Quantum Monte Carlo simulation of exciton–exciton scattering in a GaAs/AlGaAs quantum well

We present a computer simulation of exciton–exciton scattering in a quantum well. Specifically, we use quantum Monte Carlo techniques to study the bound and continuum states of two excitons in a 10 nm wide GaAs/Al_0.3Ga_0.7As quantum well. From these bound and continuum states we extract the momentum-dependent phase shifts for s-wave scattering. A surprising finding of this work is that a commonly studied effective-mass model for excitons in a 10 nm quantum well actually supports two bound biexciton states. The second, weakly bound state may dramatically enhance exciton–exciton interactions. We also fit our results to a hard-disk model and indicate directions for future work.     

Study of nonlinear absorption in GaAs/AlGaAs multiple quantum wells using the reflection Z-scan

The room-temperature third order nonlinearities in GaAs/AlGaAs multiple quantum wells have been studied using reflection Z-scan technique. Band-filling effect is considered to be the major nonlinear mechanism of the nonlinear absorption. A model to calculate the absorption coefficient of quantum wells in the nonlinear regime is presented.      

Measurements of far-infrared intersubband absorption linewidths in GaAs/AlGaAs quantum wells as a function of temperature and charge density

Intersubband transitions in doped quantum wells are of fundamental scientific interest, as well as technological interest for potential devices. One of the important characteristics of the transitions is their linewidth. We present the results of linear absorption spectroscopy on a coupled double asymmetric QW structure. Using a backgated sample, we can independently vary the charge density and DC field at the well. The absorption lines appear to be homogeneously broadened. The lines appear to become lifetime broadened for temperatures above 70 K.     

耦合量子阱中激子凝聚的研究进展

近年来，半导体量子阱中激子的玻色一爱因斯坦凝聚研究取得了很大进展．实验上利用耦合量子阱间接激子中电子和空穴在空间上的分离，显著提高了激子的冷却速度和寿命，成功地把激子冷却到1K以下，观察到了激子的准凝聚状态，并且在强激光照射下，发现了随光照强度增强而增大的激子发光环和环上形成的有规则斑点图案，引起了广泛的兴趣和重视．理论研究表明，发光环的出现是电子和空穴在量子阱中的反常输运行为造成的，但环上形成规则斑点的物理机理目前尚不清楚．文章介绍了这方面的实验背景和形成激子环的物理图像，指出了理论研究中存在的问题，并对解决问题的方案进行了讨论．     

Minimum of oscillator strength of excitons in ultra-narrow GaAs/AlGaAs quantum wells:: Theory and experiment

A theoretical and experimental study of the exciton in ultra-narrow quantum wells (QWs) is performed. A crossover from strong (separate localization of electron and hole levels) to weak confinement (with localization of excitonic center of mass) is predicted to occur for decreasing thickness, and is characterized by a minimum of the oscillator strength per unit area. Cathodoluminescence measurements performed on a series of GaAs/Al_0.35Ga_0.65As QWs with thicknesses from one to eight monolayers show a minimum of the oscillator strength, in agreement with theory, and indicate that the crossover from strong to weak confinement occurs at a thickness of about three monolayers for this composition.     

Diffusion process of electrons injected from STM tip into AlGaAs/GaAs quantum wells

We have investigated the electron diffusion process in Al_0.3Ga_0.7As/GaAs quantum well (QW) structures by means of scanning tunneling microscope light emission (STM-LE) spectroscopy. The optical measurements were performed on a cleaved (1 1 0) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants. From comparison with Monte Carlo simulations for hot electron relaxation, we found that the intervalley scattering from the Γ valley to the L and X valleys has the most significant effect on the diffusion process of the injected electrons.     

AlGaAs/GaAs多量子阱结构中受激载流子的飞秒弛豫特性

本文介绍采用飞秒饱和吸收测量技术研究Al_xGa_(1-x)As/GaAs多量子阱结构中受激载流子的超快弛豫特性.当激发光子能量大于样品势垒层能带隙时,受激产生于势垒层和势阱层连续态中的载流子分别在130和30fs时间内离开受激态,弛豫至准平衡态.势垒中的载流子被捕至势阱束缚态的情况主要发生于热载流子的冷却和复合过程的皮秒级时间内.     

Radiative coupling of intersubband transitions in GaAs/AlGaAs multiple quantum wells

Radiative coupling of resonantly excited intersubband transitions in GaAs/AlGaAs multiple quantum wells can have a strong impact on the coherent nonlinear optical response, as is shown by phase and amplitude resolved propagation studies of ultrashort electric field transients. Upon increasing the driving field amplitude, strong radiative coupling leads to a pronounced self-induced absorption, followed by a bleaching due to the onset of delayed Rabi oscillations. A many-particle theory including light propagation effects accounts fully for the experimental results.     

Optically induced level anticrossing in undoped GaAs/AlGaAs coupled double quantum wells

We report a photoluminescence detected anticrossing of the energy levels in an undoped asymmetric coupled-double-quantum-well buried in a p-i-n structure. Due to the built-in electric field, the quantum wells are tilted in such a way that the symmetric energy level is higher than that of the antisymmetric one in the conduction band. Keeping the laser excitation energy below the barrier, with increasing laser power, the level anticrossing and the quantum confined Stark effect were observed due to decreasing built-in electric field by the photogenerated electron and hole pairs.     

Acoustic phonon dephasing in shallow GaAs/Ga Al As single quantum wells

The intermediate dimensionality regime is studied on a set of shallow GaAs/Ga Al As single quantum wells. Such heterostructures exhibit 2D strong excitonic electroabsorption together with near 3D fast transport properties. We report dephasing time measurements of the heavy-hole exciton and we show that the acoustic phonon contribution decreases with to a value in good agreement with theoretical predictions for GaAs bulk.     

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.     

Long-range spatial correlations in the exciton energy distribution in GaAs/AlGaAs quantum wells

Variations in the width of a quantum well (QW) are known to be a source of broadening of the exciton line. Using low temperature near-field optical microscopy, we have exploited the dependence of exciton energy on well width to show that in GaAs QWs, these seemingly random well-width fluctuations actually exhibit well-defined order-strong long-range correlations appearing laterally, in the plane of the QW, as well as vertically, between QWs grown one on top of the other. We show that these fluctuations are correlated with the commonly found mound structure on the surface. This is an intrinsic property of molecular beam epitaxial growth.     

Trions in quantum wells

An overview is given of our investigation of the energy levels and of the correlation functions of the negatively and positively charged excitons (also called trions) in quantum wells in the presence of a perpendicular magnetic field. A detailed comparison is made with available experimental data in III–V and II–VI semiconductor quantum wells.     

Analysis of three-wave mixing in GaAs/AlGaAs multiple quantum wells via biexciton coherence

A cascade-type three-level system of GaAs/AlGaAs multiple quantum wells (MQWs) is constructed by using biexciton coherence in the transient optical response. Then, we theoretically and numerically study the system with Maxwell–Schrödinger equations to obtain the generated three-wave mixing (TWM) field. By means of appropriately adjusting the strength of the control filed and the propagation length in the MQWs sample, the maximum efficiency of the generated TWM field can be achieved at η ≈ 80%.     

Inter-island energy transfer and in-plane exciton migration in AlGaAs/GaAs quantum wells detected by exciton dynamics

We present the results of optical, steady-state and time-resolved studies of photoluminescence and photoluminescence excitation in high-quality Al_0.3Ga_0.7As/GaAs quantum wells in which the presence of large (larger than the exciton radius) atomically flat islands can be inferred, identical to the case of interrupted MBE growth. Migration of excitons towards lower-lying energy states induced by local potential fluctuations and/or progressive localisation has been revealed and the transition rate between quantum well regions 24 to 25 monolayers thick has been derived to be 290 ps⁻¹.     

Phonon satellites and time-resolved studies of carrier recombination dynamics in InGaN quantum wells

We have studied the photoluminescence and time-resolved photoluminescence of a set of InGaN quantum wells with well thickness from 1 to 7.5 nm. An analysis of the phonon satellites at 5 K shows Huang–Rhys factors from 0.32 to 0.44. The increase of this factor is caused by the electron–hole separation induced by the piezoelectric field. The time-resolved photoluminescence at room temperature shows that the decay time of the 1 and 2 nm wells does not depend on the wavelength. The maximum decay time is around 600 ps for the 2, 3 and 4 nm wells. However, for the 3 and 4 nm wells a decrease of the photoluminescence decay time is observed at the highest wavelengths. This suggest the onset of a non-radiative process in these samples. The optimum well width for efficient emission for these single quantum wells was found to be 2 nm.     

Exciton behavior in GaAs/AlGaAs coupled double quantum wells with interface disorder

In this work, we present a detailed study on the optical properties of two GaAs/Al_0.35Ga_0.65As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e₁-hh₁ transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed.     

Elastic scattering of holes by acoustic phonons in thin GaAs/GaAlAs quantum wells

The rates of scattering by acoustic phonons _γi→j${\gamma }_{i\to j}$ from subband i→j$i\to j$ are calculated for holes in a narrow GaAs quantum well using the deformation potential, the Luttinger hamiltonian and the Debye approximation. At room temperature, we find that the energy dependences for both intra- and inter-subband scattering rates follow roughly the behavior of the density of states in the subband to which the hole scatters. Moreover, we study the influence of the overlap between the initial and final states, and for elastic processes we find that, unlike for the case of wide quantum wells, for narrow ones integration over the phonon transverse wavevector _qz$q_z$ should be restricted to about 4% of the bulk Brillouin zone extent. In addition the impact of the well width on _γi→j${\gamma }_{i\to j}$ is investigated.     

Microscopic approach to many-exciton complexes in self-assembled InGaAs/GaAs quantum dots

We present a numerical calculation of many-exciton complexes in self-assembled InAs/GaAs quantum dots. We apply continuum elasticity theory and atomistic valence-force-field method to calculate strain distribution, and make use of various methods, ranging from a quasi-atomistic tight-binding approach to the single-band effective-mass approximation, to obtain single-particle energy levels. The effect of strain is incorporated by the deformation potential theory. We expand multiexciton states in the basis of Slater determinants and solve the many-body problem by the configuration-interaction method. The dynamics of multiexcitons is studied by solving the rate equations, from which the excitation–power dependence of emission spectrum is obtained. The emission spectra calculated by the microscopic tight-binding approach are found to be in good agreement with those obtained by the simple effective-mass method.