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.     

Theoretical studies of polarization dependent electro-optical modulation in lattice matched and strained multi-quantum well structures

We report on polarization dependent optical absorption for excitonic and interband transitions in lattice matched (GaAs/AlGaAs) and strained (biaxial tensile strain - GaAsP/AlGaAs; biaxial compressive strain - InGaAs/AlGaAs) multiquantum well structures in the presence of transverse electric fields. The hole states are solved by using the Kohn-Luttinger Hamiltonian and using an eigenvalue technique. The effect of heavy-hole and light-hole mixing due to the strain, electric field and quantization is studied. Under biaxial tensile strain the heavy-hole and light-hole transition can coincide, leading to interesting polarization dependent effects. Results are presented for excitonic and interband transitions.     

Effect of screening by two-dimensional charge carriers on the binding energy of excitonic states in GaAs/AlGaAs quantum wells

The spectrum of excitonic excited states in GaAs/AlGaAs quantum wells of different width is studied together with its change due to the screening of electron-hole interaction by two-dimensional electrons. The exciton binding energy decreases sharply with an increase in the concentration of two-dimensional electrons. The temperature dependence of screening parameters is studied for the ground and excited excitonic states down to ultralow temperatures T=50 mK.     

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.     

Analysis of quantum well size alteration effects on slow light device based on excitonic population oscillation

In this paper we investigate the effects of quantum well size changes on slow light device properties. The principle properties such as center frequency and slow down factor of a slow light device are affected by changing the size of quantum well. In this way, the effects of quantum well size on Oscillator Strength and binding energy of exciton are considered separately. First, we investigate the variations in oscillator strength of exciton due to different quantum well size. Second, exciton binding energy level shift due to size of quantum well is investigated. According to this analysis, we have developed a new method for tuning slow light device bandwidth center frequency and slow down factor. Analysis and simulation of a basic GaAs/AlGaAs quantum wells optical slow light device based on excitonic population oscillation shows that size of quantum wells could tune both of the frequency properties and slow down factor of an optical slow light device. Simulation results show that slow down factor and oscillation strength of exciton are proportional to each other in direct manner. Moreover, decreasing the quantum well width, causes enhancement in binding energy of excitons. These achievements are useful in optical nonlinearity enhancements, all-optical signal processing applications and optical communications.     

Photoluminescence studies of GaAs/GaAlAs multiple quantum well heterostructures

The photoluminescence excited by He:Ne and Nd:YAG lasers of GaAs/Ga_0.75Al_0.25As multiple quantum well heterostructures grown by MBE was measured as a function of temperature from 4.2 K up to room temperature and for different pumping powers at constant temperature. The excitonic transitions associated with carriers confined in the quantum wells as well as other transitions associated with impurities either already present in the substrates or introduced into the samples during growth are identified in the spectra and fully characterized. From Arrhenius plots of the photoluminescence peak integrated intensities versus inverse temperature, activation energies are estimated for acceptor defects in the samples as well as for quantum well related excitonic transitions. Photoluminescence polarization experiments demonstrate a dramatic manifestation of the selection rules governing heavy hole and light hole optical transitions in quantum wells.     

Exciton-photon strong-coupling regime for a single quantum dot embedded in a microcavity

We report on the observation of the strong-coupling regime between the excitonic transition of a single GaAs quantum dot and a discrete optical mode of a microdisk microcavity. Photoluminescence is performed at various temperatures to tune the quantum dot exciton with respect to the optical mode. At resonance, we observe a clear anticrossing behavior, signature of the strong-coupling regime. The vacuum Rabi splitting amounts to 400 microeV and is twice as large as the individual linewidths.     

半导体量子阱中激子波函数及其 Fourier系数的计算和应用

利用准玻色子方法发展的激子动力学方程是研究半导体纳米结构中激子超快动力学的有效理论手段. 为了将这种方法应用于半导体量子阱, 需要知道量子阱中的激子波函数及其在动量空间的表示, 从而得到激子动力学方程中所必须的系数. 详细讨论了理想和实际量子阱中的激子波函数, 特别是其在动量空间的表示, 并进一步讨论了激子动力学方程中所必须系数的计算方法. 通过求解这些系数, 对量子阱中因激子密度变化而引起的太赫兹脉冲作用下激子能级间跃迁过程中的非线性效应进行了理论预测, 得到了与实验符合很好的结果.     

Optical detection of an electric field in a GaAs/AlGaAs <Emphasis Type="Italic">n</Emphasis>-<Emphasis Type="Italic">i</Emphasis>-<Emphasis Type="Italic">n</Emphasis> heterostructure with double quantum wells

Processes occurring when a static transverse electric field is applied to a GaAs/AlGaAs n-i-n heterostructure with single quantum wells and asymmetric tunnel-coupled double quantum wells have been investigated by optical methods. The difference between the energies of exciton transitions for quantum wells of different widths makes it possible to attribute the observed photoluminescence peaks to particular pairs of wells or particular single quantum wells. The local electric field for each quantum well has been determined in terms of the Stark shift and splitting of exciton lines in a wide range of external voltage. A qualitative model has been proposed to explain the nonmonotonic distribution of the electric field over the depth of the heterostructure.     

Electron-hole correlation singularity in optical spectra of modulation doped GaAsAlGaAs quantum wells

By studying the temperature dependence of absorption and luminescence in modulation doped GaAsAlGaAs quantum wells, we show experimentally that the lowest energy absorption peak has a behavior strikingly different from the corresponding excitonic peak in undoped quantum wells, which is alos unexpected in the frame of the single particle picture. We find good qualitative agreement with the many-body model that attributes this “correlation singularity” to the interaction between the photoexcited hole and the sea of electrons.     

Relaxation and dephasing of multiexcitons in semiconductor quantum dots

We measure the dephasing time of ground-state excitonic transitions in InGaAs quantum dots under electrical injection in the temperature range from 10 to 70 K. Electrical injection into the barrier region results in a pure dephasing of the excitonic transitions. Once the injected carriers fill the electronic ground state, the biexciton to exciton transition is probed and a correlation of the exciton and biexciton phonon scattering mechanisms is found. Additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation.     

Comparison of thermally detected optical absorption and electromodulation spectra of GaAs/AlGaAs quantum wells

We report for the first time a comparative study of GaAs/AlGaAs quantum well (QW) spectra obtained by thermally detected optical absorption (TD-OA) and photo- (PR) and electroreflectance (ER) experiments, respectively. The excitonic transition energies, obtained at low temperature with these methods, agree within 1 meV. It is demonstrated, using samples with different AlGaAs overlayer thicknesses, that the shape of the TD-OA response is strongly influenced by interference effects for QW's grown on absorbing substrates. A complete PR lineshape analysis yields phase angles for all transitions, whose differences can be attributed to the change of the AlGaAs front barrier thicknesses. ER investigations following successive top layer removal confirm this interpretation.     

InGaAs/GaAs单量子阱PL谱的温度变化特性

采用分子束外延方法制备了InGaAs/GaAs单量子阱,利用自组装的光致荧光探测系统,对其进行了光致荧光谱研究。考察了不同温度下荧光峰波长、峰形的影响。研究结果表明:高温时荧光主要是源于带—带间载流子跃迁,而在低温时则来源于束缚在量子阱中激子的跃迁。     

Ultrafast all optical modulation based on intersubband transition in semiconductor quantum wells

Ultrafast modulation of interband-resonant light by intersubband-resonant light in n-doped GaAs/AlGaAs and GaN/AlGaN quantum wells was investigated by femtosecond pump-probe technique. A planar-type AlGaAs/GaAs modulation device shows a modulation speed of ～1 ps at room temperature. The observed modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ～1 pJ when a waveguide-type device structure is utilized. The feasibility of the all-optical modulation in GaN/AlGaN quantum wells is also investigated. The intersubband carrier relaxation time, which mainly determines the modulation speed, is measured and is found to be extremely fast (130–170 fs). The results indicate that the optical modulation at a bit rate of over 1 Tb/s will be possible by utilizing the intersubband transition in GaN/AlGaN quantum wells. The modulation efficiency in GaN/AlGaN quantum wells is also discussed in comparison with that in GaAs/AlGaAs quantum wells.     

A study of GaInP/GaAs interfaces: metallurgical coupling of successive quantum wells

We present a study of GaInP/GaAs interfaces by means of photoluminescence (PL) of multi quantum wells (MQW), embedded in GaInP, or asymmetric structures having an AlGaAs barrier GaInP/GaAs/AlGaAs. The PL energies of quantum wells were compared with calculations based on the transfer matrix envelope function approximation, well suited for asymmetric structures. GaInP/GaAs/AlGaAs MQW structures (GaInP grown first) are in reasonably good agreement with calculations. Reverse ones, AlGaAs/GaAs/GaInP, present a lower PL energy than calculated. But the agreement with theory is recovered on single quantum well samples, or in MQW when the GaInP thickness is increased up to 100 nm. We interpret this phenomenon as a diffusion of arsenic atoms from the next GaAs well through the GaInP barrier. Arsenic atoms exchange with phosphorus atoms at the GaInP-on-GaAs interface of the former well, leading to a small gap strained InGaAs region responsible for the lowering of PL energies.     

Interference of polariton waves in structures with wide GaAs/AlGaAs quantum wells

The optical reflection spectra of semiconductor GaAs/AlGaAs structures with wide quantum wells are studied experimentally. A theoretical analysis of the spectra is performed in terms of the exciton-polariton model in the approximation of quantum confinement of the exciton center of mass with regard to the contributions of both heavy and light excitons to the crystal polarization. The applicability range of the theory of the center-of-mass confinement for GaAs/AlGaAs heterostructures is estimated. It is established that, for quantum wells more than 180 nm wide, the interference effects observed in the reflection spectra of polariton waves are reproduced, to a good accuracy, by theoretical calculations based on the quantum confinement of the exciton center of mass. For quantum-well widths less than 150 nm, the experimental results are described better by the model of quantum confinement of electrons and holes.     

Two-photon absorption processes in GaAs/Al x Ga1−x As quantum wells

Summary The two-photon absorption spectra of GaAs/AlGaAs multiple quantum well and superlattice structures have been experimentally investigated by means of the nonlinear luminescence technique in different polarization configurations. A strong excitonic effect overlapping the interband two-photon spectrum has been found and the selection rules for the excitonic transitions have shown to greatly change for different polarizations. The comparison of linear and nonlinear absorption measurements provides important information on the excited states of excitons in multiple quantum wells. Work partially supported by M.P.I.     

Gated spin relaxation in (1 1 0)-oriented quantum wells

Growth, photoluminescence characterisation and time-resolved optical measurements of electron spin dynamics in (1 1 0)-oriented GaAs/AlGaAs quantum wells are described. Conditions are given for MBE growth of good-quality quantum wells, judged by the width of low-temperature excitonic photoluminescence. At 170 K the electron spin relaxation rate in (1 1 0)-oriented wells shows a 100-fold reduction compared to equivalent (1 0 0)-oriented wells and also a 10-fold increase with applied electric field from 20 to 80 kV cm⁻¹. There is evidence for similar dramatic effects at 300 K. Spin relaxation is field independent below 20 kV cm⁻¹ reflecting quantum well asymmetry. The results indicate the achievability of voltage-gateable quantum well spin memory time longer than 10 ns at room temperature simultaneously with high electron mobility.