108 research outputs found
Radiative cascades in charged quantum dots
We measured, for the first time, two photon radiative cascades due to
sequential recombination of quantum dot confined electron hole pairs in the
presence of an additional spectator charge carrier. We identified direct, all
optical cascades involving spin blockaded intermediate states, and indirect
cascades, in which non radiative relaxation precedes the second recombination.
Our measurements provide also spin dephasing rates of confined carriers.Comment: 4 pages, 3 figure
Radiative cascade from quantum dot metastable spin-blockaded biexciton
We detect a novel radiative cascade from a neutral semiconductor quantum dot.
The cascade initiates from a metastable biexciton state in which the holes form
a spin-triplet configuration, Pauli-blockaded from relaxation to the
spin-singlet ground state. The triplet biexciton has two photon-phonon-photon
decay paths. Unlike in the singlet-ground state biexciton radiative cascade, in
which the two photons are co-linearly polarized, in the triplet biexciton
cascade they are crosslinearly polarized. We measured the two-photon
polarization density matrix and show that the phonon emitted when the
intermediate exciton relaxes from excited to ground state, preserves the
exciton's spin. The phonon, thus, does not carry with it any which-path
information other than its energy. Nevertheless, entanglement distillation by
spectral filtering was found to be rather ineffective for this cascade. This
deficiency results from the opposite sign of the anisotropic electron-hole
exchange interaction in the excited exciton relative to that in the ground
exciton.Comment: 6 pages, 4 figure
Coulomb interactions in single, charged self-assembled quantum dots: radiative lifetime and recombination energy
We present results on the charge dependence of the radiative recombination
lifetime, Tau, and the emission energy of excitons confined to single
self-assembled InGaAs quantum dots. There are significant dot-to-dot
fluctuations in the lifetimes for a particular emission energy. To reach
general conclusions, we present the statistical behavior by analyzing data
recorded on a large number of individual quantum dots. Exciton charge is
controlled with extremely high fidelity through an n-type field effect
structure, providing access to the neutral exciton (X0), the biexciton (2X0)
and the positively (X1+) and negatively (X1-) charged excitons. We find
significant differences in the recombination lifetime of each exciton such
that, on average, Tau(X1-) / Tau(X0) = 1.25, Tau(X1+) / Tau(X0) = 1.58 and
Tau(2X0) / Tau(X0) = 0.65. We attribute the change in lifetime to significant
changes in the single particle hole wave function on charging the dot, an
effect more pronounced on charging X0 with a single hole than with a single
electron. We verify this interpretation by recasting the experimental data on
exciton energies in terms of Coulomb energies. We show directly that the
electron-hole Coulomb energy is charge dependent, reducing in value by 5-10% in
the presence of an additional electron, and that the electron-electron and
hole-hole Coulomb energies are almost equal.Comment: 8 pages, 7 figures, submitted to Phys. Rev.
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
Screening nuclear field fluctuations in quantum dots for indistinguishable photon generation
A semiconductor quantum dot can generate highly coherent and
indistinguishable single photons. However, intrinsic semiconductor dephasing
mechanisms can reduce the visibility of two-photon interference. For an
electron in a quantum dot, a fundamental dephasing process is the hyperfine
interaction with the nuclear spin bath. Here we directly probe the consequence
of the fluctuating nuclear spins on the elastic and inelastic scattered photon
spectra from a resident electron in a single dot. We find the nuclear spin
fluctuations lead to detuned Raman scattered photons which are distinguishable
from both the elastic and incoherent components of the resonance fluorescence.
This significantly reduces two-photon interference visibility. However, we
demonstrate successful screening of the nuclear spin noise which enables the
generation of coherent single photons that exhibit high visibility two-photon
interference.Comment: 5 pages, 4 figures + Supplementary Informatio
Electro-elastic tuning of single particles in individual self-assembled quantum dots
We investigate the effect of uniaxial stress on InGaAs quantum dots in a
charge tunable device. Using Coulomb blockade and photoluminescence, we observe
that significant tuning of single particle energies (~ -0.5 meV/MPa) leads to
variable tuning of exciton energies (+18 to -0.9 micro-eV/MPa) under tensile
stress. Modest tuning of the permanent dipole, Coulomb interaction and
fine-structure splitting energies is also measured. We exploit the variable
exciton response to tune multiple quantum dots on the same chip into resonance.Comment: 16 pages, 4 figures, 1 table. Final versio
Dressed excitonic states and quantum interference in a three-level quantum dot ladder system
We observe dressed states and quantum interference effects in a strongly
driven three-level quantum dot ladder system. The effect of a strong coupling
field on one dipole transition is measured by a weak probe field on the second
dipole transition using differential reflection. When the coupling energy is
much larger than both the homoge-neous and inhomogeneous linewidths an
Autler-Townes splitting is observed. Striking differences are observed when the
transitions resonant with the strong and weak fields are swapped, particularly
when the coupling energy is nearly equal to the measured linewidth. This result
is attributed to quantum interference: a modest destructive or constructive
interference is observed depending on the pump / probe geometry. The data
demonstrate that coher-ence of both the bi-exciton and the exciton is
maintained in this solid-state system, even under intense illumina-tion, which
is crucial for prospects in quantum information processing and non-linear
optical devices.Comment: 8 pages, 6 figures, submitted to New Journal of Physic
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