comment: 5 pages, 4 figure
Field Localization and Enhancement of Phase Locked Second and Third Harmonic Generation in Absorbing Semiconductor Cavities
We predict and experimentally observe the enhancement by three orders of
magnitude of phase mismatched second and third harmonic generation in a GaAs
cavity at 650nm and 433nm, respectively, well above the absorption edge. Phase
locking between the pump and the harmonics changes the effective dispersion of
the medium and inhibits absorption. Despite hostile conditions the harmonics
become localized inside the cavity leading to relatively large conversion
efficiencies. Field localization plays a pivotal role and ushers in a new class
of semiconductor-based devices in the visible and UV ranges
Hvilken betydning har nærledelse for utvikling av sykepleierens handle- og dømmekraft i hjemmebasert omsorg?
Studien tar utgangspunkt i dybdeintervjuer med ledere med fag- og personalansvar og gruppeintervjuer med ansatte i hjemmebasert omsorg, i 3 kommuner i Nord -Trøndelag. Gjennom deres historier og beskrivelser fra arbeidshverdagen, ser vi nærmere på hva som kreves og forventes av sykepleieren i hjemmebasert omsorg, og hva som er spesielt med hjemmebasert omsorg som kommunal tjeneste. Vi ser nærmere på hva ledere og ansatte fremhever som betydningsfullt ved ledelse i praksis, for å bidra til kunnskapsutvikling i tjenesten, og hvordan dette synliggjøres i arbeidshverdagen. Studien viser at ledelse gjennom samlokalisering med de ansatte gir lederen oversikt over faglige og personalrelaterte utfordringer, noe som gir mulighet til å løse utfordringene der og da, eller på lengre sikt. Studien viser også at lederens profesjonsutdanning bidrar til en dypere forståelse for utfordringsbildet og innholdet i tjenesten, som har stor betydning for å legge til rette for og fatte beslutninger om kunnskapsutvikling og ressursfordeling
Unstable and stable regimes of polariton condensation
Modulational instabilities play a key role in a wide range of nonlinear
optical phenomena, leading e.g. to the formation of spatial and temporal
solitons, rogue waves and chaotic dynamics. Here we experimentally demonstrate
the existence of a modulational instability in condensates of cavity
polaritons, arising from the strong coupling of cavity photons with quantum
well excitons. For this purpose we investigate the spatiotemporal coherence
properties of polariton condensates in GaAs-based microcavities under
continuous-wave pumping. The chaotic behavior of the instability results in a
strongly reduced spatial and temporal coherence and a significantly
inhomogeneous density. Additionally we show how the instability can be tamed by
introducing a periodic potential so that condensation occurs into negative mass
states, leading to largely improved coherence and homogeneity. These results
pave the way to the exploration of long-range order in dissipative quantum
fluids of light within a controlled platform.Comment: 7 pages, 5 figure
Engineering spin-orbit coupling for photons and polaritons in microstructures
One of the most fundamental properties of electromagnetism and special
relativity is the coupling between the spin of an electron and its orbital
motion. This is at the origin of the fine structure in atoms, the spin Hall
effect in semiconductors, and underlies many intriguing properties of
topological insulators, in particular their chiral edge states. Configurations
where neutral particles experience an effective spin-orbit coupling have been
recently proposed and realized using ultracold atoms and photons. Here we use
coupled micropillars etched out of a semiconductor microcavity to engineer a
spin-orbit Hamiltonian for photons and polaritons in a microstructure. The
coupling between the spin and orbital momentum arises from the polarisation
dependent confinement and tunnelling of photons between micropillars arranged
in the form of a hexagonal photonic molecule. Dramatic consequences of the
spin-orbit coupling are experimentally observed in these structures in the
wavefunction of polariton condensates, whose helical shape is directly visible
in the spatially resolved polarisation patterns of the emitted light. The
strong optical nonlinearity of polariton systems suggests exciting perspectives
for using quantum fluids of polaritons11 for quantum simulation of the
interplay between interactions and spin-orbit coupling.Comment: main text: pages 1-11 (4 figures); supplementary material: pages
12-28 (9 figures
Observation of slow light in the noise spectrum of a vertical external cavity surface emitting laser
The role of coherent population oscillations is evidenced in the noise
spectrum of an ultra-low noise lasers. This effect is isolated in the intensity
noise spectrum of an optimized single-frequency vertical external cavity
surface emitting laser. The coherent population oscillations induced by the
lasing mode manifest themselves through their associated dispersion that leads
to slow light effects probed by the spontaneous emission present in the
non-lasing side modes.Comment: accepted for publication in Phys. Rev. Let
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