Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.     

Quantum cascade transitions in nanostructures

In this article we review the physical characteristics of quantum cascade transitions (QCTs) in various nanoscopic systems. The quantum cascade laser which utilizes such transitions in quantum wells is a brilliant outcome of quantum engineering that has already demonstrated its usefulness in various real-world applications. After a brief introduction to the background of this transition process, we discuss the physics behind these transitions in an externally applied magnetic field. This has unravelled many intricate phenomena related to intersubband resonance and electron relaxation modes in these systems. We then discuss QCTs in a situation where the quantum wells in the active regions of a quantum cascade structure are replaced by quantum dots. The physics of quantum dots is a rapidly developing field with its roots in fundamental quantum mechanics, but at the same time, quantum dots have tremendous potential applications. We first present a brief review of those aspects of quantum dots that are likely to be reflected in a quantum-dot cascade structure. We then go on to demonstrate how the calculated emission peaks of a quantum-dot cascade structure with or without an external magnetic field are correlated with the properties of quantum dots, such as the choice of confinement potentials, shape, size and the low-lying energy spectra of the dots. Contents PAGE 1 Introduction 456 2 Intersubband transitions in quantum wells 458 3 Quantum cascade transitions 462 3.1. Basic principles 462 3.1.1. Minibands and minigaps 464 3.1.2. Vertical transitions 464 3.1.3. GaAs/AlGaAs quantum cascade lasers 464 3.1.4. QCLs based on superlattice structures 465 3.1.5. Type-II quantum cascade lasers 466 3.1.6. Recent developments 466 3.2. Applications: sense-ability and other qualities 466 4 Quantum cascade transitions in novel situations 467 4.1. External magnetic field 467 4.1.1. Parallel magnetic field 468 4.1.2. Many-body effects: depolarization shift 470 4.1.3. The role of disorder 471 4.1.4. Tilted magnetic field 475 4.2. Magneto-transport experiments and phonon relaxation 479 4.3. Magneto-optics experiment and phonon relaxation 484 5 A brief review of quantum dots 485 5.1. From three- to zero-dimensional systems 485 5.2. Making the dots 487 5.2.1. Lithographic patterning 487 5.2.2. Self-assembled quantum dots 488 5.3. Shell filling in quantum dots 489 5.4. Electron correlations: spin states 490 5.5. Anisotropic dots 491 5.6. Influence of an external magnetic field 491 5.6.1. The Fock diagram 491 5.6.2. The no-correlation theorem 492 5.6.3. Correlation effects and magic numbers 492 5.6.4. Spin transitions 493 5.7. Quantum dots in novel systems 494 5.8. Potential applications of quantum dots 494 5.8.1. Single-electron transistors (SETs) 494 5.8.2. Single-photon detectors 494 5.8.3. Single-photon emitters 495 5.8.4. Quantum-dot lasers 495 6 Quantum cascade transitions in quantum-dot structures 496 6.1. Quantum dots versus quantum wells 496 6.2. QCT with rectangular dots 497 6.2.1. Vertical transitions 500 6.2.2. Diagonal transitions 501 6.3. QCT in a parabolic dot 504 6.4. Magnetic field effects on intersubband transitions 506 6.5. Mid-IR luminescence from a QD cascade device 512 7 Summary and open questions 513 Acknowledgements 515 References 515     

Optical characterization of quantum dots entrained in microstructured optical fibers

We present a study of the basic optical properties of colloidal CdSe/ZnS core/shell quantum dots entrained in a microstructured optical fiber. Quantum dots suspended in heptane were pulled into the holes surrounding the solid core of a microstructured optical fiber of the holey fiber class via capillary action and are found to remain in the fiber. In this experiment, a laser coupled into the fiber photoexcited quantum dots along the length of the fiber. Quantum dot emission was observed to couple into the fiber core and propagate along the fiber. To investigate the use of such a system in fiber-based light generation or amplification, a second laser overlapping the low-energy portion of the quantum dot emission was simultaneously coupled into the fiber. We observed apparent amplification of this light when photoexciting the quantum dots well above their bandedge.     

Electron Spins in Quantum Dots as Quantum Bits

The creation, coherent manipulation, and measurement of spins in nanostructures open up completely new possibilities for electronics and information processing, among them quantum computing and quantum communication. We review our theoretical proposal for using electron spins in quantum dots as quantum bits, explaining why this scheme satisfies all the essential requirements for quantum computing. We include a discussion of the recent measurements of surprisingly long spin coherence times in semiconductors. Quantum gate mechanisms in laterally and vertically tunnel-coupled quantum dots and methods for single-spin measurements are introduced. We discuss detection and transport of electronic EPR pairs in normal and superconducting systems.     

量子点增强硅基探测成像器件的研究进展

硅基探测成像器件具有可靠性高、易集成和成本低等优点,是目前应用最广泛的探测成像器件。随着人工智能和无人驾驶等技术的日益发展,对探测成像器件提出了更高的要求,而硅基探测成像器件性能的提升成为重要的研究方向。量子点具有吸收系数大、光谱可调、发光效率高和易集成等优点,是一类优异的光谱转换和光调制材料。利用量子点材料可调制的光学特性,可以对硅基探测成像器件的功能进行拓展,从而实现紫外响应增强、红外响应拓展、紫外偏振探测和多光谱成像等功能。经过多年的研究,这一领域已经取得了一定的进展,部分技术展现出较好的应用前景。本文介绍了量子点增强硅基探测器在紫外探测、红外成像、偏振探测和多光谱成像方面的研究进展,希望能够引起国内学术界和工业界的关注和重视。     

应用级联运动方程方法研究并联双量子点的量子纠缠

Quantum dots comprise a type of quantum impurity system. The entanglement and coherence of quantum states are significantly influenced by the strong electron-electron interactions among impurities and their dissipative coupling with the surrounding environment. Competition between many-body effects and transfer couplings plays an important role in determining the entanglement among localized impurity spins. In this work, we employ the hierarchical-equations-of-motion approach to explore the entanglement of a strongly correlated double quantum dots system. The relation between the total system entropy and those of subsystems is also investigated.     

图形衬底对多周期InGaAs量子点自组装生长的影响

量子点器件技术广泛应用于量子计算和光电器件上.成核位置的均匀性、有序性和尺寸一致性,可以有效提高光电器件性能.为了实现阵列量子点的可控性,本文采用湿法刻蚀制备图形化衬底,理论上解释了铟原子在图形化衬底上成核现象,产生有序的量子点分布特征,发现图形衬底的缺陷诱导在平台边缘和沟壑边缘成核,形成较大的量子点.在Stranski-Krastanow模式下图形衬底制备多周期量子点,发现多周期生长可以弱化台阶结构对量子点分布的限制作用.     

“Spherical” quantum dots

Quantum dots with a three-dimensional confining potential, i.e. “spherical” quantum dots, are considered with inclusion of electron-electron interaction (a quantum analog of the Thomson atom). The energy spectrum of two-electron parabolic quantum dots has been determined by numerical diagonalization of the full Hamiltonian in a one-particle basis. Fiz. Tverd. Tela (St. Petersburg) 40, 2134–2135 (November 1998)     

The Fluorescence Bioassay Platforms on Quantum Dots Nanoparticles

In this paper, we present the optical properties and the platforms on fluorescent quantum dots for biological labeling, biomedical engineering and biosensor in molecular imaging. Quantum dots possess several properties that make them very attractive for fluorescent tagging: broad excitation spectrum, narrow emission spectrum, precise tunability of their emission peak, longer fluorescence lifetime than organic fluorophores and negligible photobleaching. We describe how to take such advantages of quantum dots to develop the technology and employ it to build assay platforms. Finally, ultrasensitivity, multicolor, and multiplexing of the technology of semiconductor quantum dots open up promising and interesting possibilities for bioassay platform.     

基于量子点的荧光型太阳能聚光器

近年来,量子点在结构可控、光谱调节和光学稳定方面的研究进展,表明基于量子点的聚光器件表现出优于基于传统有机染料分子的光输出性能。量子点聚光器成为目前量子点研究领域的新方向。量子点在宏量制备和绿色制备方面的深入研究,使得量子点的制造成本逐步降低,基于量子点的聚光器具有光电转换效率和成本上的优势。本文综述了量子点聚光器的研究进展,主要包括荧光型聚光器的优点、聚光器对量子点光学性质的要求、器件制备的工艺和器件的性能表征方法。重点阐述了量子点的太阳光吸收能力、荧光量子产率和重吸收等关键因素对聚光器件性能的影响,同时介绍了该领域目前最新的研究方向,展望了廉价太阳能窗户在未来城镇建筑上的潜在应用。     

固态量子计算

量子计算机拥有比经典计算机更强大的计算能力，人们普遍认为，量子计算机最终将会在固态系统中实现，文章介绍了三种固态量子计算机的方案，它们分别基于固态核振磁共振，超导结和量子点。     

Modified spontaneous emission and qubit entanglement from dipole-coupled quantum dots in a photonic crystal nanocavity

The modified spontaneous emission dynamics of two photon-coupled quantum dots in a planar-photonic crystal are theoretically investigated. Based on a photon Green function technique for quantizing the electromagnetic fields in arbitrary surroundings, pronounced vacuum Rabi oscillations and dipole-dipole interactions are self-consistently incorporated and are shown to result in a high degree of quantum-bit entanglement. Quantum dots with different optical dipole moments are also found to yield a very rich display of quantum dynamics and offer several advantages over coupling identical atoms.     

量子点液晶显示背光技术

量子点材料兼具极高的色纯度、发光颜色可调以及的荧光量子产率高等特点,已成为显示领域中的明星材料,在提升显示器件的色域方面具有巨大潜力。基于量子点材料的液晶显示背光技术是目前量子点材料在显示器件中的主流应用方向,引起了学术界和工业界的广泛关注。本文将综述量子点液晶显示背光技术的研究进展,主要包括量子点材料的选择、背光结构的应用以及材料复合与封装技术的发展现状,重点介绍了目前产业界广泛关注的量子点光学膜技术,特别是国内自主知识产权的低成本钙钛矿量子点光学膜技术,由于其具备广色域(124%NTSC)、易加工、低成本等特点,已成为具有成长潜力的技术路线。     

Equilibrium ensembles of quantum dots in atomically inhomogeneous pentagonal nanowires

A theoretical model has been proposed for describing the relaxation of elastic stresses in an atomically inhomogeneous pentagonal nanowire due to the formation of quantum dots in the form of precipitates of the second phase. Quantum dots have been considered as finite-height coaxial cylindrical inclusions subjected to intrinsic axial dilatation and located along the axis of the nanowire. The optimum shape and sizes of the quantum dots have been calculated for specified nanowire parameters. It has been shown that such quantum dots can form different equilibrium periodic structures in the pentagonal nanowires.     

Fluorinated Boron Nitride Quantum Dots: A New 0D Material for Energy Conversion and Detection of Cellular Metabolism

Quantum dots encompass a broad spectrum of optical, catalytic, and electrochemical properties bringing in novel applications in catalysis, imaging, displays, and optoelectronics. Herein, the unanticipated broad‐spectrum light absorption and high fluorescence quantum yield in fluorinated boron nitride (FBN) quantum dots are discussed. A heterostructure of FBN quantum dots with a wide‐bandgap semiconductor, titania nanotube arrays, exhibits high photocatalytic activity as evidenced by high external quantum efficiency extending from ultraviolet to green region of the solar spectrum (≈24% at 400 nm). The high activity is confirmed using photoelectrochemical hydrogen evolution experiments. Further, it is demonstrated that high fluorescence quantum yield could be tapped for the detection of glycolytic activity in cancer cells compared to normal cells. This finding could shift the paradigm of molecular detection using quantum dots. The 0D structure and the gap states introduced through fluorination are believed to be responsible for these unprecedented characteristics of boron nitride.     

Influence of dopant concentration on electrical quantum transport behaviors in junctionless nanowire transistors

We discuss the random dopant effects in long channel junctionless transistor associated with quantum confinement effects. The electrical measurement reveals the threshold voltage variability induced by the random dopant fluctuation.Quantum transport features in Hubbard systems are observed in heavily phosphorus-doped channel. We investigate the single electron transfer via donor-induced quantum dots in junctionless nanowire transistors with heavily phosphorusdoped channel, due to the formation of impurity Hubbard bands. While in the lightly doped devices, one-dimensional quantum transport is only observed at low temperature. In this sense, phonon-assisted resonant-tunneling is suppressed due to misaligned levels formed in a few isolated quantum dots at cryogenic temperature. We observe the Anderson-Mott transition from isolate electron state to impurity bands as the doping concentration is increased.     

Experimental realization of highly efficient broadband coupling of single quantum dots to a photonic crystal waveguide

We present time-resolved spontaneous emission measurements of single quantum dots embedded in photonic crystal waveguides. Quantum dots that couple to a photonic crystal waveguide are found to decay up to 27 times faster than uncoupled quantum dots. From these measurements beta-factors of up to 0.89 are derived, and an unprecedented large bandwidth of 20 nm is demonstrated. This shows the promising potential of photonic crystal waveguides for efficient single-photon sources. The scaled frequency range over which the enhancement is observed is in excellent agreement with recent theoretical proposals taking into account that the light-matter coupling is strongly enhanced due to the significant slow-down of light in the photonic crystal waveguides.     

Nonlinear thermoelectric response due to energy-dependent transport properties of a quantum dot

Quantum dots are useful model systems for studying quantum thermoelectric behavior because of their highly energy-dependent electron transport properties, which are tunable by electrostatic gating. As a result of this strong energy dependence, the thermoelectric response of quantum dots is expected to be nonlinear with respect to an applied thermal bias. However, until now this effect has been challenging to observe because, first, it is experimentally difficult to apply a sufficiently large thermal bias at the nanoscale and, second, it is difficult to distinguish thermal bias effects from purely temperature-dependent effects due to overall heating of a device. Here we take advantage of a novel thermal biasing technique and demonstrate a nonlinear thermoelectric response in a quantum dot which is defined in a heterostructured semiconductor nanowire. We also show that a theoretical model based on the Master equations fully explains the observed nonlinear thermoelectric response given the energy-dependent transport properties of the quantum dot.     

Preparation of Nanocomposites Based on PMMA and CdSe,CdSe/CdS Quantum Dots

Russian Physics Journal - Nanocomposites based on PMMA and CdSe, CdSe/CdS quantum dots were developed. Quantum dots (QD) of CdSe, CdSe/CdS with sizes from 2.0 nm to 4.0 nm were obtained by...     

The coherent time evolution of two coupled quantum dots in a two-mode cavity

For two coupled identical quantum dots in a two-mode cavity, we determine the conditions of two-photon and single-photon resonance. It is shown that the exciton-phonon interaction reduces the Rabi frequencies of each model and the Förster interaction between double quantum dots even at absolute zero temperature. The exciton-phonon interaction also makes a contribution to the static exciton-exciton dipole interaction energy. Furthermore, the additional interactions can modify the conditions of photon resonance significantly. A more realistic case of two nonidentical quantum dots is also considered. The influence of parameter misfits on the quantum system is discussed.Received: 8 July 2004, Published online: 3 November 2004PACS: 73.21.La Quantum dots - 71.35.-y Excitons and related phenomena - 42.50.Pq Cavity quantum electrodynamics; micromasers