Chain Of Quantum Dots Research Articles

Oscillation threshold of a chain of spasers

A model of resonance interaction of a chain of periodically spaced metal nanoparticles with a chain of quantum dots is developed. The conditions leading to oscillation in a chain of spasers in the vicinity of narrow plasmon resonances are determined. It is shown that the lowest oscillation threshold can be achieved under anomalously small values of population inversion. In this case, the range of oscillation frequency variation upon changing the resonance frequency of quantum dots can be substantially narrower that the plasmon-resonance linewidth.

Electro-optic switch based on near-field-coupled quantum dots

The propagation of exciton polaritons in near-field-coupled quantum-dot (QD) chains is modeled by a density-matrix formalism. It is shown that at least for low-temperature operation it is possible using electronically controlled switching by the quantum-confined Stark effect in such QD chains to rival and outperform room-temperature CMOS electronics in footprint and switch energy, though not in speed.

Ge/Si Heterostructures with Dense Chains of Stacked Quantum Dots

An overview of new results on growth and characterization of Ge/Si(001) heterostructures with dense chains of stacked Ge quantum dots is reported. Ge hut nucleation and growth at low temperatures is discussed on the basis of results obtained by high resolution scanning tunneling microscopy and in-situ reflected high-energy electron diffraction. Atomic-level models of nucleating and growing huts are considered. Recent data of high resolution transmission electron microscopy are presented focusing on long chains of Ge quantum dots synthesised in silicon matrix by means of molecular beam epitaxy at low-temperature mode. Approaches to formation of three dimensional ordered arrays of Ge cluster and Ge quantum dot crystals are considered in special detail.

Electronic spectrum and localization of electronic states in aperiodic quantum dot chains

The electronic energy spectra of aperiodic Thue-Morse, Rudin-Shapiro, and double-periodic quantum dot chains are investigated in the tight-binding approximation. The dependence of the spectrum on all parameters of a “mixed” aperiodic chain model is studied: the electronic energy at quantum dots and the hopping integrals. The electronic degree of localization in the chains under consideration is determined by analyzing the inverse participation ratio. Its spectral distribution and the dependence of the band-averaged degree of localization on these model parameters have been calculated. It is shown that a transition of the system’s sites to a resonant state in which the degree of electron localization decreases, while an overlap between the subbands occurs in the spectrum is possible when the parameters are varied.

Growth and temperature dependent photoluminescence of InGaAs quantum dot chains

We report a study of growth and photoluminescence from a single stack of MBE-grown In0.4Ga0.6As quantum dot chains. The InGaAs epilayers were grown at a low temperature so that the resulting surfaces remain flat with platelets even though their thicknesses exceed the critical thickness of the conventional Stranski–Krastanov growth mode. The flat InGaAs layers were then annealed at elevated temperatures to induce the formation of quantum dot chains. A reflection high energy electron diffraction study suggests that, when the annealing temperature is at or below 480°C, the surface of growth front remains flat during the periods of annealing and growth of a 10nm thick GaAs capping layer. Surprisingly, transmission electron microscopy images do indicate the formation of quantum dot chains, however, so the dot-chains in those samples may form from precursory platelets during the period of temperature ramping and subsequent capping with GaAs due to intermixing of group III elements. The optical emission from the quantum dot layer demonstrates that there is a critical annealing temperature of 480–500°C above which the properties of the low temperature growth approach are lost, as the optical properties begin to resemble those of quantum dots produced by the conventional Stranski–Krastanov technique.

Quantum photovoltaics in wire-dot-wire junctions

We developed an effective tight-binding modeling for photovoltaic junctions made of a finite quantum dot chain connected to two semi-infinite quantum wires. We simulated I-V responses under resonant monochromatic illumination in the case of a two-dot junction by means of the Green's function technique. We thus showed a striking property in these dot-wire architectures: the photocurrent increases under bias.

Discrete solitons in an array of quantum dots

We develop a theory for the interaction of classical light fields with an a chain of coupled quantum dots (QDs), in the strong-coupling regime, taking into account the local-field effects. The QD chain is modeled by a one-dimensional (1D) periodic array of two-level quantum particles with tunnel coupling between adjacent ones. The local-field effect is taken into regard as QD depolarization in the Hartree-Fock-Bogoliubov approximation. The dynamics of the chain is described by a system of two discrete nonlinear Schr\"{o}dinger (DNLS) equations for local amplitudes of the probabilities of the ground and first excited states. The two equations are coupled by a cross-phase-modulation cubic terms, produced by the local-field action, and by linear terms too. In comparison with previously studied DNLS systems, an essentially new feature is a phase shift between the intersite-hopping constants in the two equations. By means of numerical solutions, we demonstrate that, in this QD chain, Rabi oscillations (RO) self-trap into stable bright\textit{\ Rabi solitons} or \textit{Rabi breathers}. Mobility of the solitons is considered too. The related behavior of observable quantities, such as energy, inversion, and electric-current density, is given a physical interpretation. The results apply to a realistic region of physical parameters.

Anisotropic electro-optic effect on InGaAs quantum dot chain modulators

We investigated the anisotropic electro-optic (EO) effect on InGaAs quantum dot (QD) chain modulators. The linear EO coefficients were determined as 24.3 pm/V (33.8 pm/V) along the [011] direction and 30.6 pm/V (40.3 pm/V) along the [011¯] direction at 1.55 μm (1.32 μm) operational wavelength. The corresponding half-wave voltages (Vπs) were measured to be 5.35 V (4.35 V) and 4.65 V (3.86 V) at 1.55 μm (1.32 μm) wavelength. This is the first report on the anisotropic EO effect on QD chain structures. These modulators have 3 dB bandwidths larger than 10 GHz.

Bosonic transport through a chain of quantum dots

The particle transport through a chain of quantum dots coupled to two bosonic reservoirs is studied. For the case of reservoirs of non-interacting bosonic particles, we derive an exact set of stochastic differential equations, whose memory kernels and driving noise are characterised entirely by the properties of the reservoirs. Going to the Markovian limit an analytically solvable case is presented. The effect of interparticle interactions on the transient behaviour of the system, when both reservoirs are instantaneously coupled to an empty chain of quantum dots, is approximated by a semiclassical method, known as the Truncated Wigner approximation. The steady-state particle flow through the chain and the mean particle occupations are explained via the spectral properties of the interacting system.

Optimal control for electron shuttling

In this paper we apply an optimal control technique to derive control fields that transfer an electron between ends of a chain of donors or quantum dots. We formulate the transfer as an optimal steering problem, and then derive the dynamics of the optimal control. A numerical algorithm is developed to effectively generate control pulses. We apply this technique to transfer an electron between sites of a triple quantum dot and an ionized chain of phosphorus dopants in silicon. Using the optimal pulses for the spatial shuttling of phosphorus dopants, we then add hyperfine interactions to the Hamiltonian and show that a 500 G magnetic field will transfer the electron spatially as well as transferring the spin components of two of the four hyperfine states of the electron-nuclear spin pair.

Electron transport in quantum dot chains: Dimensionality effects and hopping conductance

Detailed experimental and theoretical studies of lateral electron transport in a system of quantum dot chains demonstrate the complicated character of the conductance within the chain structure due to the interaction of conduction channels with different dimensionalities. The one-dimensional character of states in the wetting layer results in an anisotropic mobility, while the presence of the zero-dimensional states of the quantum dots leads to enhanced hopping conductance, which affects the low-temperature mobility and demonstrates an anisotropy in the conductance. These phenomena were probed by considering a one-dimensional model of hopping along with band filling effects. Differences between the model and the experimental results indicate that this system does not obey the simple one-dimensional Mott's law of hopping and deserves further experimental and theoretical considerations.

Growth and characterization of cooperative quantum dot chains in quaternary InAsSbP material system

The cooperative quantum dot chains (CQDCs) are grown from In-As-Sb-P quaternary liquid phase on InAs(100) substrate with a deviation of surface orientation from (100) of about 0.3° along [010] direction. The wet chemical etching is utilized to create an additional artificial disorientation of the substrate. AFM investigations show that CQDCs mainly consist of central coupled InAsSb quantum dot (QD) sub-chains surrounded by InAsP-leaf chains. Cooperative chains have a ∼120 nm total width, over 5 μm length and directed along [010]. The separation between QDs within sub-chains is about 40 nm. The red shift of CQDCs’ absorption edge is detected.

Adaptive tuning of Majorana fermions in a quantum dot chain

We suggest a way to overcome the obstacles that disorder and high density of states pose to the creation of unpaired Majorana fermions in one-dimensional systems. This is achieved by splitting the system into a chain of quantum dots, which are then tuned to the conditions under which the chain can be viewed as an effective Kitaev model, so that it is in a robust topological phase with well-localized Majorana states in the outermost dots. The tuning algorithm that we develop involves controlling the gate voltages and the superconducting phases. Resonant Andreev spectroscopy allows us to make the tuning adaptive, so that each pair of dots may be tuned independently of the other. The calculated quantized zero bias conductance serves then as a natural proof of the topological nature of the tuned phase.

Association of scattering matrices in quantum networks

Algorithms based on operations that associate scattering matrices in series or in parallel (analogous to impedance association in a classical circuit) are developed here. We exemplify their application by calculating the total scattering matrix of several types of quantum networks, such as star graphs and a chain of chaotic quantum dots, obtaining results with good agreement with the literature. Through a computational-time analysis we compare the efficiency of two algorithms for the simulation of a chain of chaotic quantum dots based on series association operations of (i) two-by-two centers and (ii) three-by-three ones. Empirical results point out that the algorithm (ii) is more efficient than (i) for small number of open scattering channels. A direct counting of floating point operations justifies quantitatively the superiority of the algorithm (i) for large number of open scattering channels.

Selective area growth of InAs nanostructures on faceted GaAs microstructures by migration enhancedepitaxy

Selective area growth of InAs nanostructures on faceted GaAs microstructures by migration enhanced epitaxy is studied. By an optimization of the growth conditions and a study of the facet development, well-defined GaAs microstructures with facets mostly belonging to the crystal plane families {01n}, {11n} and (001) are fabricated. Subsequently deposited InAs shows a preferred nucleation on (001) top facets as well as on {012}/{013} and {011} facet boundaries. By the understanding of the InAs nucleation mechanism, precisely positioned In(Ga)As/GaAs nanostructures such as quantum dots and quantum dot chains are achieved. To investigate the interface properties, transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements are performed. The measurements reveal a relatively high incorporation of Ga into the InAs nanostructures even for InAs deposited at a low growth temperature of 470°C.

Temperature dependence of photoluminescence for site-controlled InAs/GaAs quantum dot chains

We study the temperature dependence of the photoluminescence (PL) from InAs quantum dot chains (QDC) grown by MBE on [011]- and [011¯]-oriented UV nanoimprint lithography processed groove patterns. We observe an increase of PL intensity from the [011¯]-oriented QDCs within the temperature range from 20–70K, which is attributed to thermally activated carrier transport from small quantum dots accumulated on the sidewalls of the [011¯]-oriented grooves to the quantum dots at the bottom of the groove. We utilize a rate equation model to quantitatively analyze the carrier transfer mechanism. Furthermore, we show that the defect related carrier loss mechanism, which accounts for weak PL quenching at low temperatures, is similar for QDCs and self-assembled quantum dots (SAQD) that were used as a reference. The carrier loss mechanism that causes the rapid quenching of SAQD PL at high temperatures is identified as exciton escape, while for the QDCs it is either single carrier escape or escape of uncorrelated electron–hole pairs. This result reveals a significant difference in the carrier dynamics of site-controlled QDCs and SAQDs.

Thermoelectric effects of a laterally coupled double-quantum-dot structure

We investigate the thermoelectric properties of a laterally coupled double-quantum-dot structure. For this structure, a one-dimensional quantum dot (QD) chain between two leads forms a main channel for electron transmission, and each QD in the chain laterally couples to an additional QD. It is found that at low temperature, similar insulating bands emerge around the antiresonant points in the electronic and thermal conductance spectra. And, the edges of the insulating bands become steep rapidly with the increase of QD numbers. What’s interesting is that striking thermoelectric effect exists in the energy region where the insulating bands appear. Furthermore, with the formation of the insulation bands, the magnitude of the Seebeck coefficient becomes stable, whereas the thermoelectric efficiency is increased. By plotting the Lorentz number spectrum, we observe that in such a structure, the Lorentz number strongly violates the Wiedemann-Franz law in the insulating-band region with its maximum at the point of antiresonance. When weak intradot Coulomb interaction is taken into account, the weakened thermoelectric effect can still be improved with the increase of QD numbers.

Construction of DNA-templated nanoparticle assemblies using click DNA ligation

We report a simple and versatile strategy for the construction of DNA-templated nanoparticle (NP) assemblies using click DNA ligation. Diels–Alder cycloaddition reaction of maleic acid and sorbic acid is used for click DNA ligation. Sorbic acid-prelabelled DNA nanowires with micron-scale length were synthesized and acted as templates for the construction of NP assemblies. Au NPs were orderly assembled onto the DNA nanowires using the click DNA ligation to form DNA-templated Au NP chains. Furthermore, DNA-templated CdTe quantum dot (QD) chains were synthesized using the method and characterized by confocal fluorescence microscopy (CFM). The DNA-templated CdTe QD chains were functionalized with cell-binding aptamer as the recognition element. The aptamer-functionalized CdTe QD chains, acting as a novel linear-shaped cellular probe, could be used for cancer cell imaging. Target cells could be rapidly and easily distinguished from a mixture of multiple cells using the cellular probe. The click DNA ligation method offers a convenient and efficient way to synthesize DNA-templated NP assemblies, and may become a powerful tool in other DNA-based studies.

Mixed states in Rabi waves and quantum nanoantennas

The mixed states of Rabi waves in a one-dimensional chain of two-level quantum dots (QDs) with tunnel coupling between neighbor QDs are theoretically considered. The propagation of Rabi waves is described by a $2\ifmmode\times\else\texttimes\fi{}2$ statistical operator depending on two spatial and one temporal variables. For a statistical operator the generalized Bloch equations are derived. The eigenmodes of the statistical operator are considered, the general solution of the initial problem is obtained, and the frequency spectrum of the induced current is investigated. The high-frequency part of the spectrum corresponds to a Mollow triplet, but is distinct from the last one as it is the continuous spectral band located on both sides of the transition frequency. The low-frequency part is caused by tunnel current and includes a dc-component and continuous ac-band in the vicinity of the Rabi oscillation frequency. The application of Rabi waves to the electrically controlled quantum nanoantennas of terahertz range is considered.

Fabrication of a self-aligned cross-wire quantum-dot chain light emitting diode by molecular beam epitaxial regrowth

The fabrication of a cross-wire p–i–n light emitting diode (LED) by molecular beam epitaxial overgrowth on mesa-patterned GaAs(100) substrates is presented. Micron-wide mesa stripes fabricated by standard photolithography are subsequently narrowed to sub-micron dimensions by GaAs overgrowth due to net migration towards the mesa top. Chains of InAs quantum dots (QDs) can then be grown in a self-aligned manner on top of the narrow GaAs ridge mesa, forming the active region of the QD-chain LED. The kinetics of the overgrowth is discussed and the electroluminescence operation of the LED is presented.