Pair Of Quantum Dots Research Articles

Efficient Quantum Dot−Quantum Dot and Quantum Dot−Dye Energy Transfer in Biotemplated Assemblies

CdSe semiconductor nanocrystal quantum dots are assembled into nanowire-like arrays employing microtubule fibers as nanoscale molecular "scaffolds." Spectrally and time-resolved energy-transfer analysis is used to assess the assembly of the nanoparticles into the hybrid inorganic biomolecular structure. Specifically, we demonstrate that a comprehensive study of energy transfer between quantum dot pairs on the biotemplate and, alternatively, between quantum dots and molecular dyes embedded in the microtubule scaffold comprises a powerful spectroscopic tool for evaluating the assembly process. In addition to revealing the extent to which assembly has occurred, the approach allows determination of particle-to-particle (and particle-to-dye) distances within the biomediated array. Significantly, the characterization is realized in situ, without need for further sample workup or risk of disturbing the solution-phase constructs. Furthermore, we find that the assemblies prepared in this way exhibit efficient quantum dot-quantum dot and quantum dot-dye energy transfer that affords faster energy-transfer rates compared to densely packed quantum dot arrays on planar substrates and to small-molecule-mediated quantum dot-dye couples, respectively.

Insight into optical properties of strain-free quantum dot pairs

Self-assembled GaAs/AlGaAs quantum dot pairs (QDPs) are grown by molecular beam epitaxy using high temperature droplet epitaxy technique. A typical QDP consists of dual-size quantum dots as observed based on atomic force microscopy image. The average height of quantum dot is 5.7 nm for the large quantum dots and 4.6 nm for the small ones. The average peak-to-peak distance of the two dots is about 75 nm. The optical properties of GaAs QDPs are studied by measuring excitation power-dependent and temperature-dependent photoluminescence. Unique photoluminescence properties have been observed from both excitation power-dependent and temperature-dependent measurements. Excitation power-dependent as well as temperature-dependent PL measurements have suggested lateral exciton transfer in the QDPs.

Self‐induced coherence in a single pair of quantum dots

AbstractWe study self‐induced coherence (SIC) in a system composed of two coupled quantum dots (QDs). SIC consists in a coherent transfer of excitation between two systems (atoms or QDs) resulting from their collective interaction with the quantum electromagnetic vacuum. This leads to population trapping in a delocalized, optically inactive state. We focus on the effect of a difference in transition energies and coupling between the two emitters on the evolution of exciton occupation in the two QD system.

Effects of hydrostatic pressure and electric field on the nonlinear optical rectification of strongly confined electron–hole pairs in GaAs quantum dots

The effects of hydrostatic pressure and applied electric field on the optical properties of single electrons and interacting electron–hole pairs in semiparabolic quantum dots are studied using the density matrix formalism and the effective-mass approximation. The binding energies of the ground and the lower three excited states of confined excitons are calculated as functions of the confining potential radius (confinement strength). The nonlinear optical rectification between the ground and low-excited states is studied as a function of the hydrostatic pressure, the applied electric field, and the confinement strength. The results show that the resonant peak of the nonlinear optical rectification can be red or blue shifted by external perturbations like hydrostatic pressure, applied electric field, and the confinement strength.

Opportunities for single hole-spin control using delocalized states of quantum dot molecules

We present three unique spin properties of holes in the delocalized states of coupled pairs of InAs quantum dots: resonant changes in the hole g factor, antisymmetric hole molecular ground states, and hole spin mixing. These three properties suggest new opportunities for spin control that are unique to holes and further support holes as a competitive candidate for the storage and manipulation of quantum information.

Near-field optical observation of electric-field-induced fluorescence switching in laterally coupled quantum dots

We report on direct observation of spatial and optical luminescence switching (on and off condition) due to collapse of the coupling conditions of quantum dots (QDs) pair. In particular, one could observe that the relative intensities between two adjacent luminescence peaks and spatial images of individual QDs located closely nearby in a high-density self-assembled QD structure underwent a reversible change with respect to an external electric field.

Clustering in a self‐assembled CdTe/ZnTe quantum dot plane revealed by inter‐dot coupling

AbstractWe study probability of coupling between randomly distributed quantum dots (QDs) in self‐assembled CdTe/ZnTe heterostructures grown with different thicknesses of CdTe formation layer. Photoluminescence excitation spectra reveal a number of QD pairs involved in inter‐dot energy transfer. The statistics of dot pairs was collected at numerous spots of samples. It shows that the observed coupling is more frequent than that expected for a uniform random distribution of QDs.

Intersublevel Infrared Photodetector with Strain-Free GaAs Quantum Dot Pairs Grown by High-Temperature Droplet Epitaxy

Normal incident photodetection at mid infrared spectral region is achieved using the intersublevel transitions from strain-free GaAs quantum dot pairs in Al(0.3)Ga(0.7)As matrix. The GaAs quantum dot pairs are fabricated by high temperature droplet epitaxy, through which zero strain quantum dot pairs are obtained from lattice matched materials. Photoluminescence, photoluminescence excitation optical spectroscopy, and visible-near-infrared photoconductivity measurement are carried out to study the electronic structure of the photodetector. Due to the intersublevel transitions from GaAs quantum dot pairs, a broadband photoresponse spectrum is observed from 3 to 8 microm with a full width at half-maximum of approximately 2.0 microm.

Evolution of Various Nanostructures and Preservation of Self-Assembled InAs Quantum Dots During GaAs Capping

The fabrication of nanostructures such as quantum rods (QRDs), quantum dot pairs (QDPs), bridged QDPs, and dimpled QDs (DQDs) is achieved by an application of a shallow GaAs layer using solid source molecular beam epitaxy (MBE). More specifically, the shape transition and evolution process as well as the preservation of original dome shape of self-assembled InAs quantum dots (QDs) are studied during the thin capping of GaAs layers in terms of morphology and optical property. As QDs are required to be capped for their applications and optical characterizations, it is critical to understand the mechanism during the capping process. QDs go through a significant shape transition and eventually evolve into various nanostructures at temperatures between 500°C and 400°C, while the original dome shape can be maintained at temperatures below 360°C. The shape evolutions are mainly driven by diffusion process induced by redistribution of surface chemical potential and strain matrix. Photoluminescence (PL) results show a blue-shift with increasing capping temperature for a fixed capping thickness, which adequately matches with morphological evolution.

InAs Quantum Dots on Nanopatterned GaAs (001) Surface: The Growth, Optical Properties, and Device Implementation

This work reviews the growth, optical properties, and device implementation of InAs semiconductor quantum dots (QDs) selectively nucleated atop nano-faceted GaAs pyramidal structures grown by metal-organic chemical-vapor deposition. This site-controlled QD growth technique offers unique features and benefits subsequent device applications. First, GaAs pyramids, formed on a SiO2-masked and nanopatterned (001) GaAs substrate, are characterized by well-defined equilibrium crystal shapes (ECS). The controlled ECS formation is crucial to subsequent patterned QD (PQD) nucleation, which is highly preferential toward {11n} planes due to superior QD energy minimization. Second, PQD optical properties are thoroughly studied by monitoring room temperature (RT) photoluminescence (PL) and power-dependent low-temperature PL to verify the crystalline quality and the three-dimensional quantum confinement. The PQD ensembles formed on different GaAs pyramids, with different PL behavior, indicate lateral coupling within QD clusters and characteristics of isolated QD pairs. Then, PQDs embedded in a InGaAs quantum well, the so-called dot-in-the-well (DWELL) structure, are compared to a self-assembled QD (SAQD) DWELL structure, and shown to have a better emission efficiency, due to the distributed QW thickness profile on the faceted GaAs pyramid. Finally, broad-area light emitting diodes based on the PQD active region are fabricated. Linear current-voltage characteristics are observed with sharp turn-on, low leakage current and low forward resistance. Electroluminescence spectra show PQD intraband structure and low quenching of emission from 77 K to room temperature. Light-current measurements demonstrate external quantum efficiency per PQD comparable to SAQDs, despite the two regrowth steps in the fabrication processes.

Effect of imperfections on the tunneling enhancement phenomenon in symmetric double quantum dots

The conductance of a pair of quantum dots, coupled through a tunnel barrier and connected to two external leads, exceeds the conductance of the tunnel barrier alone (tunneling enhancement effect) if the device is symmetrical, while it strongly decreases if the symmetry is destroyed. This device could then be used to implement a sensitive detector of symmetry breaking quantities, such as magnetic fields. We present a numerical study of the robustness of this phenomenon to the presence of imperfections. We find that, while a realistic amount of edge roughness in the depletion gates defining the structure does not compromise the enhancement effect significantly, and also lithographic errors can be compensated by properly tuning the voltages applied to the gates, the presence of randomly located ionized dopants in the heterostructure can strongly degrade the conductance enhancement and thus particular care has to be taken in terms of cleanliness and mobility of the heterostructure.

Emission properties of single InAs/GaAs quantum dot pairs and molecules grown in GaAs nanoholes

We have studied the lateral coupling between InAs/GaAs quantum dot pairs embedded in a field-effect structure. Quantum dot pairs and molecules have been identified by the correlated evolution of the Coulomb blockade features of each QD in the pair. This behaviour is largely distorted in the presence of resonant coupling states in the QD molecule. Single QD voltage evolution shows a crossover in the lineshape profile, which is associated to Spectral Diffusion processes due to residual charged environment.

Ultra‐fast nano‐optics

AbstractUltra‐fast nano‐optics is a comparatively young and rapidly growing field of research aiming at probing, manipulating and controlling ultrafast optical excitations on nanometer length scales. This ability to control light on nanometric length and femtosecond time scales opens up exciting possibilities for probing dynamic processes in nanostructures in real time and space. This article gives a brief introduction into the emerging research field of ultrafast nano‐optics and discusses recent progress made in it. A particular emphasis is laid on the recent experimental work performed in the authors' laboratories. We specifically discuss how ultrafast nano‐optical techniques can be used to probe and manipulate coherent optical excitations in individual and dipole‐coupled pairs of quantum dots, probe the dynamics of surface plasmon polariton excitations in metallic nanostructures, generate novel nanometer‐sized ultrafast light and electron sources and reveal the dipole interaction between excitons and surface plasmon polaritons in hybrid metal‐semiconductor nanostructures. Our results indicate that such hybrid nanostructures carry significant potential for realizing novel nano‐optical devices such as ultrafast nano‐optical switches as well as surface plasmon polariton amplifiers and lasers.

Tuning the emission profiles of various self-assembled InxGa1−xAs nanostructures by rapid thermal annealing

Tuning the emission profiles of various novel InxGa1−xAs nanostructures, such as quantum rods, quantum dot pairs (QDPs), bridged QDPs, dimpled quantum dots (QDs), and low-temperature-capped QDs, is demonstrated by postgrowth rapid thermal annealing. Specifically, improved optical properties, such as a much narrower full width at half maximum of 16 meV and a continuous blueshift, are demonstrated. The enhanced optical properties are attributed to the interchange of In and Ga atoms induced by both defect-assisted intermixing and strain-assisted intermixing. These results can find applications as an optical enhancement in nanostructures is critical for the improvements on device functionality.

A theoretical study of exciton energy levels in laterally coupled quantum dots

A theoretical study of the electronic and optical properties of laterally coupled quantumdots, under applied magnetic fields perpendicular to the plane of the dots, is presented.The exciton energy levels of such laterally coupled quantum-dot systems, together with thecorresponding wavefunctions and eigenvalues, are obtained in the effective-massapproximation by using an extended variational approach in which the magnetoexcitonstates are simultaneously obtained. One achieves the expected limits of one single quantumdot, when the distance between the dots is zero, and of two uncoupled quantum dots, whenthe distance between the dots is large enough. Moreover, present calculations—withappropriate structural dimensions of the two-dot system—are shown to be in agreementwith measurements in self-assembled laterally aligned GaAs quantum-dot pairs andnaturally/accidentally occurring coupled quantum dots in GaAs/GaAlAs quantumwells.

Theory of nonlinear optical response of ensembles of double quantum dots

We study theoretically the time-resolved four-wave mixing (FWM) response of an ensemble of pairs of quantum dots undergoing radiative recombination. At short (picosecond) delay times, the response signal shows beats that may be dominated by the subensemble of resonant pairs, which gives access to the information on the interdot coupling. At longer delay times, the decay of the FWM signal is governed by two rates which result from the collective interaction between the two dots and the radiation modes. The two rates correspond to the subradiant and super-radiant components in the radiative decay. Coupling between the dots enhances the collective effects and makes them observable even when the average energy mismatch between the dots is relatively large.

Sub-picosecond All-optical Switching of Exciton Spin Polarization in Nanocrystal Quantum Dots Toward Terabit Communication

We report all-optical polarization switching based on resonant excitation of spin-polarized e-h pairs in CdSe nanocrystal quantum dots at room temperature. Pump-probe experiments have been performed using the same and the opposite configurations of circular polarizations. Comparing the co- and the cross-circular polarization cases, it is quite evident that selectively-excited spins are flipped to a balanced state within 1 ps and that during this time, the two spin states are equally populated. The all-optical switching results are found to be in close agreement with pump-probe experimental results. The optical switching technique proposed in the present investigation should be extremely useful for future ultrafast terabit communication systems.

Towards Quantum Computing: A Hybrid Approach That Will Unleash a Plethora of New QD Nanostructures, Bringing Us a Step Further to Laterally Coupled QDs

While the properties of individual quantum dots (QDs) are deemed attractive for applications in quantum computing, practically, they are insufficient. However, being able to group QDs to enable communication among them (quantum dot molecules—QDMs) is still extremely challenging although this field has been studied for about 20 years. Therefore, in an attempt to control the lateral ordering of QDs, researchers at the University of Arkansas have developed a simple hybrid technique to fabricate QD pairs (QDPs) and QDMs. This technique has shown promise and paves the way for investigating possible interactions between QDs, taking us one step further to realizing quantum computing.

Performance of a quantum teleportation protocol based on temporally resolved photodetection of collective spontaneous emission

Recently a conditional quantum teleportation protocol has been proposed by Chen et al. [New J. Phys. 7, 172 (2005)] which is based on the collective spontaneous emission of a photon from a pair of quantum dots. We formulate a similar protocol for collective emission from a pair of atoms, one of which is entangled with a single mode of an optical cavity. We focus on the performance of the protocol as characterized by the fidelity of the teleported state and the overall success probability. We consider a strategy employing temporally resolved photodetection of the emitted photon in order to distinguish super-radiant from subradiant emission on the basis of a single detected photon. We find the fidelity approaches unity as the spacing of the atoms becomes much smaller than the emission wavelength with a success probability of 0.25 assuming perfect collection and detection of the emitted light. Incorporating finite collection and detection efficiency we find the protocol will still exceed a fidelity of 2/3 provided that the combined collection and detection efficiency is at least 3/4. We describe a cavity QED realization which could allow these requirements to be met using current experimental techniques.

A quantification of the interaction and spatial ordering in nano-arrays

This paper reports on the use of a local order measure to quantify the spatial ordering of a quantum dot array (QDA). By means of electron ground state energy analysis in a quantum dot pair, it is demonstrated that the length scale required for such a measure to characterize the opto-electronic properties of a QDA is of the order of a few QD radii. Therefore, as local order is the primary factor that affects the opto-electronic properties of an array of quantum dots of homogeneous size, this order was quantified through using the standard deviation of the nearest neighbor distances of the quantum dot ensemble. The local order measure is successfully applied to quantify spatial order in a range of experimentally synthesized and numerically generated arrays of nanoparticles. This measure is not limited to QDAs and has wide ranging applications in characterizing order in dense arrays of nanostructures.