Time-resolved photoluminescence spectra of high-density InGaAs/AlGaAs quantum wire structures

We investigated the temperature dependence of the time-resolved photoluminescence (PL) spectra of high-density InGaAs/AlGaAs quantum wire (QWR) distributed-feedback laser structure on a submicron grating. A red-shift of peak in the time-resolved PL after photo-excitation was observed due to the relaxation of the photo-generated excitons from the entire QWR to localized centers at 10 K. On the other hand, at 60 K, no red-shift in the time-resolved PL spectra was observed since the localization centers are thermally activated and the excitons are delocalized.     

Energy Spectra of Excitons Bound to a Neutral Acceptor in Quantum Dots

The energy spectra of the ground state for an exciton (X) trapped by a neutral acceptor (A⁰) in a quantum dot with a parabolic confinement have been calculated as a function of the electron-to-hole mass ratio σ by using the hyperspherical coordinates. We find that the (A⁰,X) complex confined in a quantum dot has in general a larger binding energy than those in a two-dimensional quantum well and a three-dimensional bulk semiconductor, and the binding energy decreases with the increase of the electron-to-hole mass ratio.     

Ground State of a Polaron in a Symmetric Triangular Quantum Well

We study the effects of electron-phonon interaction on the electron ground state in a symmetric triangular quantum well, and calculate the ground state energy of an electron in the GaAs/Al0.96Ga0.04As triangular quantum well including the effects of the interaction between electrons and confined LO phonons by using a modified Lee-Low-Pines variational method. The electron wavefunction in the triangular well is chosen as the Airy function. The numerical results are given and discussed.     

InAs 单量子点中级联辐射光子的关联测量

利用分子束外延生长InAs单量子点样品,温度为5 K时,测量了单量子点中单、双激子自发辐射的荧光(PL)光谱.研究了单、双激子发光强度随激发功率的变化及对应发光峰的偏振特性和精细结构劈裂.基于Hanbury-Brown Twiss(HBT) 实验,测量了单、双激子间发光光谱的关联函数,证实了其发光过程为级联发射过程. 关键词： InAs 单量子点 单、双激子 荧光光谱 级联辐射     

Spontaneous light emission from fractional-dimensional excitons

We apply the concept of fractional-dimensional excitons to study the process of light emission in quantum confined systems. We focus on a single parameter , known as the degree of dimensionality and which is related to the exciton coherence volume. We compute rates of light emission due to free excitons in GaAs/Al_xGa_1-xAs quantum wells as functions of and link them to experimental observations. The rates are compared with those of quantum well excitons embedded in a microcavity.     

Near-field optical mapping of exciton wave functions in a GaAs quantum dot

Near-field photoluminescence imaging spectroscopy of naturally occurring GaAs quantum dots (QDs) is presented. We successfully mapped out center-of -mass wave functions of an exciton confined in a GaAs QD in real space due to the enhancement of spatial resolution up to 30 nm. As a consequence, we discovered that the spatial profile of the exciton emission, which reflects the shape of a monolayer-high island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. This novel technique can be extensively applied to wave function engineering in the design and the fabrication of quantum devices.     

Ground state energy of excitons in quantum dot treated variationally via Hylleraas-like wavefunction

In this work, the effects of quantum confinement on the ground state energy of a correlated electron--hole pair in a spherical and in a disc-like quantum dot have been investigated as a function of quantum dot size. Under parabolic confinement potential and within effective mass approximation Ritz's variational method is applied to Hylleraas-like trial wavefunction. An efficient method for reducing the main effort of the calculation of terms like $r^k_{\rm eh}\exp(-\lambda r_{\rm eh})$is introduced. The main contribution of the present work is the introduction of integral transforms which provide the calculation of expectation value of energy and the related matrix elements to be done analytically over single-particle coordinates instead of Hylleraas coordinates.     

掺杂ZnSe/BeTe Ⅱ型量子阱结构中带电激子的磁场效应

报道了n型掺杂ZnSe/BeTe/ZnSe Ⅱ型量子阱(type-Ⅱ QW)在极低温 (5—10 K)条件下的各种光学性质. 磁场中(Farada配置)ZnSe层的反射光谱展示了一个典型的负的带电激子(X^-)的跃迁特征. 对于空间间接光致发光(spacially indirect PL)光谱,它的主发光峰显示了一个反玻尔兹曼分布的非对称性,并且在磁场中(Voigt配置)它的峰值能量随磁场的增加而降低. 这些实验结果显示了该掺杂样品的空间间接PL是来自Ⅱ型QW结构所特有的带电激子的跃迁. 关键词： 光致发光 二维电子气 带电激子 Ⅱ型量子阱     

Exciton localization behaviour in different well width undoped GaN/Al0.07Ga0.93N nanostructures

We report results from optical spectroscopy such as photoluminescence (PL) and time resolved photo-luminescence (TRPL) techniques from different well width MOCVD grown GaN/Al_0.07Ga_0.93N MQW samples. There is evidence of localization at low temperature in all samples. The decay time of all samples becomes non-exponential when the detection energy is increased with respect to the peak of the emission. Localization of carriers (excitons) is demonstrated by the “S-shape” dependences of the PL peak energies on the temperature. The time-resolved PL spectra of the 3-nm well multi quantum wells reveal that the spectral peak position shifts toward lower energies as the decay time increases and becomes red-shifted at longer decay times. There is a gradient in the PL decay time across the emission peak profile, so that the PL process at low temperatures is a free electron-localized hole transition.     

调制n型掺杂ZnSe/BeTe Ⅱ型量子阱结构的发光特性

报道了调制n型掺杂ZnSe/BeTe/ZnSe Ⅱ型量子阱(type-II QW)在极低温至室温(14—296K)条件下的各种光学性质. 反射光谱显示了对于非掺杂样品,激子(X)的跃迁起着支配作用,而只有在掺杂样品的光谱里展示了一个典型的负的带电激子(X^-)的跃迁特征. PL光谱及其直线偏振度P_l都显著地依赖于n型掺杂量和平行于QW生长方向的外加电场. 这个特征被认为是由n型掺杂导致了内秉电场(built 关键词： 光致发光 二维电子气 带电激子 Ⅱ型量子阱     

Exciton relaxation in Ga1 − xInxAs/GaAs self-organized quantum dots

The exciton dynamics in Ga_1 − xIn_xAs/GaAs self-organized quantum dots grown on GaAs (111)B substrates are studied by the time-resolved photoluminescence (PL). We have found the intra-dot exciton relaxation by the reduction of the linewidth and peak energy and also by the energy-dependent PL rise time in the transient PL spectra. Compared with the energy relaxation in the reference quantum wells, we have confirmed that the exciton relaxation in three-dimensionally confined quantum dots is slower than in the quantum wells.     

Theory of indirect exciton photoluminescence in elevated quantum trap

Inspired by an experiment of indirect excitons photoluminescence (PL) in elevated quantum trap (High et al., 2009), we theoretically investigate the energy relaxation and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that, when increasing the laser power, the intensity reversion of two PL peaks is due to the phonon necklace effect. In addition, we use a nonlinear Schrödinger equation including attractive two-body, repulsive three-body interactions and the excitation power dependence of energy distribution to understand the exciton states. This model gives a natural account for the PL blue shift with the increase of the excitation power. This study thus provides an alternative way to understand the underlying physics of the exciton dynamics in coupled potential wells.     

LO-phonon assisted tunneling from spatially direct to indirect exciton in semimagnetic double quantum well structures

Relaxation from spatially direct to the spatially indirect exciton through ZnSe barriers of different thicknesses is investigated in (ZnCdMn)Se/ZnSe/(ZnCd)Se asymmetric double quantum wells by use of magneto-optical steady-state photoluminescence (PL) and PL excitation (PLE) experiments. The 1-LO-phonon scattering has been found to be the relevant mechanism for effective electron and hole tunneling.     

Micro-Photoluminescence Confocal Mapping of Single V-Grooved GaAs Quantum Wire

We perform the micro-photoluminescence measurement at low temperatures and a scanning optical mapping with high spatial resolution of a single V-grooved GaAs quantum wire modified by the selective ion-implantation and rapid thermally annealing. While the mapping shows the luminescences respectively from the quantum wires and from quantum well areas between quantum wires in general, the micro-photoluminescence at liquid He temperatures reveals a plenty of spectral structures of the PL band for a single quantum wire. The spectral structures are attributed to the inhomogeneity and non-uniformity of both the space structure and compositions of real wires as well as the defects nearby the interface between quantum wire and surrounding quantum well structures. All these make the excitons farther localized in quasi-zero-dimensional quantum potential boxes related to these non-uniformity and/or defects. The results also demonstrate the ability of micro-photoluminescence measurement and mapping for the characterization of both opto-electronic and structural properties of real quantum wires.     

Energy gaps for fractional quantum Hall states described by a Chern-Simons composite fermion wavefunction

The Jain's composite fermion wavefunction has proven quite succesful to describe most of the fractional quantum Hall states. Its mathematical foundation lies in the Chern-Simons field theory for the electrons in the lowest Landau level, despite the fact that such wavefunction is different from a typical mean-field level Chern-Simons wavefunction. It is known that the energy excitation gaps for fractional Hall states described by Jain's composite fermion wavefunction cannot be calculated analytically. We note that analytic results for the energy excitation gaps of fractional Hall states described by a fermion Chern-Simons wavefunction are readily obtained by using a technique originating from nuclear matter studies. By adopting this technique to the fractional quantum Hall effect we obtained analytical results for the excitation energy gaps of all fractional Hall states described by a Chern-Simons wavefunction. Received 9 March 2001     

(CdZnTe,ZnS)/ZnTe复合量子阱的光学特性研究

设计并制备了一种新型的(CdZnTe,ZnS)/ZnTe复合量子阱结构.使CdZnTe量子阱中的激子有可能在短时间内隧穿到ZnS阱层,从而达到提高光双稳器件“关”速度的目的.并通过对发光特性的研究证实在我们设计的结构中横向激子隧穿的存在,从而为进一步研究超高速光开关提供了实验依据.     

Competition between carrier recombination and tunneling in quantum dots and rings under the action of electric fields

In the presence of a stationary electric field applied in the growth direction tunneling of electrons out of the quantum dots can take place. This mechanism competes with the quantum confined Stark effect (QCSE) that produces an increase of the exciton lifetime by increasing the electric field, mainly due to a net decrease of the electron–hole wavefunction overlap. The electric field range where QCSE dominates over tunneling will be mainly determined by the size of the nanostructure along the vertical direction (height), as demonstrated in this work.     

Quantum confined luminescence in Si/SiO2 superlattices

Amorphous Si/SiO₂ superlattices with periodicities between 2 and 5 nm have now been grown on (1 00) Si wafers by several different techniques: molecular beam epitaxy, magnetron sputtering, and plasma enhanced chemical vapor deposition (PECVD). With the first two methods little or no hydrogen is incorporated during growth and visible photo-luminescence (PL) is obtained at wavelengths from 520 to 800 nm. The shift in the PL peak position with Si layer thickness is consistent with quantum confined band-to-band recombination. Annealing the sputtered superlattices at temperatures up to 1100°C results in a very bright red PL that is similar in intensity to that observed in porous Si samples. For large numbers of periods (e.g., 425) the PL is strongly modulated in intensity owing to optical interference within the superlattice. Similar quantum confined, but defect induced, PL is also observed in the PECVD grown superlattices, where the amorphous Si layers are heavily hydrogenated.     

On Jastrow wavefunctions containing attractive correlations

A quantum mechanical explanation is given in support of the form of Jastrow wavefunction introduced by De Michelis and Reatto. We propose a new, simpler wavefunction which has the correct long-range behaviour.     

Oscillator strength of type-II light-hole exciton in InxGa1−xAs/GaAs strained single quantum wells

We have investigated the excitonic properties of In_0.15Ga_0.85As/GaAs strained single quantum wells by using photoreflectance spectroscopy and a variational calculation method. We clearly detected the photoreflectance signal of the type-II light-hole exciton, which consists of an electron confined in the InGaAs layer and a light hole located in the thick GaAs layer, in addition to the type-I heavy-hole exciton confined in the InGaAs layer. The calculated results of the overlap integral of the envelope function in the type-II light-hole exciton predict that the oscillator strength is remarkably enhanced with decreasing the InGaAs-layer thickness. This is demonstrated by the layer-thickness dependence of the photoreflectance intensity of the type-II light-hole exciton.