520 research outputs found
Nuclear magnetic resonance inverse spectra of InGaAs quantum dots: Atomistic level structural information
A wealth of atomistic information is contained within a self-assembled
quantum dot (QD), associated with its chemical composition and the growth
history. In the presence of quadrupolar nuclei, as in InGaAs QDs, much of this
is inherited to nuclear spins via the coupling between the strain within the
polar lattice and the electric quadrupole moments of the nuclei. Here, we
present a computational study of the recently introduced inverse spectra
nuclear magnetic resonance technique to assess its suitability for extracting
such structural information. We observe marked spectral differences between the
compound InAs and alloy InGaAs QDs. These are linked to the local biaxial and
shear strains, and the local bonding configurations. The cation-alloying plays
a crucial role especially for the arsenic nuclei. The isotopic line profiles
also largely differ among nuclear species: While the central transition of the
gallium isotopes have a narrow linewidth, those of arsenic and indium are much
broader and oppositely skewed with respect to each other. The statistical
distributions of electric field gradient (EFG) parameters of the nuclei within
the QD are analyzed. The consequences of various EFG axial orientation
characteristics are discussed. Finally, the possibility of suppressing the
first-order quadrupolar shifts is demonstrated by simply tilting the sample
with respect to the static magnetic field.Comment: Published version, 17 pages, 18 figure
High-occupancy effects and stimulation phenomena in semiconductor microcavities
This paper describes recent work on high-occupancy effects in semiconductor microcavities, with emphasis on the variety of new physics and the potential for applications that has been demonstrated recently. It is shown that the ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton-photon coupled modes, exciton polaritons, leads to a number of very interesting phenomena, which are either difficult or impossible to achieve in bulk semiconductors or quantum wells.
The very low polariton density of states enables state occupancies greater than one to be easily achieved, and hence stimulation phenomena to be realized under conditions of resonant excitation. The particular form of the lower polariton dispersion curve in microcavities allows energy and momentum conserving polariton-polariton scattering under resonant excitation. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the center of the Brillouin zone, and to a companion state at high wave vector. The stimulation phenomena lead to the formation of highly occupied states with macroscopic coherence in two specific regions of k space. The results are contrasted with phenomena that occur under conditions of nonresonant excitation. Prospects to achieve "polariton lasing" under nonresonant excitation, and high-gain, room-temperature ultrafast amplifiers and low-threshold optical parametric oscillator under resonant excitation conditions are discussed
Formation of optimal-order necklace modes in one-dimensional random photonic superlattices
We study the appearance of resonantly coupled optical modes, optical
necklaces, in Anderson localized one-dimensional random superlattices through
numerical calculations of the accumulated phase. The evolution of the optimal
necklace order m* shows a gradual shift towards higher orders with increasing
the sample size. We derive an empirical formula that predicts m* and discuss
the situation when in a sample length L the number of degenerate in energy
resonances exceeds the optimal one. We show how the \emph{extra} resonances are
pushed out to the miniband edges of the necklace, thus reducing the order of
the latter by multiples of two.Comment: 4 pages, 4 figure
Direct measurement of the hole-nuclear spin interaction in single quantum dots
We use photoluminescence spectroscopy of ''bright'' and ''dark'' exciton
states in single InP/GaInP quantum dots to measure hyperfine interaction of the
valence band hole with nuclear spins polarized along the sample growth axis.
The ratio of the hyperfine constants for the hole (C) and electron (A) is found
to be C/A~-0.11. In InP dots the contribution of spin 1/2 phosphorus nuclei to
the hole-nuclear interaction is weak, which enables us to determine
experimentally the value of C for spin 9/2 indium nuclei as C_In~-5 micro-eV.
This high value of C is in good agreement with recent theoretical predictions
and suggests that the hole-nuclear spin interaction has to be taken into
account when considering spin qubits based on holes.Comment: to be submitted to Phys Rev Let
Control of spontaneous emission from InP single quantum dots in GaInP photonic crystal nanocavities
We demonstrate semiconductor quantum dots coupled to photonic crystal cavity modes operating in the visible spectrum. We present the design, fabrication, and characterization of two dimensional photonic crystal cavities in GaInP and measure quality factors in excess of 7500 at 680 nm. We demonstrate full control over the spontaneous emission rate of InP quantum dots and by spectrally tuning the exciton emission energy into resonance with the fundamental cavity mode we observe a Purcell enhancement of similar to 8. (C) 2010 American Institute of Physics. [doi:10.1063/1.3510469
Photoluminescence of the incompressible Laughlin liquid: Excitons, charged excitons, and fractionally charged excitons
The photoluminescence (PL) of a two-dimensional electron gas (2DEG) in a high
magnetic field is studied as a function of the filling factor and the
separation d between the electron layer and the valence hole. Depending on the
magnitude of d relative to the magnetic length lambda, two distinct regimes in
the response of the 2DEG to the valence hole occur, with different elementary
emission processes contributing to the PL spectrum. At d<lambda ("strong
coupling" regime), the hole binds one or two electrons to form an exciton (X)
or one of three possible charged exciton (X-) states, a spin-singlet or one of
two spin-triplets. At d>lambda ("weak coupling" regime), the hole decouples or
binds one or two Laughlin quasielectrons to form fractionally charged excitons
(FCX's). The binding energies as well as the emission energies and intensities
of all X- and FCX states are calculated.Comment: 9 pages, 6 figures, sumbitted to physica status solidi (b
Inversion of exciton level splitting in quantum dots
The demonstration of degeneracy of exciton spin states is an important step toward the production of entangled photon pairs from the biexciton cascade. We measure the fine structure of exciton and biexciton states for a large number of single InAs quantum dots in a GaAs matrix; the energetic splitting of the horizontally and vertically polarized components of the exciton doublet is shown to decrease as the exciton confinement decreases, crucially passing through zero and changing sign. Thermal annealing is shown to reduce the exciton confinement, thereby increasing the number of dots with splitting close to zero
Voltage controlled nuclear polarization switching in a single InGaAs quantum dot
Sharp threshold-like transitions between two stable nuclear spin
polarizations are observed in optically pumped individual InGaAs self-assembled
quantum dots embedded in a Schottky diode when the bias applied to the diode is
tuned. The abrupt transitions lead to the switching of the Overhauser field in
the dot by up to 3 Tesla. The bias-dependent photoluminescence measurements
reveal the importance of the electron-tunneling-assisted nuclear spin pumping.
We also find evidence for the resonant LO-phonon-mediated electron
co-tunneling, the effect controlled by the applied bias and leading to the
reduction of the nuclear spin pumping rate.Comment: 5 pages, 2 figures, submitted to Phys Rev
Size-dependence of anisotropic exchange interaction in InAs/GaAs quantum dots
A comprehensive study of the exchange interaction between charge carriers in
self-organized InAs/GaAs quantum dots is presented. Single quantum-dot
cathodoluminescence spectra of quantum dots of different sizes are analyzed.
Special attention is paid to the energetic structure of the charged excited
exciton (hot trion). A varying degree of intermixing within the hot trion
states leads to varying degrees of polarization of the corresponding emission
lines. The emission characteristics change from circularly polarized for small
quantum dots to elliptically polarized for large quantum dots. The findings are
explained by a change of magnitude of the anisotropic exchange interaction and
compared to the related effect of fine-structure splitting in the neutral
exciton and biexciton emission.Comment: 4 pages, 3 figures, to be published in phys. stat. sol (b),
proceedings of the QD2006, May 1-5 2006, Chamonix-Mont-Blanc, Franc
Optically tunable nuclear magnetic resonance in a single quantum dot
We report optically detected nuclear magnetic resonance (ODNMR) measurements on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we make direct measurements of the inhomogeneous Knight field from a photoexcited electron which acts on the nuclei in the dot. The resulting shifts of the NMR peak can be optically controlled by varying the electron occupancy and its spin orientation, and lead to strongly asymmetric line shapes at high optical excitation. The all-optical control of the NMR line shape will enable position-selective control of small groups of nuclear spins inside a dot
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