762 research outputs found
Voltage-controlled electron-hole interaction in a single quantum dot
The ground state of neutral and negatively charged excitons confined to a
single self-assembled InGaAs quantum dot is probed in a direct absorption
experiment by high resolution laser spectroscopy. We show how the anisotropic
electron-hole exchange interaction depends on the exciton charge and
demonstrate how the interaction can be switched on and off with a small dc
voltage. Furthermore, we report polarization sensitive analysis of the
excitonic interband transition in a single quantum dot as a function of charge
with and without magnetic field.Comment: Conference Proceedings, Physics and Applications of Spin-Related
Phenomena in Semiconductors, Santa Barbara (CA), 2004. 4 pages, 4 figures;
content as publishe
Voltage-Controlled Optics of a Quantum Dot
We show how the optical properties of a single semiconductor quantum dot can
be controlled with a small dc voltage applied to a gate electrode. We find that
the transmission spectrum of the neutral exciton exhibits two narrow lines with
eV linewidth. The splitting into two linearly polarized
components arises through an exchange interaction within the exciton. The
exchange interaction can be turned off by choosing a gate voltage where the dot
is occupied with an additional electron. Saturation spectroscopy demonstrates
that the neutral exciton behaves as a two-level system. Our experiments show
that the remaining problem for manipulating excitonic quantum states in this
system is spectral fluctuation on a eV energy scale.Comment: 4 pages, 4 figures; content as publishe
Absorption and photoluminescence spectroscopy on a single self-assembled charge-tunable quantum dot
We have performed detailed photoluminescence (PL) and absorption spectroscopy
on the same single self-assembled quantum dot in a charge-tunable device. The
transition from neutral to charged exciton in the PL occurs at a more negative
voltage than the corresponding transition in absorption. We have developed a
model of the Coulomb blockade to account for this observation. At large
negative bias, the absorption broadens as a result of electron and hole
tunneling. We observe resonant features in this regime whenever the quantum dot
hole level is resonant with two-dimensional hole states located at the capping
layer-blocking barrier interface in our structure.Comment: 6 pages, 6 figure
Optical detection of single electron spin resonance in a quantum dot
We demonstrate optically detected spin resonance of a single electron
confined to a self-assembled quantum dot. The dot is rendered dark by resonant
optical pumping of the spin with a coherent laser. Contrast is restored by
applying a radio frequency (rf) magnetic field at the spin resonance. The
scheme is sensitive even to rf fields of just a few micro-T. In one case, the
spin resonance behaves exactly as a driven 3-level quantum system (a
lambda-system) with weak damping. In another, the dot exhibits remarkably
strong (67% signal recovery) and narrow (0.34 MHz) spin resonances with
fluctuating resonant positions, evidence of unusual dynamic processes of
non-Markovian character.Comment: 4 pages, 5 figure
The Geometrical Structure of 2d Bond-Orientational Order
We study the formulation of bond-orientational order in an arbitrary two
dimensional geometry. We find that bond-orientational order is properly
formulated within the framework of differential geometry with torsion. The
torsion reflects the intrinsic frustration for two-dimensional crystals with
arbitrary geometry. Within a Debye-Huckel approximation, torsion may be
identified as the density of dislocations. Changes in the geometry of the
system cause a reorganization of the torsion density that preserves
bond-orientational order. As a byproduct, we are able to derive several
identities involving the topology, defect density and geometric invariants such
as Gaussian curvature. The formalism is used to derive the general free energy
for a 2D sample of arbitrary geometry, both in the crystalline and hexatic
phases. Applications to conical and spherical geometries are briefly addressed.Comment: 22 pages, LaTeX, 4 eps figures Published versio
Formation of the internal structure of solids under severe action
On the example of a particular problem, the theory of vacancies, a new form
of kinetic equations symmetrically incorporation the internal and free energies
has been derived. The dynamical nature of irreversible phenomena at formation
and motion of defects (dislocations) has been analyzed by a computer
experiment. The obtained particular results are extended into a thermodynamic
identity involving the law of conservation of energy at interaction with an
environment (the 1st law of thermodynamics) and the law of energy
transformation into internal degree of freedom (relaxation). The identity is
compared with the analogous Jarzynski identity. The approach is illustrated by
simulation of processes during severe plastic deformation, the Rybin kinetic
equation for this case has been derived.Comment: 9 pages, 5 figure
Contrast in transmission spectroscopy of a single quantum dot
We perform transmission spectroscopy on single quantum dots and examine the
effects of a resident carriers spin, the incident laser spot size,
polarization, and power on the experimental contrast. We demonstrate a factor
of 4 improvement in the maximum contrast by using a solid immersion lens to
decrease the spot area. This increase yields a maximum signal to noise ratio of
2000 Hz-1/2, which will allow for MHz detection frequencies. We anticipate that
this improvement will allow further investigation of spectral fluctuation and
open up the feasibility for an all-optical read-out of an electron spin in a
quantum dot
A well-balanced scheme for two-fluid flows in variable cross-section ducts
International audienceWe propose a finite volume scheme for computing two-fluid flows in variable cross-section ducts. Our scheme satisfies a well-balanced property. It is based on the VFRoe approach. The VFRoe variables are the Riemann invariants of the stationnary wave and the cross-section. In order to avoid spurious pressure oscillations, the well-balanced approach is coupled with an ALE (Arbitrary Lagrangian Eulerian) technique at the interface and a random sampling remap
Asymmetry to symmetry transition of Fano line-shape: Analytical derivation
An analytical derivation of Fano line-shape asymmetry ratio has been
presented here for a general case. It is shown that Fano line-shape becomes
less asymmetric as \q is increased and finally becomes completely symmetric in
the limiting condition of q equal to infinity. Asymmetry ratios of Fano
line-shapes have been calculated and are found to be in good consonance with
the reported expressions for asymmetry ratio as a function of Fano parameter.
Application of this derivation is also mentioned for explanation of asymmetry
to symmetry transition of Fano line-shape in quantum confined silicon
nanostructures.Comment: 3 figures, Latex files, Theoretica
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