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Showing papers on "Quantum dot published in 2007"


Journal ArticleDOI
TL;DR: The two-photon luminescence microscopy imaging of human breast cancer cells with internalized carbon dots with pulsed laser excitation in the near-infrared is demonstrated.
Abstract: Carbon nanoparticles upon simple surface passivation exhibit bright photoluminescence. Reported here is a new finding that these carbon dots are also strongly two-photon luminescent with pulsed laser excitation in the near-infrared. The experimentally measured two-photon absorption cross-sections are comparable to those of the high-performance semiconductor quantum dots already available in the literature. The two-photon luminescence microscopy imaging of human breast cancer cells with internalized carbon dots is demonstrated.

1,902 citations


Journal ArticleDOI
15 Nov 2007-Nature
TL;DR: This work demonstrates a cavity-free, broadband approach for engineering photon–emitter interactions via subwavelength confinement of optical fields near metallic nanostructures and shows that efficient coupling is accompanied by more than 2.5-fold enhancement of the quantum dot spontaneous emission, in good agreement with theoretical predictions.
Abstract: Control over the interaction between single photons and individual optical emitters is an outstanding problem in quantum science and engineering. It is of interest for ultimate control over light quanta, as well as for potential applications such as efficient photon collection, single-photon switching and transistors, and long-range optical coupling of quantum bits. Recently, substantial advances have been made towards these goals, based on modifying photon fields around an emitter using high-finesse optical cavities. Here we demonstrate a cavity-free, broadband approach for engineering photon-emitter interactions via subwavelength confinement of optical fields near metallic nanostructures. When a single CdSe quantum dot is optically excited in close proximity to a silver nanowire, emission from the quantum dot couples directly to guided surface plasmons in the nanowire, causing the wire's ends to light up. Non-classical photon correlations between the emission from the quantum dot and the ends of the nanowire demonstrate that the latter stems from the generation of single, quantized plasmons. Results from a large number of devices show that efficient coupling is accompanied by more than 2.5-fold enhancement of the quantum dot spontaneous emission, in good agreement with theoretical predictions.

1,412 citations


Journal ArticleDOI
TL;DR: Providing the missing link between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy-transfer pairs, and light-emitting sources in nanoscale optoelectronics.
Abstract: Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. These molecular metals exhibit highly polarizable transitions and scale in size according to the simple relation EFermi/N1/3, predicted by the free-electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons, and these conduction-electron transitions are the low-number limit of the plasmon—the collective dipole oscillations occurring when a continuous density of states is reached. Providing the missing link between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy-transfer pairs, and light-emitting sources in nanoscale optoelectronics.

1,169 citations


Journal ArticleDOI
TL;DR: In this paper, a solution-processable core-shell quantum dots with a CdSe core and a ZnS or CdS/ZnS shell were used as emissive layers in the devices.
Abstract: Quantum-dot-based LEDs are characterized by pure and saturated emission colours with narrow bandwidth, and their emission wavelength is easily tuned by changing the size of the quantum dots. However, the brightness, efficiency and lifetime of LEDs need to be improved to meet the requirements of commercialization in the near future. Here, we report red, orange, yellow and green LEDs with maximum luminance values of 9,064, 3,200, 4,470 and 3,700 cd m−2, respectively, the highest values reported so far. Solution-processable core–shell quantum dots with a CdSe core and a ZnS or CdS/ZnS shell were used as emissive layers in the devices. By optimizing the thicknesses of the constituent layers of the devices, we were able to develop quantum-dot-based LEDs with improved electroluminescent efficiency (1.1–2.8 cd A−1), low turn-on voltages (3–4 V) and long operation lifetimes. These findings suggest that such quantum-dot-based LEDs will be promising for use in flat-panel displays.

1,046 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed spin qubits in graphene quantum dots and showed that in an array of many qubits it is possible to couple any two of them via Heisenberg exchange with the others being decoupled by detuning.
Abstract: The main characteristics of good qubits are long coherence times in combination with fast operating times. It is well known that carbon-based materials could increase the coherence times of spin qubits, which are among the most developed solid-state qubits. Here, we propose how to form spin qubits in graphene quantum dots. A crucial requirement to achieve this goal is to find quantum-dot states where the usual valley degeneracy in bulk graphene is lifted. We show that this problem can be avoided in quantum dots based on ribbons of graphene with armchair boundaries. The most remarkable new feature of the proposed spin qubits is that, in an array of many qubits, it is possible to couple any two of them via Heisenberg exchange with the others being decoupled by detuning. This unique feature is a direct consequence of the quasi-relativistic spectrum of graphene.

962 citations


Journal ArticleDOI
TL;DR: CdSe semiconductor nanocrystals and single-crystal ZnO nanowires are combined to demonstrate a new type of quantum-dot-sensitized nanowire solar cell that exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuits voltages of 0.5-0.6 V when illuminated with 100 mW/ cm2 simulated AM1.5 spectrum.
Abstract: We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot−nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5−0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50−60% were also obtained.

957 citations


Journal ArticleDOI
24 May 2007-Nature
TL;DR: This work develops core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures), which breaks the exact balance between absorption and stimulated emission, and allows for optical amplification due to single excitons.
Abstract: Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size—owing to the quantum-confinement effect—and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron–hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant (∼100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons. Semiconductor nanocrystals have very good light-emitting properties, so have potential as optical amplification media that can be easily processed with solution-based techniques: possible applications include optical interconnects in microelectronics, lab-on-a-chip technologies and quantum information processing. The problem with these structures is that at least two excitons (bound electron–hole pairs) need to be present in a nanocrystal before optical gain can be achieved, and this limits performance. In effect, the excitons annihilate each other before optical amplification can occur. This obstacle has now been overcome using nanocrystals with cores and shells made from different semiconductor materials, constructed in such a way that electrons and holes are separated from each other. This makes optical gain based on single excitons possible, significantly enhancing their promise as a practical optical material for laser applications. Semiconductor nanocrystals seem good candidates for 'soft' optical gain media, but optical gain and lasing is hard to achieve owing to a fundamental optical effect, which involves the problem that at least two excitons need to be present in a nanocrystal to achieve gain, and this limits performance. Here the problem is circumvented by designing nanocrystals with cores and shells made from different semiconductor materials, and in such a way that electrons and holes are separated from each other: this makes possible optical gain based on single excitons, thereby significantly enhancing the promise of semiconductor nanocrystals as practical optical materials for a wide range of lasing applications.

895 citations


Journal Article
TL;DR: The experimental realization of single electron spin rotations in a double quantum dot is reported, demonstrating the feasibility of operating single-electron spins in a quantum dot as quantum bits.
Abstract: The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.

865 citations


Journal ArticleDOI
TL;DR: The specific topics discussed here include the structure of NC electronic states, spectral signatures of multiexcitons in transient absorption and photoluminescence, exciton-exciton interaction energies, Auger recombination, and carrier multiplication.
Abstract: Because of the strong spatial confinement of electronic wave functions and reduced dielectric screening, the effects of carrier-carrier Coulomb interactions are greatly enhanced in semiconductor nanocrystals (NCs) compared with those in bulk materials. These interactions open a highly efficient decay channel via Auger recombination, which represents a dominant recombination pathway for multiexcitons in NCs. Furthermore, strong Coulomb coupling between charge carriers leads to extremely efficient direct photogeneration of multiexcitons by single photons via carrier (or exciton) multiplication. This review focuses on spectral and dynamical properties of multiexcitons in semiconductor NCs. The specific topics discussed here include the structure of NC electronic states, spectral signatures of multiexcitons in transient absorption and photoluminescence, exciton-exciton interaction energies, Auger recombination, and carrier multiplication. This chapter also briefly reviews the implications of multiexciton effects for practical technologies, such as NC lasing and photovoltaics.

829 citations


Journal ArticleDOI
TL;DR: Femtosecond transient absorption studies indicate that the rate constant for electron transfer from the thermalized s-state of CdSe quantum dots increases with decreasing particle size, which can be easily modulated by controlling the particle size.
Abstract: Electron injection from excited CdSe quantum dots into TiO2 nanoparticles can be easily modulated by controlling the particle size. Femtosecond transient absorption studies indicate that the rate constant for electron transfer from the thermalized s-state of CdSe quantum dots increases with decreasing particle size. The energy difference between the conduction bands of the two semiconductor systems acts as a driving force for the electron transfer in the normal Marcus region. An increase in the interparticle electron transfer rate constant by 3 orders of magnitude (from ∼107 to 1010 s-1) has been achieved by decreasing the CdSe particle diameter from 7.5 to 2.4 nm.

815 citations


Journal ArticleDOI
16 Sep 2007-Nature
TL;DR: Measurements provide both a method for probing the cavity–quantum dot system and a step towards the realization of quantum devices based on coherent light scattering and large optical nonlinearities from quantum dots in photonic crystal cavities.
Abstract: Solid-state cavity quantum electrodynamics (QED) systems offer a robust and scalable platform for quantum optics experiments and the development of quantum information processing devices. In particular, systems based on photonic crystal nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed the observation of weak and strong coupling regimes of interaction between the photonic crystal cavity and a single quantum dot in photoluminescence. In the weak coupling regime, the quantum dot radiative lifetime is modified; in the strong coupling regime, the coupled quantum dot also modifies the cavity spectrum. Several proposals for scalable quantum information networks and quantum computation rely on direct probing of the cavity–quantum dot coupling, by means of resonant light scattering from strongly or weakly coupled quantum dots. Such experiments have recently been performed in atomic systems and superconducting circuit QED systems, but not in solid-state quantum dot–cavity QED systems. Here we present experimental evidence that this interaction can be probed in solid-state systems, and show that, as expected from theory, the quantum dot strongly modifies the cavity transmission and reflection spectra. We show that when the quantum dot is coupled to the cavity, photons that are resonant with its transition are prohibited from entering the cavity. We observe this effect as the quantum dot is tuned through the cavity and the coupling strength between them changes. At high intensity of the probe beam, we observe rapid saturation of the transmission dip. These measurements provide both a method for probing the cavity–quantum dot system and a step towards the realization of quantum devices based on coherent light scattering and large optical nonlinearities from quantum dots in photonic crystal cavities.

Journal ArticleDOI
TL;DR: In this article, a first principle, theoretical study of MoS2 nanoparticles is presented, which provides a unified explanation of measured photoluminescence spectra and recent STM measurements as a function of size.
Abstract: We present a first principle, theoretical study of MoS2 nanoparticles that provides a unified explanation of measured photoluminescence spectra and recent STM measurements as a function of size. In addition, our calculations suggest ways to engineer the electronic properties of these systems so as to obtain direct band gap 3D layered nanoparticles or Mo doped metallic nanowires. In particular, we show that single sheet MoS2 nanoparticles up to ∼3.4 nm show no appreciable quantum confinement effects. Instead, their electronic structure is entirely dominated by surface states near the Fermi level. In 3D nanoparticles, we found a strong dependence of their electronic properties on layer stacking and distance, and we suggest that the observed photoluminescence variation as a function of size originates from the number of planes composing the system. The number of these planes and their distance can be tuned to engineer clusters with direct band gaps, at variance with the bulk. Our results also suggest ways to...

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate an all-optical modulator in which efficient interaction between two light beams at different wavelengths is achieved by converting them into co-propagating surface plasmon polaritons interacting by means of a thin layer of CdSe quantum dots (QDs).
Abstract: Photonics is a promising candidate technology for information processing, communication and data storage. Essential building blocks, such as logic elements and modulators, have been demonstrated. However, because of weak nonlinear light–matter interactions, these components typically require high power densities and large interaction volumes, limiting their application in dense chip-based integration. A solution may be found in surface plasmon polaritons (SPPs), guided electromagnetic waves that propagate with high field confinement along a metal–dielectric interface. We demonstrate an all-optical modulator in which efficient interaction between two light beams at different wavelengths is achieved by converting them into co-propagating SPPs interacting by means of a thin layer of CdSe quantum dots (QDs). The high SPP field confinement and high QD-absorption cross-section enable optical modulation at low power densities (~10^2 W cm^(-2)) in micrometre-scale planar devices.

Journal ArticleDOI
TL;DR: In this article, inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV-vis−NIR spectrophotometry and transmission electron microscopy to determine the nanocrystal composition and molar extinction coefficient ϵ of colloidal PbSe quantum dot (Q-PbSe) suspensions.
Abstract: Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis−NIR spectrophotometry and transmission electron microscopy to determine the nanocrystal composition and molar extinction coefficient ϵ of colloidal PbSe quantum dot (Q-PbSe) suspensions. The ICP-MS results show a nonstoichiometric Pb/Se ratio, with a systematic excess of lead for all samples studied. The observed ratio is consistent with a faceted spherical Q-PbSe model, composed of a quasi stoichiometric Q-PbSe core terminated by a Pb surface shell. At high photon energies, we find that ϵ scales with the nanocrystal volume, irrespective of the Q-PbSe size. From ϵ, we calculated a size-independent absorption coefficient. Its value is in good agreement with the theoretical value for bulk PbSe. At the band gap, ϵ is size-dependent. The resulting absorption coefficient increases quadratically with decreasing Q-PbSe size. Calculations of the oscillator strength of the first optical transition are in good agreement with theoretical ...


Journal ArticleDOI
TL;DR: The results indicate that nonradiative quenching of the QD emission by proximal Au-NPs is due to long-distance dipole-metal interactions that extend significantly beyond the classical Förster range, in agreement with previous studies using organic dye-Au-NP donor-acceptor pairs.
Abstract: Luminescent quantum dots (QDs) were proven to be very effective fluorescence resonance energy transfer donors with an array of organic dye acceptors, and several fluorescence resonance energy transfer based biosensing assemblies utilizing QDs have been demonstrated in the past few years. Conversely, gold nanoparticles (Au-NPs) are known for their capacity to induce strong fluorescence quenching of conventional dye donors. Using a rigid variable-length polypeptide as a bifunctional biological linker, we monitor the photoluminescence quenching of CdSe-ZnS QDs by Au-NP acceptors arrayed around the QD surface, where the center-to-center separation distance was varied over a broad range of values (approximately 50-200 Angstrom). We measure the Au-NP-induced quenching rates for such QD conjugates using steady-state and time-resolved fluorescence measurements and examine the results within the context of theoretical treatments based on the Forster dipole-dipole resonance energy transfer, dipole-metal particle energy transfer, and nanosurface energy transfer. Our results indicate that nonradiative quenching of the QD emission by proximal Au-NPs is due to long-distance dipole-metal interactions that extend significantly beyond the classical Forster range, in agreement with previous studies using organic dye-Au-NP donor-acceptor pairs.

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrated an approach to sensitized type solar cells, based on TiO2 inverse opal and the use of CdSe quantum dots (QDs) as sensitizers.
Abstract: The authors have demonstrated an approach to sensitized-type solar cells, based on TiO2 inverse opal and the use of CdSe quantum dots (QDs) as sensitizers. CdSe QDs were grown in situ on TiO2 inverse opal electrodes, utilizing a chemical bath deposition method. All of the photovoltaic performances, including short circuit photocurrent density, open circuit voltage, fill factor, and efficiency, were significantly improved by surface modification with ZnS and fluoride ions. A power conversion efficiency of about 2.7% has been attained, under solar illumination of 100mW∕cm2. This value is relatively high for metal oxide solar cells, sensitized with semiconductor QDs.

Journal ArticleDOI
TL;DR: Fluorescence intensity can be enhanced by a factor of up to 108 compared with quantum dots on an unpatterned surface by fabricating two-dimensional photonic crystal slabs that operate at visible wavelengths and engineering their leaky modes so that they overlap with the absorption and emission wavelengths.
Abstract: Colloidal quantum dots display a wide range of novel optical properties that could prove useful for many applications in photonics. Here, we report the enhancement of fluorescence emission from colloidal quantum dots on the surface of two-dimensional photonic crystal slabs. The enhancement is due to a combination of high-intensity near fields and strong coherent scattering effects, which we attribute to leaky eigenmodes of the photonic crystal. By fabricating two-dimensional photonic crystal slabs that operate at visible wavelengths and engineering their leaky modes so that they overlap with the absorption and emission wavelengths of the quantum dots, we demonstrate that the fluorescence intensity can be enhanced by a factor of up to 108 compared with quantum dots on an unpatterned surface.

Journal ArticleDOI
TL;DR: Recognition of a biotin pattern by d-dots conjugated with avidine was carried to illustrate the suitability of these efficient (about 40% PL quantum yield), stable, small, and water-soluble d-Dots as biomedical labeling reagents.
Abstract: Mn2+-doped ZnSe quantum dots (Mn:ZnSe d-dots) with a tunable photoluminescence (PL) peak position were made to be water soluble by coating them with a monolayer of mercaptopropionic acid, a very short hydrophilic thiol. If the dopant centers were located close to the surface, thiol-coating partially quenched the PL. With about 2−3 monolayers of pure ZnSe on the surface, the PL of d-dots was actually enhanced upon thiol coating. When the doping centers were placed reasonably inside a d-dot, with about four monolayers of pure ZnSe between the doping centers and the surface ligands, the thiol ligands did not quench the PL of the d-dots, even though they did completely quench the PL of intrinsic ZnSe quantum dots. The overall size of such d-dots/ligand complex is only about 7−8 nm, implying an excellent permeability in biological issues. These d-dots were found to be exceptionally stable against continuous UV radiation in air for at least 25 days. They were also stable in boiling water with air bubbling under...

Journal ArticleDOI
25 May 2007-Science
TL;DR: On-demand single-electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier using an electron analog of the single-photon gun to prove useful for the use of quantum bits in ballistic conductors.
Abstract: We report on the electron analog of the single-photon gun. On-demand single-electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission was triggered by the application of a potential step that compensated for the dot-charging energy. Depending on the barrier transparency, the quantum emission time ranged from 0.1 to 10 nanoseconds. The single-electron source should prove useful for the use of quantum bits in ballistic conductors. Additionally, periodic sequences of single-electron emission and absorption generate a quantized alternating current.

Journal ArticleDOI
TL;DR: In this article, the InAs/InP quantum dash (QD) materials for lasers and amplifiers, and QD device performance with particular interest in optical communication are summarized.
Abstract: This paper summarizes recent advances on InAs/InP quantum dash (QD) materials for lasers and amplifiers, and QD device performance with particular interest in optical communication. We investigate both InAs/InP dashes in a barrier and dashes in a well (DWELL) heterostructures operating at 1.5 mum. These two types of QDs can provide high gain and low losses. Continuous-wave (CW) room-temperature lasing operation on ground state of cavity length as short as 200 mum has been achieved, demonstrating the high modal gain of the active core. A threshold current density as low as 110 A/cm2 per QD layer has been obtained for infinite-length DWELL laser. An optimized DWELL structure allows achieving of a T0 larger than 100 K for broad-area (BA) lasers, and of 80 K for single-transverse-mode lasers in the temperature range between 25degC and 85degC. Buried ridge stripe (BRS)-type single-mode distributed feedback (DFB) lasers are also demonstrated for the first time, exhibiting a side-mode suppression ratio (SMSR) as high as 45 dB. Such DFB lasers allow the first floor-free 10-Gb/s direct modulation for back-to-back and transmission over 16-km standard optical fiber. In addition, novel results are given on gain, noise, and four-wave mixing of QD-based semiconductor optical amplifiers. Furthermore, we demonstrate that QD Fabry-Perot (FP) lasers, owing to the small confinement factor and the three-dimensional (3-D) quantification of electronic energy levels, exhibit a beating linewidth as narrow as 15 kHz. Such an extremely narrow linewidth, compared to their QW or bulk counterparts, leads to the excellent phase noise and time-jitter characteristics when QD lasers are actively mode-locked. These advances constitute a new step toward the application of QD lasers and amplifiers to the field of optical fiber communications

Journal ArticleDOI
TL;DR: Light emitting devices (LEDs) with a broad spectral emission generated by electroluminescence from a mixed-monolayer of red, green, and blue emitting colloidal quantum dots (QDs) in a hybrid organic/inorganic structure are demonstrated.
Abstract: We demonstrate light emitting devices (LEDs) with a broad spectral emission generated by electroluminescence from a mixed-monolayer of red, green, and blue emitting colloidal quantum dots (QDs) in a hybrid organic/inorganic structure. The colloidal QDs are reproducibly synthesized and yield high luminescence efficiency materials suitable for LED applications. Independent processing of the organic charge transport layers and the QD luminescent layer allows for precise tuning of the emission spectrum without changing the device structure, simply by changing the ratio of different color QDs in the active layer. Spectral tuning is demonstrated through fabrication of white QD-LEDs that exhibit external quantum efficiencies of 0.36% (Commission Internationale de l'Eclairage) coordinates of (0.35, 0.41) at video brightness, and color rendering index of 86 as compared to a 5500 K blackbody reference.

Journal ArticleDOI
TL;DR: In this article, the authors present experimental evidence for a long-range electromagnetic interaction between laterally arranged quantum dot (QD) systems and show that the QDs do not behave like independent objects as long as they form an ensemble of QDs.
Abstract: In 1954, Dicke pointed out that the description of a spontaneously radiating gas has to include the fact that all atoms or molecules interact with a common radiation field1. Consequently, the individual particles may not be considered as independent sources of radiation. In this regard, the question arises of whether quantum dot (QD) systems may also exhibit signatures of cooperative radiation and hence have to be considered as coupled quantum systems. Here, we present experimental evidence for a long-range electromagnetic interaction between laterally arranged QDs. The experimental results suggest that the QDs do not behave like independent objects as long as they form an ensemble of QDs. By removing QDs from the sample, we found that the coupling was reduced. The range of interaction is shown to be at least 150 nm. This may therefore provide a mechanism to couple discrete quantum objects on a large scale.

Journal ArticleDOI
TL;DR: In this article, instead of water, alcohol was used as a solvent in a chemical bath deposition process for the in situ synthesis of CdS quantum dots onto mesoporous TiO2 films.
Abstract: Alcohol, instead of water, was used as a solvent in a chemical bath deposition process for the in situ synthesis of CdS quantum dots onto mesoporous TiO2 films. Due to low surface tension, the alcohol solutions have high wettability and superior penetration ability on the mesoscopic TiO2 film, leading to a well-covered CdS on the surface of mesopores. The CdS-sensitized TiO2 electrode prepared using the alcohol system not only has a higher incorporated amount of CdS but also greatly inhibits the recombination of injected electrons. The efficiency of a CdS quantum-dots-sensitized solar cell prepared using the present method is as high as 1.84% under the illumination of one sun (AM1.5, 100mW∕cm2).

Journal ArticleDOI
06 Dec 2007-Nature
TL;DR: Strong coupling, the regime of coherent quantum interactions, is demonstrated through observation of vacuum Rabi splitting in the transmitted and reflected signals from the cavity, and the fibre coupling method is used to examine the system’s steady-state nonlinear properties.
Abstract: Cavity quantum electrodynamics, the study of coherent quantum interactions between the electromagnetic field and matter inside a resonator, has received attention as both a test bed for ideas in quantum mechanics and a building block for applications in the field of quantum information processing. The canonical experimental system studied in the optical domain is a single alkali atom coupled to a high-finesse Fabry–Perot cavity. Progress made in this system has recently been complemented by research involving trapped ions, chip-based microtoroid cavities, integrated microcavity-atom-chips, nanocrystalline quantum dots coupled to microsphere cavities, and semiconductor quantum dots embedded in micropillars, photonic crystals and microdisks. The last system has been of particular interest owing to its relative simplicity and scalability. Here we use a fibre taper waveguide to perform direct optical spectroscopy of a system consisting of a quantum dot embedded in a microdisk. In contrast to earlier work with semiconductor systems, which has focused on photoluminescence measurements, we excite the system through the photonic (light) channel rather than the excitonic (matter) channel. Strong coupling, the regime of coherent quantum interactions, is demonstrated through observation of vacuum Rabi splitting in the transmitted and reflected signals from the cavity. The fibre coupling method also allows us to examine the system's steady-state nonlinear properties, where we see a saturation of the cavity–quantum dot response for less than one intracavity photon. The excitation of the cavity–quantum dot system through a fibre optic waveguide is central to applications such as high-efficiency single photon sources, and to more fundamental studies of the quantum character of the system.

Journal ArticleDOI
TL;DR: It is shown that it is possible to confine massless Dirac fermions in a monolayer graphene sheet by inhomogeneous magnetic fields, which allows one to design mesoscopic structures in graphene by magnetic barriers, e.g., quantum dots or quantum point contacts.
Abstract: Because of Klein tunneling, electrostatic potentials are unable to confine Dirac electrons. We show that it is possible to confine massless Dirac fermions in a monolayer graphene sheet by inhomogeneous magnetic fields. This allows one to design mesoscopic structures in graphene by magnetic barriers, e.g., quantum dots or quantum point contacts.

Journal ArticleDOI
TL;DR: In this article, reproducible fabrication of InP−InAsP nanowire light-emitting diodes in which electron−hole recombination is restricted to a quantum-dot-sized InAsP section is reported.
Abstract: We report reproducible fabrication of InP−InAsP nanowire light-emitting diodes in which electron−hole recombination is restricted to a quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the quantum dot with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically driven quantum optics experiments. We have investigated the operation of these nanoLEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for single photon applications.

Journal ArticleDOI
TL;DR: In this paper, a quantum-dot-based single-photon source with a measured singlephoton emission rate of 4.0MHz (31MHz into the first lens, with an extraction efficiency of 38%) due to the suppression of exciton dark states was demonstrated.
Abstract: Optoelectronic devices that provide non-classical light states on demand have a broad range of applications in quantum information science1, including quantum‐key‐distribution systems2, quantum lithography3 and quantum computing4. Single-photon sources5,6 in particular have been demonstrated to outperform key distribution based on attenuated classical laser pulses7. Implementations based on individual molecules8, nitrogen vacancy centres9 or dopant atoms10 are rather inefficient owing to low emission rates, rapid saturation and the lack of mature cavity technology. Promising single-photon-source designs combine high-quality microcavities11 with quantum dots as active emitters12. So far, the highest measured single-photon rates are ∼ 200 kHz using etched micropillars13,14. Here, we demonstrate a quantum-dot-based single-photon source with a measured single-photon emission rate of 4.0 MHz (31 MHz into the first lens, with an extraction efficiency of 38%) due to the suppression of exciton dark states. Furthermore, our microcavity design provides mechanical stability, and voltage-controlled tuning of the emitter/mode resonance and of the polarization state.

Journal ArticleDOI
TL;DR: The structure of MOF-5 is considered as being constructed of discrete semiconductor Zn4O13 quantum dots stabilized and interconnected by terephthalate linkers.
Abstract: The structure of MOF-5 is considered as being constructed of discrete semiconductor Zn4O13 quantum dots stabilized and interconnected by terephthalate linkers. Terephthalate can absorb light and se...

Journal ArticleDOI
TL;DR: This work presents proof that the emission from a strongly-coupled QD- microcavity system is dominated by a single quantum emitter.
Abstract: We observe antibunching in the photons emitted from a strongly coupled single quantum dot and pillar microcavity in resonance. When the quantum dot was spectrally detuned from the cavity mode, the cavity emission remained antibunched, and also anticorrelated from the quantum dot emission. Resonant pumping of the selected quantum dot via an excited state enabled these observations by eliminating the background emitters that are usually coupled to the cavity. This device demonstrates an on-demand single-photon source operating in the strong coupling regime, with a Purcell factor of 61+/-7 and quantum efficiency of 97%.