Electron spin beats in InGaAs/GaAs quantum dots

Time-resolved picosecond spectroscopy is used for the first time to study optical orientation and spin dynamics of carriers in self-organized In(Ga)As/GaAs quantum-dot (QD) arrays. Optical orientation of carriers created by 1.2 ps light pulses, both in the GaAs matrix and wetting layer, and captured by QDs is found to last a few hundreds of picosecond. The saturation of electron ground state at high-excitation-light intensity leads to electron polarization in excited states close to 100% and to its vanishing in ground state. Electron-spin quantum beats in a transverse magnetic field are observed for the first time in semiconductor QDs. We thus determine the quasi-zero-dimensional electron g factor in In_0.5Ga_0.5As/GaAs QDs to be: |g _⊥|=0.27±0.03. Fiz. Tverd. Tela (St. Petersburg) 41, 871–874 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Electron dynamics in modulation p-doped InGaAs/GaAs quantum dots

We investigate the electron dynamics of p-type modulation doped and undoped InGaAs/GaAs quantum dots using up-conversion photoluminescence at low temperature and room temperature. The rise time of the p-doped sample is significantly shorter than that of the undoped at low temperature. With increasing to room temperature the undoped sample exhibits a decreased rise time whilst that of the doped sample does not change. A relaxation mechanism of electron-hole scattering is proposed in which the doped quantum dots exhibit an enhanced and temperature independent relaxation due to excess built-in holes in the valence band of the quantum dots. In contrast, the rise time of the undoped quantum dots decreases significantly at room temperature due to the large availability of holes in the ground state of the valence band. Furthermore, modulation p-doping results in a shorter lifetime due to the presence of excess defects.     

Excitonic energy shell structure of self-assembled InGaAs/GaAs quantum dots

Performing optical spectroscopy of highly homogeneous quantum dot arrays in ultrahigh magnetic fields, an unprecedently well resolved Fock-Darwin spectrum is observed. The existence of up to four degenerate electronic shells is demonstrated where the magnetic field lifts the initial degeneracies, which reappear when levels with different angular momenta come into resonance. The resulting level shifting and crossing pattern also show evidence of many-body effects such as the mixing of configurations and exciton condensation at the resonances.     

Room-temperature broadband emission of an InGaAs/GaAs quantum dots laser

We report the first demonstration to our knowledge of an ultrabroad emission laser using InGaAs/GaAs quantum dots by cycled monolayer deposition. The device exhibits a lasing wavelength coverage of approximately 40 nm at an approximately 1160 nm center wavelength at room temperature. The broadband signature results from the superposition of quantized lasing states from highly inhomogeneous dots.     

Photoluminescence saturation in InGaAs/GaAs single quantum wells

Saturation of the photoluminescence associated with the 11H transition in the InGaAs single quantum wells is observed under high intensity optical excitation. At the onset of saturation, a spill-over of the photoluminescence occurs into the GaAs cladding layers as the excitation intensity is increased. The measurements are used to determine a limiting value of the quantum efficiency of the quantum-well associated photoluminescence.     

Control of vertically coupled InGaAs/GaAs quantum dots with electric fields

Controllable interactions that couple quantum dots are a key requirement in the search for scalable solid state implementations for quantum information technology. From optical studies of excitons and corresponding calculations, we demonstrate that an electric field on vertically coupled pairs of In(0.6)Ga(0.4)As/GaAs quantum dots controls the mixing of the exciton states on the two dots and also provides controllable coupling between carriers in the dots.     

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.     

State-filling and magneto-photoluminescence of excited states in InGaAs/GaAs self-assembled quantum dots

We present the first radiative lifetime measurements and magneto-photoluminescence results of excited states in InGaAs/GaAs semiconductor self-assembled quantum dots. By increasing the photo-excitation intensity, excited state interband transitions up ton= 5 can be observed in the emission spectrum. The dynamics of the interband transitions and the inter-sublevel relaxation in these zero-dimensional energy levels lead to state-filling of the lower-energy states, allowing the quasi-Fermi level to be raised by more than 200 meV due to the combined large inter-sublevel spacing and the low density of states. The decay time of each energy level obtained under various excitation conditions is used to evaluate the inter-sublevel thermalization time. Finally, the emission spectrum of the dots filled with an average of about eight excitons is measured in magnetic fields up to 13 Tesla. The dependences of the spectrum as a function of carrier density and magnetic field are compared to calculations and interpreted in terms of coherent many-exciton states and their destruction by the magnetic field.     

Infrared-light propagation and storing through two lateral tunnel-coupled InGaAs/GaAs quantum dots

Dynamic propagation and storing-retrieving of a weak infrared (IR)-light pulse in a coupled semiconductor double-quantum-dot (SDQD) structure are studied theoretically with feasible parameters. Two characteristic features are found. First, it is shown that, with a constant control field, the SDQD medium is transparent to the probe pulse which propagates over sufficiently long distances. Furthermore, the pulse shape remains unchanged on propagation. Second, with a time-varying control field, we are able to store and retrieve the probe pulse in this four-subband SDQD medium by adiabatically switching off and on the control field. Such a four-subband SDQD system for storing and retrieving coherent information is much more practical than its atomic counterpart as a result of its flexible design and the controllable (tunable) interference strength and thus provides a new possibility for technological applications in quantum information science in the SDQD solid-state nanostructure.     

Exciton line broadening in strained InGaAs/GaAs single quantum wells

We report the first observation of well-resolved exciton peaks in the room-temperature absorption spectrum of the strained In_0.20Ga_0.80As/GaAs Single Quantum-Well (SQW) structure. The best fit of the exciton resonances gives the conduction-band offset ratioQ _c=0.70±0.05. The strength of the exciton-phonon coupling is determined from linewidth analysis and is found to be much larger than that of strained InGaAs/GaAs MQW structures.     

Pulsed optically detected NMR of single GaAs/AlGaAs quantum wells

While nuclear magnetic resonance (NMR) is one of the most important experimental tools for the analysis of bulk materials, the low sensitivity of conventional NMR makes it unsuitable for the investigation of small structures. We introduce an experimental scheme that makes NMR spectra of single, nanometer-sized quantum wells possible with excellent sensitivity and selectivity while avoiding the spectral broadening associated with some alternative techniques. The scheme combines optical pumping and pulsed radiofrequency excitation of the nuclei with time-resolved detection of the free induction decay through the polarization of the photoluminescence.     

Direct formation of InGaAs/GaAs quantum dots during submonolayer epitaxies from molecular beams

Direct formation of InGaAs/GaAs quantum dots is studied comparatively by Reflection High Energy Electron Diffraction, Scanning Tunneling Microscopy and Photoluminescence methods. It is found that submonolayer migration enhanced epitaxy growth mode allows a reduction in the dispersion of quantum dots size distribution as compared to submonolayer molecular beam epitaxy. In situ and ex situ results obtained are in agreement with each other and give a conceptual kinetic picture of the growth processes during epitaxy from molecular beams.     

Emission and strain in InGaAs/GaAs quantum wells with InAs quantum dots obtained at different temperatures

The photoluminescence (PL), its temperature dependence and X ray diffraction (XRD) have been studied in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs), obtained with the variation of QD growth temperatures (470–535 °C). The increase of QD growth temperatures is accompanied by the enlargement of QD lateral sizes (from 12 up to 28 nm) and by the shift non monotonously of PL peak positions. The fitting procedure has been applied for the analysis of the temperature dependence of PL peaks. The obtained fitting parameters testify that in studied QD structures the process of In/Ga interdiffusion between QDs and capping/buffer layers takes place partially. However this process cannot explain the difference in PL peak positions.     

Exciton fine structure splitting of single InGaAs self-assembled quantum dots

We show how the resonant absorption of the ground state neutral exciton confined in a single InGaAs self-assembled quantum dot can be directly observed in an optical transmission experiment. A spectrum of the differential transmitted intensity is obtained by sweeping the exciton energy into resonance with laser photons exploiting the voltage induced Stark-shift. We describe the details of this experimental technique and some example results which exploit the 1 μeV spectral resolution. In addition to the fine structure splitting of the neutral exciton and an upper bound on the homogeneous linewidth at 4.2 K, we also determine the transition electric dipole moment.     

Inelastic scattering processes in GaAs/n-AlGaAs selectively doped heterojunctions with InGaAs quantum dots

Inelastic scattering processes of the two-dimensional electron gas (2DEG) in both normal and inverted n-AlGaAs/GaAs heterojunction FET structures have been studied, for the case where InGaAs dots are embedded in the vicinity of GaAs channel. By analyzing the magnetoresistance data, the inelastic scattering time τ_in is determined as a function of the concentration N_2D of 2D electrons and shown to be reduced by 10–40% by the presence of InGaAs dots. By investigating a GaAs/n-AlGaAs inverted heterojunction FET with embedded InGaAs dots, we have varied the percentage P_oc of charged dots filled with an electron and found that τ_in decreases as P_oc increases, indicating that the inelastic scattering rate of 2DEG by charged dots is higher than that by the neutral ones.     

Photoexcited carrier dynamics in aligned InAs/GaAs quantum dots grown on strain-relaxed InGaAs layers

Carrier dynamics in aligned InAs/GaAs quantum dots (QDs) grown on cross-hatched patterns induced by metastable In_xGa_1−xAs layers have been studied by time-resolved photoluminescence. The low-temperature carrier lifetimes were found to be of the order of 100–200 ps and determined by carrier trapping and nonradiative recombination. Comparisons with control “nonaligned” InAs QDs show remarkable differences in dependence of peak PL intensities on excitation power, and in PL decay times dependences on both temperature and excitation intensities. Possible origin of traps, which determine the carrier lifetimes, is discussed.     

Magneto-optical single dot spectroscopy of GaSb/GaAs type II quantum dots

We have investigated magneto-optical properties of GaSb/GaAs self-assemble type II quantum dots by single dot spectroscopy in magnetic field. We have observed clear Zeeman splitting and diamagnetic shift of GaSb/GaAs quantum dots. The diamagnetic coefficient ranges from 5 to 30 μeV/T². The large coefficient and their large distribution are attributed to the size inhomogeneity and electron localization outside the dot. The g-factor of GaSb/GaAs quantum dots is slightly larger than that of similar type I InGaAs/GaAs quantum dots. In addition, we find almost linear relationship between the diamagnetic coefficient and the g-factor. The linear increase of g-factor with diamagnetic coefficient is due to an increase of spin-orbit interaction with dot size.     

Exciton optical absorption in disordered,strained InGaAs/GaAs single quantum wells

The absorption coefficient spectrum of undoped, disordered InGaAs/GaAs single quantum wells is calculated within the parabolic band approximation in this compressively strained structure. Results are presented for light propagating normal to and along the plane of the quantum well, taking into consideration the 1S-like exciton and all bound states, including the 2D enhancement Sommerfeld factor. The results presented here show that the exciton peaks in TE polarization remain constant with disordering and exhibit a larger wavelength shift than in TM polarization. The absorption edge in disordered InGaAs/GaAs, which is a function of the strain and interdiffusion, can be tailored to the desired wavelength around 1.0 μm. These results can be of interest in the design of photonic devices on a single substrate.     

InGaAs/GaAs量子链的光学特性研究

研究了InGaAs/GaAs量子链的稳态和瞬态光谱特性,特别是载流子的动力学过程.实验发现荧光寿命有很强的探测能量依赖关系,荧光寿命随发光能量的增加而减小;实验还发现,当激发功率较小时,荧光寿命随激发功率增大而增大,当激发功率足够大时,荧光寿命趋于饱和.这些结果清楚地表明,在量子链结构中,参与发光的载流子之间存在明显的耦合和输运现象,进一步分析表明,这种输运主要是由于载流子沿量子链方向的耦合造成的.发光的偏振特性研究进一步证实了载流子沿量子链方向输运过程. 关键词： InGaAs/GaAs 量子点 量子链     

Photo-pumped lasing from low density self-assembled InGaAs/GaAs quantum dots within a high-Q vertical cavity

A high-Q cavity containing three layers of self-assembled InGaAs/GaAs quantum dots has been prepared. Emission occurs in the wavelength range 1275–1285 nm over the wafer surface. We have observed lasing thresholds in the power-in-versus-power-out characteristics, with associated changes in the angular emission profile, when the structure is optically pumped CW at 300 K. At high pump powers spectrally resolved lateral modes are seen in the emission spectra of a planar cavity and this is discussed in terms of the index change induced by the pump laser.