Ay Andrei Silov

Spatial structure of an individual Mn acceptor in GaAs.

The wave function of a hole bound to an individual Mn acceptor in GaAs is spatially mapped by scanning tunneling microscopy at room temperature and an anisotropic, crosslike shape is observed. The spatial structure is compared with that from an envelope-function, effective mass model and from a tight-binding model. This demonstrates that anisotropy arising from the cubic symmetry of the GaAs crystal produces the crosslike shape for the hole wave function. Thus the coupling between Mn dopants in GaMnAs mediated by such holes will be highly anisotropic.

Spatial Structure of Mn-Mn Acceptor Pairs in GaAs

The local density of states of Mn-Mn pairs in GaAs is mapped with cross-sectional scanning tunneling microscopy and compared with theoretical calculations based on envelope-function and tight-binding models. These measurements and calculations show that the crosslike shape of the Mn-acceptor wave function in GaAs persists even at very short Mn-Mn spatial separations. The resilience of the Mn-acceptor wave function to high doping levels suggests that ferromagnetism in GaMnAs is strongly influenced by impurity-band formation. The envelope-function and tight-binding models predict similarly anisotropic overlaps of the Mn wave functions for Mn-Mn pairs. This anisotropy implies differing Curie temperatures for Mn delta-doped layers grown on differently oriented substrates.

G-factors and diamagnetic coefficients of electrons, holes, and excitons in InAs/InP quantum dots

The electron, hole, and exciton g factors and diamagnetic coefficients have been calculated using envelope-function theory for cylindrical InAs/InP quantum dots in the presence of a magnetic field parallel to the dot symmetry axis. A clear connection is established between the electron g factor and the amplitude of those valence-state envelope functions that possess nonzero orbital momentum associated with the envelope function. The dependence of the exciton diamagnetic coefficients on the quantum dot height is found to correlate with the energy dependence of the effective mass. Calculated exciton g factor and diamagnetic coefficients, constructed from the values associated with the electron and hole constituents of the exciton, match experimental data well, however including the Coulomb interaction between the electron and hole states improves the agreement. Remote-band contributions to the valence-band electronic structure, included perturbatively, reduce the agreement between theory and experiment.

Size-dependent exciton g factor in self-assembled InAs/InP quantum dots

We have studied the size dependence of the exciton g factor in self-assembled InAs/InP quantum dots. Photoluminescence measurements on a large ensemble of these dots indicate a multimodal height distribution. Cross-sectional scanning tunneling microscopy measurements have been performed and support the interpretation of the macrophotoluminescence spectra. More than 160 individual quantum dots have systematically been investigated by analyzing single dot magnetoluminescence between 1200 and 1600 nm. We demonstrate a strong dependence of the exciton g factor on the height and diameter of the quantum dots, which eventually gives rise to a sign change of the g factor. The observed correlation between exciton g factor and the size of the dots is in good agreement with calculations. Moreover, we find a size-dependent anisotropy splitting of the exciton emission in zero magnetic field.

Controlling polarization anisotropy of site-controlled InAs/InP (100) quantum dots

We report on the shape and polarization control of site-controlled multiple and single InAs quantum dots (QDs) on InP pyramids grown by selective-area metal-organic vapor phase epitaxy. With increasing growth temperature the QDs elongate causing strong linear polarization of the photoluminescence. With reduced pyramid base/pyramid top area/QD number, the degree of polarization decreases, attributed to the symmetric pyramid top, reaching zero for single QDs grown at lower temperature. This control of linear polarization is important for entangled photon sources operating in the 1.55 μm wavelength region.

Spin-orbit-induced circulating currents in a semiconductor nanostructure.

Circulating orbital currents produced by the spin-orbit interaction for a single electron spin in a quantum dot are explicitly evaluated at zero magnetic field, along with their effect on the total magnetic moment (spin and orbital) of the electron spin. The currents are dominated by coherent superpositions of the conduction and valence envelope functions of the electronic state, are smoothly varying within the quantum dot, and are peaked roughly halfway between the dot center and edge. Thus the spatial structure of the spin contribution to the magnetic moment (which is peaked at the dot center) differs greatly from the spatial structure of the orbital contribution. Even when the spin and orbital magnetic moments cancel (for g=0) the spin can interact strongly with local magnetic fields, e.g., from other spins, which has implications for spin lifetimes and spin manipulation.

Circular polarization of luminescence caused by the current in quantum wells

The degree of circular polarization of photoluminescence from an n-type III–V-based [001] quantum well (QW) is calculated under an electric current flow in the well plane. It is shown that mixing of the states of light and heavy holes leads to circular polarization of photoluminescence during the propagation of light in the plane of the structure. The role of various terms that are linear in the wave vector in the electron energy spectrum is analyzed for the effects of spin orientation and emergence of circular polarization of radiation in the electric field.

Design of composite InAsP/InGaAs quantum wells for a 1.55 mu-m polarization independent semiconductor optical amplifier

We investigate a composite InAsP/InGaAs quantum well in which an 8 nm tensile strained InGaAs well is surrounded by two compressively strained InAsP layers which feature a 70:30 conduction band offset ratio. The composite quantum well is found to provide a high TM differential gain. The InAsP layers provide strain compensation while simultaneously shifting the band gap to the relevant 1.55 μm wavelength region and increasing the electron confinement. Composite InAsP/InGaAs quantum wells are a promising candidate for realizing a polarization independent semiconductor optical amplifier at 1.55 μm.

Geometric and compositional influences on spin-orbit induced circulating currents in nanostructures

Circulating orbital currents, originating from the spin-orbit interaction, are calculated for semiconductor nanostructures in the shape of spheres, disks, spherical shells, and rings for the electron ground state with spin oriented along a symmetry axis. The currents and resulting orbital and spin magnetic moments, which combine to yield the effective electron g factor, are calculated using a recently introduced formalism that allows the relative contributions of different regions of the nanostructure to be identified at zero magnetic field. For all these spherically or cylindrically symmetric hollow or solid nanostructures, independent of material composition and whether the boundary conditions are hard or soft, the dominant orbital current originates from intermixing of valence-band states in the electron ground state, circulates within the nanostructure, and peaks approximately halfway between the center and edge of the nanostructure in the plane perpendicular to the spin orientation. For a specific material composition and confinement character, the confinement energy and orbital moment are determined by a single size-dependent parameter for spherically symmetrical nanostructures, whereas they can be independently tuned for cylindrically symmetric nanostructures.

Activating photonic crystal membrane nanocavities by infiltrating with liquid crystals or luminescent colloidal nanocrystals

Liquid crystal (LC, Merk 5 CB) is infiltrated into active, InAs quantum dots embedded, InGaAsP membrane type nanocavities to investigate the possible effect of the LC orientation on active cavity tuning. The tuning is demonstrated thermally and thermo-optically. The thermal tuning showed that the cavity modes can be tuned in opposite directions and exhibits a sudden change at the clearing temperature. The mechanism relies on the existence of both ordinary and extraordinary refractive indices of the liquid crystal due to its molecular alignment inside the voids. It shows that the electric field distribution of cavity modes can have a substantial component parallel to the LC director. The average electric field orientation with respect to the LC orientation can be mode dependent, so that different modes can be dominated by either branch of the LCs refractive index. Thermo-optic tuning of the modes is obtained when the power of the excitation laser is increased from 40 μW to 460 μW. A large and a reversible blueshift of more than 10 nm of the cavity modes is observed which is attributed to temperature induced liquid transport. InGaAsP type of nanocavities, without InAs quantum dots were infiltrated with PbSe colloidal quantum dots to obtain a comparison of internal light sources either in the semiconductor or in the holes.