A. M. Alcalde

Robust states in semiconductor quantum dot molecules

Semiconductor quantum dots coherently driven by pulsed laser are fundamental physical systems which allow studying the dynamical properties of confined quantum states. These systems are attractive candidates for a solid-state qubit, which open the possibility for several investigations in quantum-information processing. In this work we study the effects of a specific decoherence process, the spontaneous emission of excitonic states, in a quantum dot molecule. We model our system considering a three-level Hamiltonian and solve the corresponding master equation in the Lindblad form. Our results show that the spontaneous emission associated with the direct exciton helps to build up a robust indirect exciton state. This robustness against decoherence allows potential applications in quantum memories and quantum gate architectures. We further investigate several regimes of physical parameters, showing that this process is easily controlled by tuning of external fields.

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.

Interaction of a focused laser beam and a coaxial powder jet in laser surface processing

Laser cladding, laser alloying, and laser free-form manufacturing are promising techniques where a laser beam is frequently used in conjunction with a coaxial powder jet for surface treating or rapid manufacturing. In the present work, a computational model was developed and used to simulate several experimental setups in order to optimize the processing conditions. More specifically, the model was used to calculate the distribution of particles temperature and the transmitted laser beam power as a function of the position of the origin of the powder jet in relation to the converging lens that focuses the laser beam. Two cases were analyzed, the first assuming that the powder density varies with the inverse of the square of the distance, and the second one considering a Gaussian powder density distribution. The intensity distribution in the laser beam was assumed to be Gaussian. In the present work, it was shown that when the characteristic parameters of the laser beam and of the powder jet are kept constant, the distance between the lens and the powder origin determines the particles temperature distribution and the laser beam attenuation and it is an important aspect in the optimization of energy transfer in the process.Laser cladding, laser alloying, and laser free-form manufacturing are promising techniques where a laser beam is frequently used in conjunction with a coaxial powder jet for surface treating or rapid manufacturing. In the present work, a computational model was developed and used to simulate several experimental setups in order to optimize the processing conditions. More specifically, the model was used to calculate the distribution of particles temperature and the transmitted laser beam power as a function of the position of the origin of the powder jet in relation to the converging lens that focuses the laser beam. Two cases were analyzed, the first assuming that the powder density varies with the inverse of the square of the distance, and the second one considering a Gaussian powder density distribution. The intensity distribution in the laser beam was assumed to be Gaussian. In the present work, it was shown that when the characteristic parameters of the laser beam and of the powder jet are kept const...

Manipulation of g-factor in diluted magnetic semiconductors quantum dots: Optical switching control

We propose an optical switch based on a spin tuning mechanism in diluted-magnetic semiconductor quantum dots. At certain critical magnetic field, Bc, the Zeeman spin-splitting energies can cross leading to a zero value of the effective electron g-factor and the Fermi level undergoes a spin-flip transition. Magneto-optical switching is obtained for magnetic fields below and above Bc. Correlations between Bc, confinement shapes, dot sizes, and host material compositions have been established within well-defined temperature and magnetic impurity composition ranges. The generality of the presented theoretical framework allows for its application to magnetic field controlled quantum dot arrays, and spin-injection among others.

Surface phonons modes: a tool to determine the quantum dot morphology

We report here theoretical and experimental studies on the spatial confinement of phonons in ternary CdSxSe1ix nanocrystals embedded in a glass matrix formed by the composites SiO2-Na2CO3-B2O3-Al2O3 doped with CdO, S and Se. We determined the morphologic characteristics of the nanocrystals by analyzing the dependence of surface phonon modes on the geometrical parameters. The calculated frequencies are compared with values from Raman spectra of CdSxSe1ix nanocrystals grown under different thermal treatments. A good correlation between experimental and calculated CdS-like and CdSe-like surface optical modes is observed. Raman selection rules and their connection with the nature of the surface optical phonons is discussed.

Finite element method for electronic properties of semiconductor nanocrystals

The finite element method (FEM) has been implemented in order to investigate the electronic structure of spherical quantum dots (SQDs) in an external magnetic field. The Schrodinger equation has been discretized by means of Galerkin’s weighted residue method with a nonuniform mesh of triangular elements. Unlike other approaches, the computational effort required to obtain converged results is independent of the strength of the magnetic field. Since the basis functions are given by strictly local polynomials in real space, FEM allows a controlled convergence of the solutions. The effects of the diamagnetic term on the energy levels and their reordering produced by state crossing for semiconductor metal oxide quantum dots in alkaline aqueous colloids, and CdTe SQDs embedded in a glass matrix, have been discussed. The efficiency and accuracy of FEM have been shown by its successful applications to a single SQD, two-coupled SQDs, and a hydrogenic impurity in SQDs.

Exchange interaction and the tunneling induced transparency in coupled quantum dots

We investigate the optical response of quantum dot molecules coherently driven by polarized laser light. Our description includes the splitting in excitonic levels caused by isotropic and anisotropic exchange interactions. We consider interdot transitions mediated by hole tunneling between states with the same total angular momentum and between bright and dark exciton states as allowed by spin-flip hopping between the dots in the molecule. Using realistic experimental parameters we demonstrate that the excitonic states coupled by tunneling exhibit a rich and controllable optical response. We show that through the appropriate control of an external electric field and light polarization, the tunneling coupling establishes an efficient destructive quantum interference path that creates a transparency window in the absorption spectra whenever states of appropriate symmetry are mixed by the carrier tunneling. We explore the relevant parameter space that allows probing this phenomenon in experiments. Controlled variation in applied field and laser detuning would allow the optical characterization of spin-preserving and spin-flip hopping amplitudes in such systems by measuring the width of the tunneling-induced transparency windows.

Magneto-optical properties in IV-VI lead-salt semimagnetic nanocrystals

We present a systematic study of lead-salt nanocrystals (NCs) doped with Mn. We have developed a theoretical simulation of electronic and magneto-optical properties by using a multi-band calculation including intrinsic anisotropies and magnetic field effects in the diluted magnetic semiconductor regime. Theoretical findings regarding both broken symmetry and critical phenomena were studied by contrasting two different host materials (PbSe and PbTe) and changing the confinement geometry, dot size, and magnetic doping concentration. We also pointed out the relevance of optical absorption spectra modulated by the magnetic field that characterizes these NCs.

Phonon modulation of the spin-orbit interaction as a spin relaxation mechanism in InSb quantum dots

We calculate the spin relaxation rates in a parabolic InSb quantum dots due to the spin interaction with acoustical phonons. We considered the deformation potential mechanism as the dominant electron-phonon coupling in the Pavlov-Firsov spin-phonon Hamiltonian. By studying suitable choices of magnetic field and lateral dot size, we determine regions where the spin relaxation rates can be practically suppressed. We analyze the behavior of the spin relaxation rates as a function of an external magnetic field and mean quantum dot radius. Effects of the spin admixture due to Dresselhaus contribution to spin-orbit interaction are also discussed.

Excitonic entanglement of protected states in quantum dot molecules

Abstract The entanglement of an optically generated electron–hole pair in artificial quantum dot molecules is calculated considering the effects of decoherence by interaction with environment. Since the system evolves into mixed states and due to the complexity of energy level structure, we use the negativity as entanglement quantifier, which is well defined in D ⊗ D ′ composite vector spaces. By a numerical analysis of the non-unitary dynamics of the exciton states, we establish the feasibility of producing protected entangled superposition by an appropriate tuning of bias electric field, F . A stationary state with a high value of negativity (high degree of entanglement) is obtained by fine tuning of F close to a resonant condition between indirect excitons. We also found that when the optical excitation is approximately equal to the electron tunneling coupling, Ω / T e ∼ 1 , the entanglement reaches a maximum value. In front of the experimental feasibility of the specific condition mentioned before, our proposal becomes an useful strategy to find robust entangled states in condensed matter systems.