94 research outputs found

### Spin Photovoltaic Effect in Quantum Wires with Rashba Interaction

We propose a mechanism for spin polarized photocurrent generation in quantum
wires. The effect is due to the combined effect of Rashba spin-orbit
interaction, external magnetic field and microwave radiation. The
time-independent interactions in the wire give rise to a spectrum asymmetry in
k-space. The microwave radiation induces transitions between spin-splitted
subbands, and, due to the peculiar energy dispersion relation, charge and spin
currents are generated at zero bias voltage. We demonstrate that the generation
of pure spin currents is possible under an appropriate choice of external
control parameters

### Radiation-induced current in quantum wires with side-coupled nano-rings

Photocurrent generation is studied in a system composed of a quantum wire
with side-coupled quantum rings. The current generation results from the
interplay of the particular geometry of the system and the use of circularly
polarized radiation. We study the energy-momentum conservation for optical
transitions involving electrons moving forwards and backwards in the wire. Due
to the lack of time-reversal symmetry in the radiation, the optical transitions
depend on the direction of motion of the electrons, leading to a current at
zero bias voltage. The photocurrent increases with the number of rings within a
wide range of physical parameters. A weak non-linear dependence of the current
in the number of rings, related to quantum interference effects, is also
predicted. This geometry suggests a scalable method for the generation of
sizeable photocurrents based on nanoscale components.Comment: 7 pages, 6 figure

### Control of asymmetric Hopfield networks and application to cancer attractors

The asymmetric Hopfield model is used to simulate signaling dynamics in
gene/transcription factor networks. The model allows for a direct mapping of a
gene expression pattern into attractor states. We analyze different control
strategies aiming at disrupting attractor patterns using selective local fields
representing therapeutic interventions. The control strategies are based on the
identification of signaling $bottlenecks$, which are single nodes or strongly
connected clusters of nodes that have a large impact on the signaling. We
provide a theorem with bounds on the minimum number of nodes that guarantee
controllability of bottlenecks consisting of strongly connected components. The
control strategies are applied to the identification of sets of proteins that,
when inhibited, selectively disrupt the signaling of cancer cells while
preserving the signaling of normal cells. We use an experimentally validated
non-specific network and a specific B cell interactome reconstructed from gene
expression data to model cancer signaling in lung and B cells, respectively.
This model could help in the rational design of novel robust therapeutic
interventions based on our increasing knowledge of complex gene signaling
networks

### Exciton dynamics in semiconductor confined systems

The purpose of my thesis is to provide a theoretical analysis of the dynamics of optically excited carriers in semiconductor confined systems. In particular, I will focus the investigations on the effects due to the presence of a strong electron-hole Coulomb correlation. The Coulomb interaction leads to the formation of hydrogen-like bound states called excitons. The dynamics of these states is investigated in bare and cavity embedded quantum wells, and in quantum wires. I have investigated the dynamics of the relaxation of excitons in quantum wells due to the interaction with acoustic phonons and I have reproduced the temporal evolution of the photoluminescence emission. I have explained why the decay times observed in non resonant photoluminescence experiments are much slower than the radiative recombination time of a single exciton. Furthermore, deviation from the thermal equilibrium gives a characteristic dependence of these decay times on the temperature. The build-up of the photoluminescence is related to the relaxation by phonon emission of the excited electron-hole pairs. Initially, the pairs created at high energy are not bound, and the formation of bound excitons occurs during the relaxation. I have described the exciton formation due to the emission of acoustic and optical phonons, and I have calculated the characteristic times for this process in GaAs quantum wells. The formation can be geminate or bimolecular. In the geminate formation, the exciton is directly created by the photon of the external pump by simultaneous emission of an optical phonon, while in the bimolecular formation the exciton is created from thermalized electron-hole pairs. In the first case the formation occurs only during the laser pump, while in the second case the formation depends on the total density of carriers available in the crystal. The effects of the phonon assisted formation on the overall dynamics of free carriers, excitons and of the photoluminescence are discussed. Excitons confined in a quantum well coupled with the photons modes of a semiconductor microcavity gives mixed exciton-photon states called microcavity polaritons. The dynamics of the population of polaritons, which present an energy dispersion with an upper and lower branch, shows peculiar characteristics. In particular, a bottleneck region above the minimum of the lower polariton dispersion exists. In this region the population of polaritons is accumulated, and strong deviations from the thermal equilibrium are thus produced. Moreover, for the lower polariton states, a suppression of the scattering by acoustic phonons produces a strong inhibition of the thermal broadening. Finally, the dynamics of the photoluminescence spectra in semiconductor quantum wires is investigated. The time resolved photoluminescence experiments, the radiative recombination modifies adiabatically the total density, and the hypothesis quasi-thermal equilibrium can be applied. The optical spectra probes a system which ranges from a regime of ionized electron hole plasma to a gas of weakly interacting bound excitons. In order to describe such a complex system, I have used a Green's functions technique, and I have modeled the Coulomb interaction using a contact potential. This approach allows to observe in the absorption spectra a gain region close to the excitonic resonance. Moreover, the effects of the electron-hole correlations, calculated in a numerical self consistent way, explain the absence of the shift of the photoluminescence emission due to Coulomb induced band gap renormalization

### Transient dynamics of subradiance and superradiance in open optical ensembles

We introduce a computational Maxwell-Bloch framework for investigating out of
equilibrium optical emitters in open cavity-less systems. To do so, we compute
the pulse-induced dynamics of each emitter from fundamental light-matter
interactions and self-consistently calculate their radiative coupling,
including phase inhomogeneity from propagation effects. This semiclassical
framework is applied to open systems of quantum dots with different density and
dipolar coupling. We observe that signatures of superradiant behavior, such as
directionality and faster decay, are weak for systems with extensions
comparable to $\lambda/2$. In contrast, subradiant features are robust and can
produce long-term population trapping effects. This computational tool enables
quantitative investigations of large optical ensembles in the time domain and
could be used to design new systems with enhanced superradiant and subradiant
properties.Comment: 5 pages, 5 figure

### Photovoltaic Effect in Bent Quantum Wires in the Ballistic Transport Regime

A scheme for the generation of a photocurrent in bent quantum wires is proposed. We calculate the current using a generalized Landauer-BÃ¼ttiker approach that takes into account the electromagnetic radiation. For circularly polarized light, it is demonstrated that the curvature in the bent wire induces an asymmetry in the scattering coefficients for left and right moving electrons. This asymmetry results in a current at zero bias voltage. The effect is due to the geometry of the wire which transforms the photon angular momentum into translational motion for the electrons. Possible experimental realizations of this scheme are discussed

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