Adiabatic optical entanglement between electron spins in separate quantum dots

We present an adiabatic approach to the design of entangling quantum operations with two electron spins localized in separate InAs/GaAs quantum dots via the Coulomb interaction between optically-excited localized states. Slowly-varying optical pulses minimize the pulse noise and the relaxation of the excited states. An analytic "dressed state" solution gives a clear physical picture of the entangling process, and a numerical solution is used to investigate the error dynamics. For two vertically-stacked quantum dots we show that, for a broad range of dot parameters, a two-spin state with concurrence $C>0.85$ can be obtained by four optical pulses with durations $\sim 0.1 - 1$ ns.Comment: 7 pages, 5 figure

Topologically protected excitons in porphyrin thin films

The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyze a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogenous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.Comment: Submitted early March 2014 to a journal; currently in revisio

Fast initialization of the spin state of an electron in a quantum dot in the Voigt configuration

We consider the initialization of the spin-state of a single electron trapped in a self-assembled quantum dot via optical pumping of a trion level. We show that with a magnetic field applied perpendicular to the growth direction of the dot, a near-unity fidelity can be obtained in a time equal to a few times the inverse of the spin-conserving trion relaxation rate. This method is several orders-of-magnitude faster than with the field aligned parallel, since this configuration must rely on a slow hole spin-flip mechanism. This increase in speed does result in a limit on the maximum obtainable fidelity, but we show that for InAs dots, the error is very small.Comment: 4 pages, 4 figure

A drift-diffusion model for spin-polarized transport in a two-dimensional non-degenerate electron gas controlled by spin-orbit interaction

We apply the Wigner function formalism to derive drift-diffusion transport equations for spin-polarized electrons in a III-V semiconductor single quantum well. The electron spin dynamics is controlled by the spin-orbit interaction which is linear in the momentum. In the transport regime studied, the electron momentum scattering rate is appreciably faster than the spin dynamics. A set of transport equations is defined in terms of a particle density, a spin density, and the respective fluxes. The model developed allows study of the coherent dynamics of a non-equilibrium spin polarization. As an example, we consider a stationary transport regime for a heterostructure grown along the (0, 0, 1) crystallographic direction. Due to the interplay of the Rashba and Dresselhaus spin-orbit terms, the spin dynamics strongly depends on the transport direction. The model is consistent with the results of pulse-probe measurements of the spin coherence in strained semiconductor layers. It can be useful in studying properties of spin-polarized transport and modelling spintronic devices operating in the diffusive transport regime

Monte Carlo modeling of spin injection through a Schottky barrier and spin transport in a semiconductor quantum well

We develop a Monte Carlo model to study injection of spin-polarized electrons through a Schottky barrier from a ferromagnetic metal contact into a non-magnetic low-dimensional semiconductor structure. Both mechanisms of thermionic emission and tunneling injection are included in the model. Due to the barrier shape, the injected electrons are non-thermalized. Spin dynamics in the semiconductor heterostructure is controlled by the Rashba and Dresselhaus spin-orbit interactions and described by a single electron spin density matrix formalism. In addition to the linear term, the third order term in momentum for the Dresselhaus interaction is included. Effect of the Schottky potential on the spin dynamics in a 2 dimensional semiconductor device channel is studied. It is found that the injected current can maintain substantial spin polarization to a length scale in the order of 1 micrometer at room temperature without external magnetic fields.Comment: 18 pages, 4 figures, J. Appl. Phys., accepted for publicatio

Theory of isotopic effects in the optical spectra of lanthanide ions in crystals

Physical mechanisms contributing to the isotopic structure of spectra of the insulating activated crystals induced by the isotopic disorder in the host crystal or in a system of the impurity optical centers is considered. The microscopic theory of the geometrical anharmonic isotopic effect is presented. We show that the local deformation due to a mass defect in the crystal lattice is dominant in the formation of the fine specific spectral structure in crystals with the proper inhomogeneous isotopic composition (such as 6Li c 7Li 1-cYF 4:Ho with the natural abundance of c equals 7.42% of the 6Li isotope). It contributes essentially to the observed shifts of the spectral lines in the case of different masses of the active optical centers (such as 164Er, 166Er, 168Er and 170Er isotopes in the LiYF 4 crystal) as well. Detailed calculations of crystal field parameters and of their first and second derivatives with respect to the lattice strains for the impurity lanthanide ions in lithium-yttrium double fluorides are performed within the framework of the exchange charge model and used in the estimations of the different contributions to the isotope-induced shifts of the Stark sublevels

Near-field Raman dichroism of azo-polymers exposed to nanoscale dc electrical and optical poling

© 2016 The Royal Society of Chemistry.Azobenzene-functionalized polymer films are functional materials, where the (planar vs. homeotropic) orientation of azo-dyes can be used for storing data. In order to characterize the nanoscale 3D orientation of the pigments in sub-10 nm thick polymer films we use two complementary techniques: polarization-controlled tip-enhanced Raman scattering (TERS) microscopy and contact scanning capacity microscopy. We demonstrate that the homeotropic and planar orientations of the azo-dyes are produced by applying a local dc electrical field and a resonant longitudinal optical near-field, respectively. For a non-destructive probe of the azo-dye orientation we apply a non-resonant optical near-field and compare the intensities of the Raman-active vibrational modes. We show that near-field Raman dichroism, a characteristic similar to the absorption dichroism used in far-field optics, can be a quantitative indicator of the 3D molecular orientation of the azo-dye at the nanoscale. This study directly benefits the further development of photochromic near-field optical memory that can lead to ultrahigh density information storage

Modelling for semiconductor spintronics

The authors summarise semiclassical modelling methods, including drift-diffusion, kinetic transport equation and Monte Carlo simulation approaches, utilised in studies of spin dynamics and transport in semiconductor structures. As a review of the work by the authors' group, several examples of applications of these modelling techniques are presented. © IEE, 2005

Semiclassical transport models for semiconductor spintronics

We will summarize and consider several examples of applications of semiclassical approaches used in semiconductor spintronic device modeling. These include drift-diffusion models, kinetic transport equations and Monte Carlo simulation schemes