469 research outputs found
Exciton spin-flip rate in quantum dots determined by a modified local density of optical states
The spin-flip rate that couples dark and bright excitons in self-assembled
quantum dots is obtained from time-resolved spontaneous emission measurements
in a modified local density of optical states. Employing this technique, we can
separate effects due to non-radiative recombination and unambiguously record
the spin-flip rate. The dependence of the spin-flip rate on emission energy is
compared in detail to a recent model from the literature, where the spin flip
is due to the combined action of short-range exchange interaction and acoustic
phonons. We furthermore observe a surprising enhancement of the spin-flip rate
close to a semiconductor-air interface, which illustrates the important role of
interfaces for quantum dot based nanophotonic structures. Our work is an
important step towards a full understanding of the complex dynamics of quantum
dots in nanophotonic structures, such as photonic crystals, and dark excitons
are potentially useful for long-lived coherent storage applications.Comment: 5 pages, 4 figure
Measuring the effective phonon density of states of a quantum dot
We employ detuning-dependent decay-rate measurements of a quantum dot in a
photonic-crystal cavity to study the influence of phonon dephasing in a
solid-state quantum-electrodynamics experiment. The experimental data agree
with a microscopic non-Markovian model accounting for dephasing from
longitudinal acoustic phonons, and identifies the reason for the hitherto
unexplained difference between non-resonant cavity feeding in different
nanocavities. From the comparison between experiment and theory we extract the
effective phonon density of states experienced by the quantum dot. This
quantity determines all phonon dephasing properties of the system and is found
to be described well by a theory of bulk phonons.Comment: 5 pages, 3 figures, submitte
Large quantum dots with small oscillator strength
We have measured the oscillator strength and quantum efficiency of excitons
confined in large InGaAs quantum dots by recording the spontaneous emission
decay rate while systematically varying the distance between the quantum dots
and a semiconductor-air interface. The size of the quantum dots is measured by
in-plane transmission electron microscopy and we find average in-plane
diameters of 40 nm. We have calculated the oscillator strength of excitons of
that size and predict a very large oscillator strength due to Coulomb effects.
This is in stark contrast to the measured oscillator strength, which turns out
to be much below the upper limit imposed by the strong confinement model. We
attribute these findings to exciton localization in local potential minima
arising from alloy intermixing inside the quantum dots.Comment: 4 pages, 3 figures, submitte
Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes
We have performed time-resolved spectroscopy on InAs quantum dot ensembles in
photonic crystal membranes. The influence of the photonic crystal is
investigated by varying the lattice constant systematically. We observe a
strong slow down of the quantum dots' spontaneous emission rates as the
two-dimensional bandgap is tuned through their emission frequencies. The
measured band edges are in full agreement with theoretical predictions. We
characterize the multi-exponential decay curves by their mean decay time and
find enhancement of the spontaneous emission at the bandgap edges and strong
inhibition inside the bandgap in good agreement with local density of states
calculations.Comment: 9 pages (preprint), 3 figure
Optoelectronic cooling of mechanical modes in a semiconductor nanomembrane
Optical cavity cooling of mechanical resonators has recently become a
research frontier. The cooling has been realized with a metal-coated silicon
microlever via photo-thermal force and subsequently with dielectric objects via
radiation pressure. Here we report cavity cooling with a crystalline
semiconductor membrane via a new mechanism, in which the cooling force arises
from the interaction between the photo-induced electron-hole pairs and the
mechanical modes through the deformation potential coupling. The optoelectronic
mechanism is so efficient as to cool a mode down to 4 K from room temperature
with just 50 uW of light and a cavity with a finesse of 10 consisting of a
standard mirror and the sub-wavelength-thick semiconductor membrane itself. The
laser-cooled narrow-band phonon bath realized with semiconductor mechanical
resonators may open up a new avenue for photonics and spintronics devices.Comment: 5 pages, 4 figure
Efficient out-coupling of high-purity single photons from a coherent quantum dot in a photonic-crystal cavity
We demonstrate a single-photon collection efficiency of from
a quantum dot in a low-Q mode of a photonic-crystal cavity with a single-photon
purity of recorded above the saturation power. The high
efficiency is directly confirmed by detecting up to kilocounts per
second on a single-photon detector on another quantum dot coupled to the cavity
mode. The high collection efficiency is found to be broadband, as is explained
by detailed numerical simulations. Cavity-enhanced efficient excitation of
quantum dots is obtained through phonon-mediated excitation and under these
conditions, single-photon indistinguishability measurements reveal long
coherence times reaching ns in a weak-excitation regime. Our work
demonstrates that photonic crystals provide a very promising platform for
highly integrated generation of coherent single photons including the efficient
out-coupling of the photons from the photonic chip.Comment: 13 pages, 8 figures, submitte
Action at a distance: Dependency sensitivity in a New World primate
Sensitivity to dependencies (correspondences between distant items) in sensory stimuli plays a crucial role in human music and language. Here, we show that squirrel monkeys (Saimiri sciureus) can detect abstract, non-adjacent dependencies in auditory stimuli. Monkeys discriminated between tone sequences containing a dependency and those lacking it, and generalized to previously unheard pitch classes and novel dependency distances. This constitutes the first pattern learning study where artificial stimuli were designed with the species' communication system in mind. These results suggest that the ability to recognize dependencies represents a capability that had already evolved in humans' last common ancestor with squirrel monkeys, and perhaps before. © 2013 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0/, which permits unrestricted use, provided the original author and source are credited
Near-unity coupling efficiency of a quantum emitter to a photonic-crystal waveguide
A quantum emitter efficiently coupled to a nanophotonic waveguide constitutes
a promising system for the realization of single-photon transistors,
quantum-logic gates based on giant single-photon nonlinearities, and high
bit-rate deterministic single-photon sources. The key figure of merit for such
devices is the -factor, which is the probability for an emitted single
photon to be channeled into a desired waveguide mode. We report on the
experimental achievement of for a quantum dot
coupled to a photonic-crystal waveguide, corresponding to a single-emitter
cooperativity of . This constitutes a nearly ideal
photon-matter interface where the quantum dot acts effectively as a 1D
"artificial" atom, since it interacts almost exclusively with just a single
propagating optical mode. The -factor is found to be remarkably robust
to variations in position and emission wavelength of the quantum dots. Our work
demonstrates the extraordinary potential of photonic-crystal waveguides for
highly efficient single-photon generation and on-chip photon-photon
interaction
Single-photon nonlinear optics with a quantum dot in a waveguide
Strong nonlinear interactions between photons enable logic operations for
both classical and quantum-information technology. Unfortunately, nonlinear
interactions are usually feeble and therefore all-optical logic gates tend to
be inefficient. A quantum emitter deterministically coupled to a propagating
mode fundamentally changes the situation, since each photon inevitably
interacts with the emitter, and highly correlated many-photon states may be
created . Here we show that a single quantum dot in a photonic-crystal
waveguide can be utilized as a giant nonlinearity sensitive at the
single-photon level. The nonlinear response is revealed from the intensity and
quantum statistics of the scattered photons, and contains contributions from an
entangled photon-photon bound state. The quantum nonlinearity will find
immediate applications for deterministic Bell-state measurements and
single-photon transistors and paves the way to scalable waveguide-based
photonic quantum-computing architectures
Quantitative analysis of quantum dot dynamics and emission spectra in cavity quantum electrodynamics:Paper
We present detuning-dependent spectral and decay-rate measurements to study
the difference between spectral and dynamical properties of single quantum dots
embedded in micropillar and photonic-crystal cavities. For the micropillar
cavity, the dynamics is well described by the dissipative Jaynes-Cummings
model, while systematic deviations are observed for the emission spectra. The
discrepancy for the spectra is attributed to coupling of other exciton lines to
the cavity and interference of different propagation paths towards the detector
of the fields emitted by the quantum dot. In contrast, quantitative information
about the system can readily be extracted from the dynamical measurements. In
the case of photonic crystal cavities we observe an anti crossing in the
spectra when detuning a single quantum dot through resonance, which is the
spectral signature of strong coupling. However, time-resolved measurements
reveal that the actual coupling strength is significantly smaller than
anticipated from the spectral measurements and that the quantum dot is rather
weakly coupled to the cavity. We suggest that the observed Rabi splitting is
due to cavity feeding by other quantum dots and/or multiexcition complexes
giving rise to collective emission effects.Comment: 14 pages, 5 figures, submitte
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