Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a ...technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.
Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-densities for single QD ...devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/µm
and periods ranging from several millimeters down to at least a few hundred microns. This method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer.
Quantum dots or rings are artificial nanometre-sized clusters that confine
electrons in all three directions. They can be fabricated in a semiconductor
system by embedding an island of low-bandgap ...material in a sea of material
with a higher bandgap. Quantum dots are often referred to as artificial atoms
because, when filled sequentially with electrons, the charging energies are
pronounced for particular electron numbers; this is analogous
to Hund's rules in atomic physics. But semiconductors also have a valence
band with strong optical transitions to the conduction band. These transitions
are the basis for the application of quantum dots as laser emitters,
storage devices and fluorescence markers.
Here we report how the optical emission (photoluminescence) of a single quantum
ring changes as electrons are added one-by-one. We find that the emission
energy changes abruptly whenever an electron is added to the artificial atom,
and that the sizes of the jumps reveal a shell structure.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The nonlinear Fano effect Govorov, A. O; Kroner, M; Remi, S ...
Nature (London),
01/2008, Letnik:
451, Številka:
7176
Journal Article
Recenzirano
The Fano effect is ubiquitous in the spectroscopy of, for instance, atoms, bulk solids and semiconductor heterostructures. It arises when quantum interference takes place between two competing ...optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes. The Fano effect is particularly important in the interpretation of electronic transport and optical spectra in semiconductors. Whereas Fano's original theory applies to the linear regime at low power, at higher power a laser field strongly admixes the states and the physics becomes rich, leading, for example, to a remarkable interplay of coherent nonlinear transitions. Despite the general importance of Fano physics, this nonlinear regime has received very little attention experimentally, presumably because the classic autoionization processes, the original test-bed of Fano's ideas, occur in an inconvenient spectral region, the deep ultraviolet. Here we report experiments that access the nonlinear Fano regime by using semiconductor quantum dots, which allow both the continuum states to be engineered and the energies to be rescaled to the near infrared. We measure the absorption cross-section of a single quantum dot and discover clear Fano resonances that we can tune with the device design or even in situ with a voltage bias. In parallel, we develop a nonlinear theory applicable to solid-state systems with fast relaxation of carriers. In the nonlinear regime, the visibility of the Fano quantum interferences increases dramatically, affording a sensitive probe of continuum coupling. This could be a unique method to detect weak couplings of a two-level quantum system (qubits), which should ideally be decoupled from all other states.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L = 2 becomes a bright exciton with L = 1. The spin-flip process is revealed in ...the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi sea in the back contact of our device and corresponds to the high temperature Kondo regime. We use the Anderson Hamiltonian to calculate a spin-flip rate, and we find excellent agreement with the experimental results.
We describe a micro-fabrication method to create concave features with ultra-low roughness in silica, either on optical fibers or on flat substrates. The machining uses a single CO2 laser pulse ...train. Parameters are chosen such that evaporation removes material while a low-viscosity melt layer produces excellent surface quality. A surface roughness σ ∼ 0.2 nm is regularly obtained. The concave depressions are near-spherical close to the center with radii of curvature between 20 and 2000 μm. The method allows fabrication of low-scatter micro-optical devices such as mirror substrates for high-finesse cavities or negative lenses on fiber tips, extending the range of micro-optical components.
This study is a metrological investigation of eight superconducting gravimeters that have operated in the Strasbourg gravimetric Observatory. These superconducting gravimeters include an older ...compact C026 model, a new observatory type iOSG23 and six iGravs (6, 15, 29, 30, 31, 32). We first compare the amplitude calibration of the meters using measurements from FG5 #206 absolute gravimeter (AG). In a next step we compute the amplitude calibration of all the meters by time regression with respect to iOSG23 itself carefully calibrated by numerous AG experiments. The relative calibration values are much more precise than absolute calibration for each instrument and strongly reduce any tidal residual signal. We also compare the time lags of the various instruments with respect to iOSG23, either by time cross-correlation or tidal analysis for the longest records (about 1 year). The instrumental drift behavior of the iGravs and iOSG23 is then investigated and we examine the relationships observed between gravity and body temperature measurements. Finally, we compare the noise levels of all the instruments. A three-channel correlation analysis is used to separate the incoherent (instrumental) noise from the coherent (ambient) noise. The self-noise is then compared to a model of thermal noise (Brownian motion) using the known instrumental parameters of the damped harmonic oscillator. The self-noise of iGrav instruments is well-explained by the thermal noise model at seismic frequencies (between 10
–3
and 10
–2
Hz). As expected, the self-noise of iOSG23 with a heavier sphere is also lower than that of iGravs at such frequencies.
Evidence now exists for a pathogenic role for neutrophils in acute cerebral ischemia. We have studied the patterns and temporal profile of cerebral neutrophil recruitment to areas of acute ischemic ...stroke (IS) and have attempted to correlate this with neurological status and outcome.
Patients with cortical middle cerebral artery (MCA) IS were recruited within 24 hours of clinical onset. Neutrophil recruitment was studied using indium-111 (111In) troponolate-labeled neutrophils, planar imaging, and single-photon emission computed tomography (SPECT). Volume of brain infarction was calculated from concurrent computed tomography (CT). Hematoxylin and eosin sections were obtained postmortem (n=2). Outcome was measured using Barthel, Rankin, and National Institute of Health Stroke (NIHSS) scales.
Fifteen patients were studied. Significant 111In-neutrophil recruitment to ipsilateral hemisphere, as measured by asymmetry index (AI), was demonstrated within 24 hours of onset in 9 patients; this response was heterogenous between patients and on repeated measurement attenuated over time. Histologically, recruitment was confirmed within intravascular, intramural, and intraparenchymal compartments. Interindividual heterogeneity in neutrophil response did not correlate with infarct volume or outcome. In an exploratory analysis, neutrophil accumulation appeared to correlate significantly with infarct expansion (Spearman rho=0.66; P=0.03, n=12).
Neutrophils recruit to areas of ischemic brain within 24 hours of symptom onset. This recruitment attenuates over time and is confirmed histologically. While neutrophil accumulation may be associated with either the magnitude or the rate of infarct growth, these results require confirmation in future studies.
Abstract Macrophage presence within atherosclerotic plaque is a feature of instability and a risk factor for plaque rupture and clinical events. Activated macrophages express high levels of the ...translocator protein/peripheral benzodiazepine receptor (TSPO/PBR). In this study, we investigated the potential for quantifying plaque inflammation by targeting this receptor. TSPO expression and distribution in the plaque were quantified using radioligand binding assays and autoradiography. We show that cultured human macrophages expressed 20 times more TSPO than cultured human vascular smooth muscle cells (VSMCs), the other abundant cell type in plaque. The TSPO ligands 3 H( R )-1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoquinoline carboxamide (3 H( R )-PK11195) and 3 HN-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (3 H-DAA1106) bound to the same sites in human carotid atherosclerotic plaques in vitro , and demonstrated significant correlation with macrophage-rich regions. In conclusion, our data indicate that radioisotope-labelled DAA1106 has the potential to quantify the macrophage content of atherosclerotic plaque.
We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local ...environment at the single-charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with ±5 nm resolution. The results identify and quantify the main source of charge noise in the commonly used optical field-effect devices.
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