限域量子态调控研究

电子、激子和声子等量子态在固体中的行为早已被人们所熟知. 然而，当体系的尺寸只有纳米量级的时候，已有的固体理论常常不能适用，需要新的低维物理理论的建立. 我们系统研究了低维体系限域量子态（包括电子、激子和声子）的行为对环境、应力、压力及光的响应和性质的调控. 较早认识到低维体系之显著的表面-体积比对量子态性质调控之有效性，系统地揭示了低维体系的一系列由表面和应力决定的新颖性质，证明了低维体系的表面和应力效应同量子限域效应同等重要. 本文概况了如下五个方面的结果：（1）一种使用应力效应调控电子能带结构的方法和（2）一种使用表面效应调控电子能带结构的方法（这两个方法都可将低维体系能带从间接能隙调控至直接能隙能带结构）；（3）一种低维体系表面掺杂方法，该方法将在低维体系掺杂中取代传统方法；（4）量子点表面诱导的光致异构现象；（5）基于表面自催化半导体低维结构的形成机理. 希望我们的研究工作有助于促进低维体系在光电子、纳电子、环境、能源、生物和医学等领域的应用.     

Spin dynamics and quantum transport in magnetic semiconductor quantum structures

Quantum structures derived from magnetic semiconductors serve as a powerful arena within which to study the interplay between quantum electronics and thin film magnetism. In particular, the semiconductor aspects of these flexible systems allow direct access to the electronic spin degrees of freedom using both magneto-optical as well as magneto-transport probes. Here we provide an overview of recent developments in the experimental study of II–VI magnetic semiconductor quantum structures, with particular emphasis on the dynamical behavior of field-tunable electronic spin states and spin-dependent quantum transport.     

Theory of light scattering from semiconductor quantum dots: Excitation frequency dependent emission dynamics

We present a microscopic model for the dynamic scattering and emission spectrum of a semiconductor quantum dot, after coherent optical excitation. We investigate the spectral properties and the emission dynamics of the different scattering and emission contributions considering a V-type semiconductor quantum dot model under resonant conditions and include the coupling to LO-phonons via higher order Born approximations. This theory helps identifying the different contributions to the spectrum via time resolved calculations.     

Spin excitations in few-electrons AlGaAs/GaAs quantum dots probed by inelastic light scattering

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock–Darwin levels. In QDs with very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified.     

Auger relaxation processes in semiconductor nanocrystals and quantum wells

Auger recombination rates in mesoscopic semiconductor structures have been studied as a function of energy band parameters and heterostructure size. It is shown that nonthreshold Auger processes stimulated by the presence of heteroboundaries become the dominant nonradiative recombination channel in nanometer size semiconductor structures. The size dependence of luminescence quantum yields in nanostructures and microcrystals are discussed. Auger-like collisions of electrons and heavy holes are shown to serve as “accelerators” of thermalization processes in semiconductor quantum dots.     

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.     

Optical near-field mapping of bright and dark quantum dot states

We theoretically investigate scanning near-field optical microscopy (SNOM) of semiconductor quantum dots. A general theoretical framework is developed that accounts for photo excitation and relaxation in complex dielectric environments. We find that in the near-field regime bright and dark excitonic states become mixed, opening new channels for the coupling to the electromagnetic field.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

The influences of electric field and temperature on state energies of a strong-coupling polaron in an asymmetric Gaussian potential quantum well

We investigate the electric field effect on the ground state (GS), the first-excited state (FES) and the excitation energies of a strong-coupled polaron in an asymmetric Gaussian potential (AGP) quantum well (QW) by using a variational method of the Pekar type (VMPT). By employing the quantum statistics theory (QST), we also study the temperature effects on the state energies (SE). It is found that the SE of polaron are an increasing function of temperature. The polaron's SE are decreasing functions of electric field, but the excitation energy is an increasing one.     

Bandgap calculation in Si quantum dot arrays using a genetic algorithm

In this work we present a fast and accurate genetic algorithm to determine the envelope functions and eigenenergies of the ground states of electrons and holes in low-dimensional complex semiconductor structures. We have developed the theoretical formalism of the algorithm in a general way in order to make it easy to include arbitrary nonparabolic and anisotropic band profiles in the calculations. From these results, calculation of the bandgaps of nanostructures can be carried out efficiently.Besides presenting and testing the algorithm, we calculate the ground state of electron and holes in two-dimensional quantum dot arrays, taking nonparabolicity and anisotropy into account.     

二维光子晶体量子阱的光谱特性研究

研究了二维光子晶体量子阱的光谱特性,该量子阱结构由二维正方晶格圆柱晶胞光子晶体通过移去中间位置的介质圆柱层形成。由于光子晶体中的光子禁带充当了光子运动的势垒,类似于半导体量子阱中电子的行为,在光子晶体量子阱结构中会出现量子化的光子能态。文章利用平面波展开法计算了所用光子晶体的能带结构,利用传输矩阵方法计算了量子阱结构的透射光谱。计算结果表明,在光子禁带中出现了离散的透射峰,透射峰的强度随着势垒宽度的增加而减弱,个数随着势阱宽度的增加而增加,通过计算得到了其定量关系,并且讨论了透射峰频率与势阱宽度的关系。     

Possible generation of electric multipole radiation from solid state quantum rings

The self-organized crystal growth of semiconductor quantum rings has opened a new possibility to study and exploit optical transitions between ring-shaped quantum states. In such states, orbital angular momenta of particle envelope functions are well-defined. We investigate theoretically the intraband interlevel transitions between such states and examine the possibility of electrical multipole radiations (EMRs). Selections rules due to envelope function quantum numbers are deduced. To enhance the EMR efficiency, we propose a novel coupled dot–ring structure, by which the lowest allowed EMR can be selected and manipulated, allowing the efficient radiation of multipole photons.     

Optically induced mechanical interaction between semiconductor quantum dots under an electronic resonance condition

Assuming an electronic resonance condition, we study the shape-dependence of the radiation force (RF) on a semiconductor quantum dot (QD) floating in medium and the optically induced mechanical interaction (OIMI) between two QDs with Maxwell stress tensor (MST) method, where the response fields are calculated by the improved discrete dipole approximation (DDA). Main results are as follows: (1) Properties of the RF on an isolated QD drastically change due to its shape and polarization of an incident light, which can be used for shape-selective manipulation. (2) Anomalous OIMI between two QDs arises depending on the spatial structures of internal fields, which results from the interaction between polarizations in respective QDs when they are near each other.     

Hydrogenic impurity ground state in quantum well: the envelope function revisited

We present a new variationnal method for calculating the ground state energy of an electron bound to an impurity located in a quantum well. This method relies on an envelope function which is determined exactly from a formal minimization procedure. The obtained energies are lower by as much as 10% than the ones found by the widely used free electron envelope function. Their large width limits are reached with exponentially small corrections as they should. We also find that, except for narrow wells, the shape of these exact envelope functions strongly depends on the impurity position, being consequently quite different from the usual free electron ones. In order to discuss the improvements brought by our new procedure in the most striking way, we have used a model semiconductor quantum well with infinite barrier height and simplified band structure. Extensions can be made to finite barrier and more realistic band structures, following the same technique. Received 11 December 2000     

Y2O2S:Eu nanocrystals, a strong quantum-confined luminescent system

Trivalent europium-doped yttrium oxysulfide nanocrystals synthesized using sol-gel thermolysis show significant blue shifts in the excitation bands corresponding to fundamental absorption, charge-transfer absorption. A significant blue shift observed in the fundamental absorption edge for the nanocrystals having an average crystallite size (φ) in the range 9-15 nm indicates a strong quantum confinement with a Bohr exciton radius of 5-13 nm. Also, the diffuse reflectance spectra and the corresponding Kubelka-Munk plot indicate the possibility of profound decrease in the absorption coefficient of Eu³⁺-ligand charge-transfer species necessitating further studies in this wide-gap semiconductor nanocrystalline system.     

Ultrafast Kerr rotations and zero-field dephasing time of electron spins in InAs/GaAs quantum disks

Time-resolved Kerr rotation (TRKR) measurements based on pump-probe arrangement were carried out at 5 K on the monolayer fluctuation induced InAs/GaAs quantum disks grown on GaAs substrate without external magnetic field. The lineshape of TRKR signals shows an unusual dependence on the excitation wavelength, especially antisymmetric step-shaped structures appearing when the excitation wavelength was resonantly scanned over the heavy- and light-hole subbands. Moreover, these step structures possess an almost identical decay time of ∼40 ps which is believed to be the characteristic spin dephasing time of electrons in the extremely narrow InAs/GaAs quantum disks.     

Intersubband optical transitions in one dimensional quantum wire structures

One dimensional (1D) quantum wire structures are emerging as the new generation of semiconductor nanostructures offering exciting physical properties which have significant potential for novel device applications. These structures have been the subject of intensive investigation recently including extensive theoretical and experimental studies of their interband optical properties. In this work we present the results of our study of the intersubband optical transitions in 1D semiconductor quantum wires. The crescent shaped quantum wire structures used for this research were grown on non-planar GaAs substrates. The intersubband transition energy spectra, the selection rules, and the two dimensional envelope wavefunctions were theoretically investigated by using our new LENS (local envelope states) expansion. We present recent experimental results on modulation doped V-groove quantum wires, including PL, PLE, TEM, CL, and infrared polarization resolved spectroscopy. We have observed a very unusual absorption lineshape at the far-infrared wavelengths that we assigned to phonon assisted Fano resonance in a modulation doped quantum wire structure.     

Highly efficient radiative recombination of electron–hole pairs localized at compound semiconductor quantum dots embedded in Si

We have studied the optical properties of compound semiconductor quantum dots (CSQDs) embedded in Si. Both photoluminescence and electroluminescence spectra were found to be associated with an inhomogeneously broadened band in the near-infrared. A long decay lifetime of luminescence was observed, which is in support of an indirect transition in both k- and real-space. Strong localization of electron–hole pairs was found to occur due to a deep potential well created by the built-in electric dipole at the III–V/Si interface. A Si-based light-emitting diode with GaSb-CSQDs in the active layer showed a high value of quantum efficiency. Light amplification was also observed under pulsed laser excitation.     

Diagonalizations on a correlated basis

Unsymmetrical quantum-dot systems are generally difficult to study using wave-function techniques, like quantum Monte Carlo (QMC) or exact diagonalization (ED) methods. The initial trial wave function for Monte Carlo methods is difficult to find, and the exact diagonalization method can only handle very few particles.In this article a two-dimensional semiconductor quantum dot containing a non-centered impurity ion is studied, using a new exact wave-function method. Results are analyzed and compared to density-functional-theory calculations. The computational method allows one to relax the commonly used lowest-Landau level (LLL) approximation, and it's effects are studied, e.g., on the charge and current density profiles.The method, which is a combination of QMC and ED methods, is described. It combines the scalability of Monte Carlo methods with the benefits of exact diagonalization, and allows one to accurately obtain the wave function for unsymmetrical quantum dots up to more than ten electrons. Also, excited states are accessible and are analyzed in this article.     

Formation of nanoscale heterointerfaces by selective area metalorganic vapor-phase epitaxy and their applications

We describe fabrication methods of GaAs and InAs quantum dot (QD) structures and related semiconductor nanostructures having nanoscale heterointerfaces grown by the selective area metalorganic vapor-phase epitaxial (SA-MOVPE) method on partially masked GaAs substrates. GaAs QD arrays and wire–dot coupled structures having strong lateral confinement were fabricated on appropriately designed masked substrates. InAs QDs were also formed on various kinds of GaAs pyramidal and wire structures, where site-selective formation is demonstrated by the combination of self-assembling growth mode and selective area growth. The application of QDs to single-electron transistors using SA-MOVPE is also described.