107 research outputs found
Inversion of the exciton built-in dipole moment in In(Ga)As quantum dots via nonlinear piezoelectric effect
We show that anisotropic biaxial stress can be used to tune the built-in
dipole moment of excitons confined in In(Ga)As quantum dots up to complete
erasure of its magnitude and inversion of its sign. We demonstrate that this
phenomenon is due to piezoelectricity. We present a model to calculate the
applied stress, taking advantage of the so-called piezotronic effect, which
produces significant changes in the current-voltage characteristics of the
strained diode-membranes containing the quantum dots. Finally, self-consistent
k.p calculations reveal that the experimental findings can be only accounted
for by the nonlinear piezoelectric effect, whose importance in quantum dot
physics has been theoretically recognized although it has proven difficult to
single out experimentally.Comment: 6 pages, 4 figure
Effect of second order piezoelectricity on excitonic structure of stress-tuned InGaAs/GaAs quantum dots
We study the effects of the nonlinear piezoelectricity and the In
distribution on the exciton energy, the electron-hole electric dipole moment,
and the fine-structure splitting in stress-tunable InGaAs/GaAs quantum dots
integrated onto a piezoelectric actuator. In particular, we investigate in
detail the contributions of various elements of the expansion of the electrical
polarization in terms of externally induced elastic strain on the latter two
important quantum dot properties. Based on the comparison of the effects of
first- and second-order piezoelectricity we provide a simple relation to
estimate the influence of applied anisotropic stress on the quantum dot dipole
moment for quantum dots significantly lattice mismatched to the host crystal
2.5% efficient organic plastic solar cells
We show that the power conversion efficiency of organic photovoltaic devices based on a conjugated polymer/methanofullerene blend is dramatically affected by molecular morphology. By structuring the blend to be a more intimate mixture that contains less phase segregation of methanofullerenes, and simultaneously increasing the degree of interactions between conjugated polymer chains, we have fabricated a device with a power conversion efficiency of 2.5% under AM1.5 illumination. This is a nearly threefold enhancement over previously reported values for such a device, and it approaches what is needed for the practical use of these devices for harvesting energy from sunlight.
Extracellular electrical signals in a neuron-surface junction: model of heterogeneous membrane conductivity
Signals recorded from neurons with extracellular planar sensors have a wide
range of waveforms and amplitudes. This variety is a result of different
physical conditions affecting the ion currents through a cellular membrane. The
transmembrane currents are often considered by macroscopic membrane models as
essentially a homogeneous process. However, this assumption is doubtful, since
ions move through ion channels, which are scattered within the membrane.
Accounting for this fact, the present work proposes a theoretical model of
heterogeneous membrane conductivity. The model is based on the hypothesis that
both potential and charge are distributed inhomogeneously on the membrane
surface, concentrated near channel pores, as the direct consequence of the
inhomogeneous transmembrane current. A system of continuity equations having
non-stationary and quasi-stationary forms expresses this fact mathematically.
The present work performs mathematical analysis of the proposed equations,
following by the synthesis of the equivalent electric element of a
heterogeneous membrane current. This element is further used to construct a
model of the cell-surface electric junction in a form of the equivalent
electrical circuit. After that a study of how the heterogeneous membrane
conductivity affects parameters of the extracellular electrical signal is
performed. As the result it was found that variation of the passive
characteristics of the cell-surface junction, conductivity of the cleft and the
cleft height, could lead to different shapes of the extracellular signals
Single SiGe Quantum Dot Emission Deterministically Enhanced in a High-Q Photonic Crystal Resonator
We report the resonantly enhanced radiative emission from a single SiGe
quantum dot (QD), which is deterministically embedded into a bichromatic
photonic crystal resonator (PhCR) at the position of its largest modal electric
field by a scalable method. By optimizing our molecular beam epitaxy (MBE)
growth technique, we were able to reduce the amount of Ge within the whole
resonator to obtain an absolute minimum of exactly one QD, accurately
positioned by lithographic methods relative to the PhCR, and an otherwise flat,
a few monolayer thin, Ge wetting layer (WL). With this method, record quality
(Q) factors for QD-loaded PhCRs up to are achieved. A comparison
with control PhCRs on samples containing a WL but no QDs is presented, as well
as a detailed analysis of the dependence of the resonator-coupled emission on
temperature, excitation intensity, and emission decay after pulsed excitation.
Our findings undoubtedly confirm a single QD in the center of the resonator as
a potentially novel photon source in the telecom spectral range
Resolving the temporal evolution of line broadening in single quantum emitters
Funding: H2020 European Research Council (679183); Austrian Science Fund (P29603); Seventh Framework Programme (601126); Central European Institute of Technology (7AMB17AT044); Horizon 2020 Framework Programme (731473); European Metrology Programme for Innovation and Research (17FUN06); Bundesministerium für Wissenschaft, Forschung und Wirtschaft (CZ 07 / 2017); Ministerstvo Školství, Mládeže a Telovýchovy; QuantERA (Hyper-U-P-S); QuantERA (CUSPIDOR); Linz Institute of Technology (LIT); LIT Secure and Correct Systems Lab; Bayerisches Staatsministerium für Bildung und Kultus, Wissenschaft und Kunst; Deutsche Forschungsgemeinschaft (SCHN1376 5.1).Light emission from solid-state quantum emitters is inherently prone to environmental decoherence, which results in a line broadening and in the deterioration of photon indistinguishability. Here we employ photon correlation Fourier spectroscopy (PCFS) to study the temporal evolution of such a broadening in two prominent systems: GaAs and In(Ga)As quantum dots. Differently from previous experiments, the emitters are driven with short laser pulses as required for the generation of high-purity single photons, the time scales we probe range from a few nanoseconds to milliseconds and, simultaneously, the spectral resolution we achieve can be as small as ∼ 2µeV. We find pronounced differences in the temporal evolution of different optical transition lines, which we attribute to differences in their homogeneous linewidth and sensitivity to charge noise. We analyze the effect of irradiation with additional white light, which reduces blinking at the cost of enhanced charge noise. Due to its robustness against experimental imperfections and its high temporal resolution and bandwidth, PCFS outperforms established spectroscopy techniques, such as Michelson interferometry. We discuss its practical implementation and the possibility to use it to estimate the indistinguishability of consecutively emitted single photons for applications in quantum communication and photonic-based quantum information processing.Publisher PDFPeer reviewe
Ultra-steep side facets in multi-faceted SiGe/Si(001) Stranski-Krastanow islands
For the prototypical Ge/Si(001) system, we show that at high growth temperature a new type of Stranski-Krastanow islands is formed with side facets steeper than {111} and high aspect ratio. Nano-goniometric analysis of the island shapes reveals the presence of six new facet groups in addition to those previously found for dome or barn-shaped islands. Due to the highly multi-faceted island shape and high aspect ratio, the new island types are named "cupola" islands and their steepest {12 5 3} side facet is inclined by 68°to the substrate surface. Assessing the relative stability of the new facets from surface area analysis, we find that their stability is similar to that of {113} and {15 3 23} facets of dome islands. The comparison of the different island shapes shows that they form a hierarchical class of geometrical structures, in which the lower aspect ratio islands of barns, domes and pyramids are directly derived from the cupola islands by successive truncation of the pedestal bases without facet rearrangements. The results underline the key role of surface faceting in the process of island formation, which is as crucial for understanding the island's growth evolution as it is important for device applications
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