Inelastic interaction of ultrashort pulses of polarized light with an ensemble of isolated quantum dots

A system of equations is formulated describing the evolution of a slowly varying envelope of an arbitrarily polarized ultrashort pulse of electromagnetic radiation in a medium with its resonant properties determined by an ensemble of isolated quantum dots. It is assumed that the concentration of quantum dots is small and that the whole system is equivalent to a gas of resonant four-level atoms. Particular solutions are found that correspond to the propagation of a stationary optical pulse. It is shown by numerical solution of the generalized truncated Maxwell-Bloch equations that steady-state propagation is possible only for circularly polarized light pulses, whereas the pulses of arbitrary polarization either decay and experience the dispersion-related broadening or are converted into circularly polarized solitary waves.     

Light–matter coupling in nanostructures without an inversion center

A theoretical analysis of the electronic interaction with an intense electromagnetic field in a two-level asymmetrical quantum dot is presented. As a consequence of a strong light–matter coupling in such a system, dipole radiation at the Rabi frequency turns out to be possible. Since the Rabi frequency is controlled by the strength of the coupling electromagnetic field, the effect can serve to provide frequency-tuned parametric amplification and generation of electromagnetic waves. The manifestation of the effect is discussed for group III nitride quantum dots. Terahertz emission from arrays of such quantum dots is shown to be experimentally observable.     

Emission and energy relaxation of excitons in quantum dots with disordered interfaces grown in a low-permittivity matrix

Samples of borosilicate glasses doped by CdS with concentrations smaller than 1% are studied. It is shown that, due to a disorder at interfaces of quantum dots, the main channels of emission of excitons by quantum dots are the annihilation of excitons in quantum and localized surface states, while the efficiency of interaction between the channels largely depends on the radius of quantum dots. It is found for the first time that states that form the second emission channel are not discrete energy levels in the band gap, as is usually assumed in some experimental and theoretical works, but rather form a quasi-continuous tail of the density of localized states. These localized states appear as a result of dangling bonds of outer atoms of quantum dots. Energy relaxation of carriers via localized states is the reason for a long response time of these structures to an external action and can be enhanced due to a polarization effect caused by different dielectric constants of materials of quantum dots and matrix.     

Sizes and fluorescence of cadmium sulfide quantum dots

Cadmium sulfide quantum dots have been synthesized by wet chemical deposition from an aqueous solution. The sizes of the quantum dots determined by dynamic light scattering directly in the colloidal solution and by intermittent-contact atomic force microscopy in the dry sediment agree with each other. It has been found that splitting of the fluorescence peaks of the quantum dots can be affected by the disorder of the atomic structure of cadmium sulfide quantum dots.     

Thermal conductance in a quantum waveguide modulated with quantum dots

We investigate the thermal conductance in a quantum waveguide modulated with quantum dots at low temperatures. It is found that the thermal conductance sensitively depends on the geometrical parameters of the structure and boundary conditions. When the stress-free boundary conditions are applied in the structure, the universal quantum of thermal conductance can be found regardless of the geometry details in the limit T→0. For an uniform quantum waveguide, a thermal conductance plateau can be observed at very low temperatures; while for the quantum waveguide modulated with quantum dots, the plateau disappears, instead a decrease of the thermal conductance can be observed as the temperature goes up in the low temperature region, and its magnitude can be adjusted by the radius of the quantum dot. Moreover, it is found that the quantum waveguide with two coupling quantum dots exhibits oscillatory decaying thermal conductance behavior with the distance between two quantum dots. However, when the hard-wall boundary conditions are applied, the thermal conductance displays different behaviors.     

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the quantum dots and the itinerant holes in the semiconductor valence band makes possible hole transfer between the DMS quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.Received: 17 Mars 2003, Published online: 30 January 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.30.Ds Spin waves - 75.50.Dd Nonmetallic ferromagnetic materials - 75.50.Pp Magnetic semiconductors     

Formation of plasmon pulses in the cooperative decay of excitons of quantum dots near a metal surface

The formation of pulses of surface electromagnetic waves at a metal–dielectric boundary is considered in the process of cooperative decay of excitons of quantum dots distributed near a metal surface in a dielectric layer. It is shown that the efficiency of exciton energy transfer to excited plasmons can, in principle, be increased by selecting the dielectric material with specified values of the complex permittivity. It is found that in the mean field approximation, the semiclassical model of formation of plasmon pulses in the system under study is reduced to the pendulum equation with the additional term of nonlinear losses.     

Spectrum of electron-hole states of the Si/Ge structure with Ge quantum dots

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots of various sizes are investigated. We observed optical transition lines between the hole levels of quantum dots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantum dots are successfully described by the “quantum box” model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantum dots can be controlled by changing the growth parameters.     

Preparation and characterization of nano cobalt oxide

A simple and general method has been proposed for preparing strong violet emitting CdS quantum dots, in which a ligand exchange strategy was applied to surface passivation and functionalization with good reproducibility. The resulting quantum dots showed a visible violet luminescence with emission peak centered near 423 nm and photoluminescence quantum yields reached over 30%. Additionally, different mercapto-compounds used as ligands can make different functionalized surfaces, favoring quantum dots dispersion in different media and their further applications. It was observed that the band edge emission has the main contribution to the bright violet luminescence.     

Mn掺杂ZnS量子点与孔雀石绿和隐性孔雀石绿的相互作用

研究了Mn掺杂ZnS量子点与孔雀石绿和隐性孔雀石绿的相互作用。首先采用水热法以壳聚糖为基质,利用成核掺杂原理制备量子点,然后分别采用紫外-可见吸收和荧光光谱技术研究了量子点与孔雀石绿和隐性孔雀石绿的相互作用。结果表明,向初始浓度为20 mg·L~(-1)孔雀石绿溶液中加入200 mg·L~(-1)量子点,60 min孔雀石绿的去除率约为82%,量子点可以光催化降解孔雀石绿,其反应过程符合假一级动力学的假设;量子点对隐性孔雀石绿具有荧光猝灭作用,猝灭作用为动态猝灭,相互作用力为氢键和范德华力,吉布斯自由能小于零,量子点与隐性孔雀石绿的相互作用是一个自发的过程。因此,量子点可以用于光催化降解孔雀石绿和快速无毒检测隐性孔雀石绿。     

The effect of nutation upon nonresonant propagation of electromagnetic wave in the ensemble of quantum dots

A new type of nutation oscillations is described arising only in the phase of a nonresonant electromagnetic wave (in the vicinity of its leading edge) propagating in the ensemble of quantum dots and producing electron transfer between the states of the quantum dots with approximately the same energy. It is shown that the period of the nutation oscillations can be used to determine main parameters of the nonresonant electron transfer between the quantum dots.     

Interface phonons in semiconductor nanostructures with quantum dots

The vibrational spectra of structures with InAs quantum dots in an AlGaAs matrix and AlAs quantum dots in an InAs matrix are investigated experimentally and theoretically. The Raman spectra exhibit features that correspond to transverse-optical (TO), longitudinal-optical (LO), and interface phonons. The frequencies of interface phonons in InAs and AlAs quantum dots and in an AlGaAs matrix with various concentrations of aluminum are calculated with the use of experimental values of transverse-and longitudinal-optical phonons in the approximation of a dielectric continuum. It is shown that the model of a dielectric continuum adequately describes the behavior of interface phonons in structures with quantum dots under the assumption that the quantum dots are spheroidal.     

Combined cross-sectional STM/AFM in liquid: Study of InGaAs/GaAs heterostructures with quantum wells and dots

Combined scanning tunneling and atomic force microscopy (STM/AFM) of cross-sectional cleavages in a protective liquid medium (oil) is applied to study InGaAs/GaAs heterostructures with quantum wells and dots. It is shown that the quantum wells and dots can be visualized on cleavages in both AFM and STM modes and to measure the current-voltage characteristics of the contact between an AFM probe and the cleavage surface.     

Quantum light memory using quantum dot spins in a microdisk cavity via Raman process

A solid state quantum circuit where an ensemble of self-assembled quantum dots in a microdisk cavity served as long-lived quantum light memory, is investigated. It is shown that via laser coupling Raman process, the coherent transfer between the light field (qubits) and the ensemble spin states of the quantum dots can be efficient and fast. The coherence properties of the system are analyzed, which enables us to obtain a long coherence time.     

Surface enhanced Raman scattering by CdS quantum dots

Surface enhanced Raman scattering is studied in nanostructures with CdS quantum dots formed using the Langmuir-Blodgett technology. Features due to quantum dot longitudinal optical phonons are observed in the Raman spectra of both free CdS quantum dots and such dots distributed in an organic matrix. The surface enhanced Raman scattering by nanostructures with CdS quantum dots covered by an Ag cluster film is observed experimentally. Applying Ag clusters onto the nanostructure surfaces results in a sharp (40-fold) increase in the intensity of Raman scattering by optical phonons in the quantum dots. It is shown that the dependence of surface enhanced Raman scattering on the excitation energy is resonant with a maximum at the energy corresponding to the maximum absorption coefficient of Ag clusters.     

The influence of local phonon modes in a wide-band matrix on the tunnel current-voltage characteristics of quasi-zero-dimensional structures

Experimental results on the visualization of the density of states in InAs/GaSa(001) quantum dots that were obtained by tunnel atomic-force microscopy in an ultrahigh vacuum are presented. A one-dimensional (1D) model of dissipative quantum tunneling is proposed for describing experimental current-voltage characteristics of a tunnel contact between an atomic force microscope probe and the surface of InAs/GaAs (001) quantum dots. It was found that the influence of two local modes of the wide-band matrix on the probability of 1D dissipative tunneling leads to the appearance of several randomly spaced peaks in the field dependence. It was shown that the theoretical dependence agrees qualitatively with experimental the current-voltage characteristic of the atomic force microscope tip and the surface of InAs/GaAs(001) quantum dots.     

Effect of the Semiconductor Quantum Dot Shell Structure on Fluorescence Quenching by Acridine Ligand

The main line of research in cancer treatment is the development of methods for early diagnosis and targeted drug delivery to cancer cells. Fluorescent semiconductor core/shell nanocrystals of quantum dots (e.g., CdSe/ZnS) conjugated with an anticancer drug, e.g., an acridine derivative, allow real-time tracking and control of the process of the drug delivery to tumors. However, linking of acridine derivatives to a quantum dot can be accompanied by quantum dot fluorescence quenching caused by electron transfer from the quantum dot to the organic molecule. In this work, it has been shown that the structure of the shell of the quantum dot plays the decisive role in the process of photoinduced charge transfer from the quantum dot to the acridine ligand, which is responsible for fluorescence quenching. It has been shown that multicomponent ZnS/CdS/ZnS shells of CdSe cores of quantum dots, which have a relatively small thickness, make it possible to significantly suppress a decrease in the quantum yield of fluorescence of quantum dots as compared to both the classical ZnS thin shell and superthick shells of the same composition. Thus, core/multicomponent shell CdSe/ZnS/CdS/ZnS quantum dots can be used as optimal fluorescent probes for the development of systems for diagnosis and treatment of cancer with the use of anticancer compounds based on acridine derivatives.     

红光InAlAs量子点的结构和光学性质

利用ＭＢＥ方法在（００１）衬底上成功地生长密度大、尺寸小、发红光的ＩｎＡｌＡｓ／ＡｌＧａＡｓ量子点结构。通过原子力显微镜观察表明,ＩｎＡｌＡｓ量子的密度和大小都随覆盖厚度的增加而增大;发现Ａｌ原子的表面迁移率决定ＩｎＡｌＡｓ量子点的形貌,光荧光谱证实了量子点的发光峰值在红光范围,并结合形貌的统计得到了量子点的发光峰展宽主要昌受量子点的横向尺寸影响。     

核壳型ZnS∶Cu/ZnS量子点的制备及发光性质

本文采用水相合成方法制备了ZnS∶Cu量子点并进行了ZnS壳层修饰,研究了壳层厚度对ZnS∶Cu量子点光学性质的影响,采用TEM、XRD、PL、PLE和UV-Vis等测试方法对其进行了表征。实验结果表明,合成的ZnS∶Cu/ZnS量子点为立方闪锌矿,尺寸分布均匀呈球形,分散性良好,经过壳层修饰平均粒径由2 nm增加到3.2 nm。随着ZnS壳与ZnS核量的比的增加,量子点的PLE激发峰位置和UV-Vis吸收谱线出现红移,也说明了量子点的尺寸增大,证明ZnS在ZnS∶Cu量子点的表面生长,形成了核壳结构的ZnS∶Cu/ZnS量子点。随着壳层增厚,量子点与铜离子发光中心相关的发射峰强度先增大后减小,当壳核比n_s/n_c=2.5时,发光强度达到最大。     

CdSeS合金结构量子点的多激子俄歇复合过程

多激子效应通常是指吸收单个光子产生多个激子的过程,该效应不仅可以为研究基于量子点的太阳能电池开拓新思路,还可以为提高太阳能电池的光电转换效率提供新方法.但是,超快多激子产生和复合机制尚不明确.这里以CdSeS合金结构量子点为研究对象,研究了其多激子生成和复合动力学.稳态吸收光谱显示, 510, 468和430 nm附近的稳态吸收峰,分别对应1S_(3/2)(h)-1S(e)(或1S), 2S_(3/2)(h)-1S(e)(或2S)和1P_P(3/2)(h)-1P(e)(或1P)激子的吸收带.通过飞秒时间分辨瞬态吸收光谱和纳秒时间分辨荧光光谱两种时间分辨光谱技术对CdSeS合金结构量子点的超快动力学进行了探究,结果显示, 1S激子的双激子复合时间大概是80 ps,这一时间比传统量子点的双激子复合时间(小于50 ps)延长了近一倍,结合最近发展的超快界面电荷分离技术,在激子湮灭之前将其利用起来,这一时间的延长将有很大的应用前景;其中,在2S和1P激子中除上述双激子复合外,还存在一个通过声子耦合路径的空穴弛豫过程,时间大概是5—6 ps.最后,利用纳秒时间分辨荧光光谱得到该样品体系单激子复合的时间约为200 ns.