126 research outputs found
Local disorder and optical properties in V-shaped quantum wires : towards one-dimensional exciton systems
The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires
(QWRs) by high spatial resolution spectroscopy. Scanning optical imaging of
different generations of samples shows that the localization length has been
enhanced as the growth techniques were improved. In the best samples, excitons
are delocalized in islands of length of the order of 1 micron, and form a
continuum of 1D states in each of them, as evidenced by the sqrt(T) dependence
of the radiative lifetime. On the opposite, in the previous generation of QWRs,
the localization length is typically 50 nm and the QWR behaves as a collection
of quantum boxes. These localization properties are compared to structural
properties and related to the progresses of the growth techniques. The presence
of residual disorder is evidenced in the best samples and explained by the
separation of electrons and holes due to the large in-built piezo-electric
field present in the structure.Comment: 8 figure
Anisotropy and controllable band structure in supra-wavelength polaritonic metasurfaces
In this letter we exploit the extended coherence length of mixed
plasmon/exciton states to generate active metasurfaces. For this purpose,
periodic stripes of organic dye are deposited on a continuous silver film.
Typical metasurface effects, such as effective behavior and geometry
sensitivity, are measured for periods exceeding the polaritonic wavelength by
more than one order of magnitude. By adjusting the metasurface geometry,
anisotropy, modified band structure and unidimensional polaritons are
computationally simulated and experimentally observed in reflectometry as well
as in emission.Comment: 4 figures, 1 Supplementary Material
Enhanced light extraction from InGaN/GaN quantum wells with silver gratings
We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.Peer reviewe
Mott transition from a diluted exciton gas to a dense electron-hole plasma in a single V-shaped quantum wire
We report on the study of many-body interactions in a single high quality
V-shaped quantum wire by means of continuous and time-resolved
microphotoluminescence. The transition from a weakly interacting exciton gas
when the carrier density n is less than 10^5 cm^-1 (i.e. n aX < 0.1, with aX
the exciton Bohr radius), to a dense electron-hole plasma (n > 10^6 cm^-1, i.e.
n aX > 1) is systematically followed in the system as the carrier density is
increased. We show that this transition occurs gradually : the free carriers
first coexist with excitons for n aX > 0.1, then the electron-hole plasma
becomes degenerate at n aX = 0.8. We also show that the non-linear effects are
strongly related to the kind of disorder and localization properties in the
structure especially in the low density regime.Comment: 5 figure
Magnetization of Mn_12 Ac in a slowly varying magnetic field: an ab initio study
Beginning with a Heisenberg spin Hamiltonian for the manganese ions in the
Mn_12 Ac molecule, we find a number of low-energy states of the system. We use
these states to solve the time-dependent Schrodinger equation and find the
magnetization of the molecule in the presence of a slowly varying magnetic
field. We study the effects of the field sweep rate, fourth order anisotropic
spin interactions and a transverse field on the weights of the different states
as well as the magnetization steps which are known to occur in the hysteresis
plots in this system. We find that the fourth order term and a slow field sweep
rate are crucial for obtaining prominent steps in magnetization in the
hysteresis plots.Comment: LaTeX, 11 pages, 12 eps figure
Polarization-Correlated Photon Pairs from a Single Quantum Dot
Polarization correlation in a linear basis, but not entanglement, is observed
between the biexciton and single-exciton photons emitted by a single InAs
quantum dot in a two-photon cascade. The results are well described
quantitatively by a probabilistic model that includes two decay paths for a
biexciton through a non-degenerate pair of one-exciton states, with the
polarization of the emitted photons depending on the decay path. The results
show that spin non-degeneracy due to quantum-dot asymmetry is a significant
obstacle to the realization of an entangled-photon generation device.Comment: 4 pages, 4 figures, revised discussio
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