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Register a new userSelf-assembly of laterally aligned GaAs quantum dot pairs
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We report the fabrication of self-assembled, strain-free GaAs/Al$_{0.27}$Ga$_{0.73}$As quantum dot pairs which are laterally aligned in the growth plane, utilizing the droplet epitaxy technique and the anisotropic surface potentials of the GaAs (100) surface for the migration of Ga adatoms. Photoluminescence spectra from a single quantum dot pair, consisting of a doublet, have been observed. Finite element energy level calculations of a model quantum dot pair are also presented.
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In strained heteroepitaxy, two-dimensional (2D) layers can exhibit a critical thickness at which three-dimensional (3D) islands self-assemble, relieving misfit strain at the cost of an increased surface area. Here we show that such a morphological phase transition can be induced on-demand using surfactants. We explore Bi as a surfactant in the growth of InAs on GaAs(110), and find that the presence of surface Bi induces Stranski-Krastanov growth of 3D islands, while growth without Bi always favors 2D layer formation. Exposing a static two monolayer thick InAs layer to Bi rapidly transforms the layer into 3D islands. Density functional theory calculations reveal that Bi reduces the energetic cost of 3D island formation by modifying surface energies. These 3D nanostructures behave as optically active quantum dots. This work illustrates how surfactants can enable quantum dot self-assembly where it otherwise would not occur.
Efficient coupling between solid state quantum emitters and plasmonic waveguides is important for the realization of integrated circuits for quantum information, communication and sensing. However, realization of plasmonic circuits is still scarce, particularly due to challenges associated with accurate positioning of quantum emitters near plasmonic resonators. Current pathways for the construction of plasmonic circuits involve cumbersome and costly methods such as scanning atomic force microscopy or mechanical manipulation, where individual elements are physically relocated using the scanning tip. Here, we introduce a simple, fast and cost effective chemical self-assembly method for the attachment of two primary components of a practical plasmonic circuit: a single photon emitter and a waveguide. Our method enables coupling of nanodiamonds with a single quantum emitter (the nitrogen-vacancy (NV) center) onto the terminal of a silver nanowire, by simply varying the concentration of ascorbic acid (AA) in a reaction solution. The AA concentration is used to control the extent of agglomeration, and can be optimised so as to cause preferential, selective activation of the tips of the nanowires. The nanowire-nanodiamond structures show efficient plasmonic coupling of fluorescence emission from single NV centers into surface plasmon polariton (SPP) modes, evidenced by a more than two-fold reduction in fluorescence lifetime and an increase in fluorescence intensity.
We report the fabrication and photoluminescence properties of laterally-coupled GaAs/AlGaAs quantum dots. The coupling in the quantum dot molecules is tuned by an external electric field. An intricate behavior, consisting of spectral line crossings and avoided crossings is observed for different molecules. Anticrossing patterns in the photoluminescence spectra provide direct evidence of the lateral coupling between two nearby quantum dots. A simple calculation suggests that the coupling is mediated by electron tunneling, through which the states of direct and indirect exciton are brought into resonance.
The spin polarization of electrons trapped in InAs self-assembled quantum dot ensembles is investigated. A statistical approach for the population of the spin levels allows one to infer the spin polarization from the measure values of the addition energies. From the magneto-capacitance spectroscopy data, the authors found a fully polarized ensemble of electronic spins above 10 T when $mathbf{B}parallel[001]$ and at 2.8 K. Finally, by including the g-tensor anisotropy the angular dependence of spin polarization with the magnetic field $mathbf{B}$ orientation and strength could be determined.
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 wavefunction 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.
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