Jose M. Villas-Boas

Decoherence of rabi oscillations in a single quantum dot.

We develop a realistic model of Rabi oscillations in a quantum-dot photodiode. Based in a multiexciton density matrix formulation we show that for short pulses the two-level model fails and higher levels should be taken into account. This affects some of the experimental conclusions, such as the inferred efficiency of the state rotation (population inversion) and the deduced value of the dipole interaction. We also show that the damping observed cannot be explained using constant rates with fixed pulse duration. We demonstrate that the damping observed is in fact induced by an off-resonant excitation to or from the continuum of wetting layer states. Our model describes the nonlinear decoherence behavior observed in recent experiments.

Quantum interference and control of the optical response in quantum dot molecules

We discuss the optical response of a quantum molecule under the action of two lasers fields. Using a realistic model and parameters, we map the physical conditions to find three different phenomena reported in the literature: the tunneling induced transparency, the formation of Autler-Townes doublets, and the creation of a Mollow-like triplet. We found that the electron tunneling between quantum dots is responsible for the different optical regime. Our results not only explain the experimental results in the literature but also give insights for future experiments and applications in optics using quantum dots molecules.

Coherent control of the dynamics of a single quantum-dot exciton qubit in a cavity

In this work we demonstrate theoretically how to use external laser field to control the population inversion of a single quantum dot exciton qubit in a nanocavity. We consider the Jaynes-Cummings model to describe the system, and the incoherent losses were take into account by using Lindblad operators. We have demonstrated how to prepare the initial state in a superposition of the exciton in the ground state and the cavity in a coherent state. The effects of exciton-cavity detuning, the laser-cavity detunings, the pulse area and losses over the qubit dynamics are analyzed. We also show how to use a continuous laser pumping in resonance with the cavity mode to sustain a coherent state inside the cavity, providing some protection to the qubit against cavity loss.

Spin polarized photocurrent from a single quantum dot

In this paper we model the effects of applying strong polarized light on a self‐assembled single quantum dot photodiode. We use a density matrix formalism where electron and hole tunneling is introduced by rates obtained from a microscopic description. Our analysis shows that elliptical polarized light results in spin‐polarized photocurrents, and that sharp polarization reversal can be produced with increasing pulse area. This behavior can be used as a dynamical switch to invert the spin‐polarization of the extracted current.

Using quantum state protection via dissipation in a quantum-dot molecule to solve the Deutsch problem

The wide set of control parameters and reduced size scale make semiconductor quantum dots attractive candidates to implement solid-state quantum computation. Considering an asymmetric double quantum dot coupled by tunneling, we combine the action of a laser field and the spontaneous emission of the excitonic state to protect an arbitrary superposition state of the indirect exciton and ground state. As a by-product we show how to use the protected state to solve the Deutsch problem.

Atom-mediated effective interactions between modes of a bimodal cavity

We show a procedure for engineering effective interactions between two modes in a bimodal cavity. Our system consists of one or more two-level atoms, excited by a classical field, interacting with both modes. The two effective Hamiltonians have forms similar to beam-splitter and quadratic beam-splitter interactions. We also demonstrate that the nonlinear Hamiltonian can be used to prepare an entangled coherent state, also known as a multidimensional entangled coherent state, which has been pointed out as an important entanglement resource. We show that the nonlinear interaction parameter can be enhanced considering N independent atoms trapped inside a high-finesse optical cavity.

Intraband InAs/InAlGaAs/InP Quantum Dot Detectors for the MIR

In this work, the authors presented the results of a detailed optical characterization of specially designed stacked self-assembled InAs/InGaAlAs/InGaAs/InP QDIP structures grown by metalorganic vapor phase epitaxy. The QDIP active region consists of an InGaAs quantum well grown on InGaAlAs followed by a thin InGaAlAs layer on top of which the dots are nucleated. The ternary and the quaternary material are lattice matched to InP. The dots are then covered by a thin InP barrier which helps reducing the dark current and is more convenient to be grown at the same temperature as the dots themselves.

Polarization Dependence of Photocurrent in Quantum‐Dot Infrared Photodetectors

Through polarization dependence measurements of photocurrent together with theoretical calculations we were able to identify different intersubband transitions in InAs/InGaAs/InP quantum dot structures for infrared photodetectors and observe a 2D/0D hybrid behaviour of the dot structures.

Photoluminescent Properties of InAs Quantum Dots Grown by MOVPE on an InxAlyGa1−x−yAs Layer and their Dependence on the Layer Stoichiometry

In this paper we report on the growth, using MOVPE (Metal Organic Vapor Phase Epitaxy), of a layer of InAs quantum dots (QDs) on top of quaternary layers of In0.53AlyGa0.47−yAs lattice‐matched to InP substrates, with a variable Stoichiometry due to the variation in the Al concentration. It has been recently reported that as the Al concentration in the quaternary layer increases, smaller InAs QDs are obtained, and also that the peak of the luminescence emission shifts to longer wavelengths up to around 2.1 microns. In this work we present results on the PL characterization for a set of InAs QDs/In0.53AlyGa0.43−yAs samples, for aluminum concentrations of y = 0, 0.058, 0.11 and 0.165. PL spectra were measured changing the laser excitation power and the sample temperature in the range from 16K up to room temperature. From the analysis of the PL spectra we observe that at low temperatures the emission band is composed of two contributions centered at around 0.64 and 0.673 eV which shift to higher energies as t...

Optical Properties of InAs Quantum Dots Grown on Variable Stoichiometry InxGa1−xAs and In0.53AlyGa0.43−yAs Layers

The use of InP substrates has made possible to obtain InAs QD layers with room temperature photoluminescence (PL) in the range 2.0–2.2 μm. This last result was possible because of the shift to lower energies of the InAs QD energy bandgap due to the reduction in the strain field between the InAs and InP, as compared to the case for InAs and GaAs. To study the importance in the control of the strain field between the InAs dots and the beneath layer onto which they are grown, we have used two approximations: 1) Growth of InAs QDs layers on top of InxGa1−xAs layers with variable Stoichiometry; that is, with variable In concentration.; and 2) Growth of InAs QDs layers on top of InxAlyGa1−x−yAs layers with variable Al concentration. In both cases we have used the metal organic vapor phase epitaxy growth technique to grow four‐layer structures on top of InP substrates: the first layer being an InP buffer layer, followed by an InxGa1−xAs or InxAlyGa1−x−yAs layer, then a layer of InAs dot material, and finally cov...