Optical control of spin coherence in singly charged (In,Ga)As/GaAs quantum dots

Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses.     

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.     

Optical tailoring of electron spin coherence in quantum dots

We address the precession of an ensemble of electron spins, each confined in a (In, Ga)As/GaAs self-assembled quantum dot. The quantum dot inhomogeneity is directly reflected in the precession of the optically oriented electron spins about an external magnetic field, which is subject to fast dephasing on a nanoseconds time scale. Proper periodic laser excitation allows synchronization of the electron spin precessions with the excitation cycle. The experimental conditions can be tailored such that eventually all (about a million) electron spins that are excited by the laser precess with a single frequency. In this regime the ensemble can be exploited during the single electron spin coherence times being in the microseconds range.     

Recombination in tensile-strained In0.3Ga0.7As quantum well on InP

Photoluminescence (PL) measurements under different excitation powers were carried out at low temperature on tensile-strained In_0.3Ga_0.7As single wells of 6 nm with InGaAs barriers lattice matched to InP substrate. PL measurements taken at 2 K show a main emission band at 0.762 eV probably originating from a type-II transition. The insertion of an ultrathin InAs layer at In_0.3Ga_0.7As on In_0.53Ga_0.47As interface reveals an additional feature at 0.711 eV as well as an excited-state luminescence emission at high pump powers. The InAs insertion improves heterointerface quality, which was confirmed by an increase in PL intensity.     

The temperature dependence of the photoluminescence from overgrown GaAs/Al0.3Ga0.7As quantum dots and wires

Photoluminescence measurements on GaAs/Al_0.3Ga_0.7As quantum dots and wires fabricated using electron bears lithography and reactive ion etching are reported both before and after regrowth with a layer of Al_0.4Ga_0.6As. Dots exhibit little change in luminescence efficiency from the bulk with a reduction in diameter either before or after regrowth. Surface recombination therefore appears to be suppressed. In wires, however, luminescence intensity is very sensitive to wire width, decreasing rapidly with this parameter, but recovers and becomes independent of size after overgrowth. The temperature dependence of the photoluminescence from the dots and wires showed that dots and wires less than 150nm in width luminesced to higher temperatures than the larger diameter structures and dots liminesced to higher temperatures than wires of comparable width. This suggests that there is a finite coherence area effect which increases the radiative lifetimes of excitons in the quantum structures due to the geometric constraint, in the lateral direction in the wires and in all three directions in the dots. Below 20K bound exciton luminescence dominates in the dots but not in the wires. In wires it is still possible for the excitons to diffuse to nonradiative sites within the exciton lifetime. Regrowth at 750°C causes migration of aluminium into the quantum well and causes the shape of the well to become parabolic resulting shifts in the exciton emission to shorter wavelengths, making it difficult to separate the effect of processing from those due to quantum confinement.     

Optical generation of spin coherence in single-charged (In,Ga)As/GaAs self-assembled quantum dots

The generation of electron spin coherence has been studied in n-modulation-doped (In,Ga)As/GaAs self-assembled quantum dots (QDs) which contain on average a single electron per dot. The doping has been confirmed by pump–probe Faraday rotation experiments in a magnetic field parallel to the heterostructure growth direction. For studying spin coherence, the magnetic field was rotated by 90° to the Voigt geometry, and the precession of the electron spin about the field was monitored. The coherence is generated by resonant excitation of the QDs with circularly polarized laser pulses, creating a coherent superposition of an electron, and a trion state. The efficiency of the generation can be controlled by the pulse intensity, being most efficient for (2n+1)π pulses.     

Magnetoabsorption spectra of magnetoexciton transitions in GaAs/Ga0.7Al0.3As quantum wells

The internal transitions and absorption spectra of confined magnetoexcitons in GaAs/Ga0.TA10.aAs quan- tum wells have been theoretically investigated under magnetic fields along the growth direction of the semiconductor heterostructure. The magnetoexciton states are obtained within the effective-mass ap- proximation by using a variational procedure. The trial exciton-envelope wavefunctions are described as hydrogeniclike polynomial functions. The internal transition energies are investigated by studying the allowed magnetoexcitonic transitions using terahertz radiation circularly polarized in the plane of the quantum well. The intraexcitonic magnetoabsorption to 2p^± like magnetoexciton states as functions of the coefficients are obtained for transitions from 1s-like applied magnetic field.     

Investigation of various optical transitions in GaAs/Al0.3Ga0.7As double quantum ring grown by droplet epitaxy

This work examines the optical transitions of a GaAs double quantum ring (DQR) embedded in Al_0.3Ga_0.7As matrix by photoreflectance spectroscopy (PR). The GaAs DQR was grown by droplet epitaxy (DE). The optical properties of the DQR were investigated by excitation‐intensity and temperature‐dependent PR. The various optical transitions were observed in PR spectra, whereas the photoluminescence (PL) spectrum shows only the DQR and GaAs band emissions. The various optical transitions were identified for the GaAs near‐band‐edge transition, surface confined state (SCS), DQR confined state, wetting layer (WL), spin–orbital split (E_GaAs + Δ_o), and AlGaAs band transition. PR spectroscopy can identify various optical transitions that are invisible in PL. The PR results show that the GaAs/AlGaAs DQR has complex electronic structures due to the various interfaces resulting from DE.     

Dependence of propagation loss on etching depth in Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As strip-loaded type waveguides

Al_0.3Ga_0.7As/GaAs/Al_0.3Ga_0.7As strip-loaded waveguides were fabricated using a broad-beam electron cyclotron resonance (ECR) ion source. It was found that a very smooth etching profile can be obtained by ECR ion etching and the etching rate of Al_0.3Ga_0.7As is 70 nm min^-1. The propagation losses of strip-loaded type III–V compound semiconductor waveguides with various etching depths were studied by the Fabry-Perot cavity method. It was observed that the reflectance at the cleavage increases slightly with etching depth for TE polarization. The propagation loss is measured as 1.5 dB cm^-1 for etching depth of 0.7 m, less than 1 dB cm^-1 for 0.8 m, and 3.5 dB cm^-1 for 1.1 m.     

对称GaAs/Al0.3Ga0.7As双量子阱中激子的束缚能

在有效质量近似下采用简单的尝试波函数变分地计算了对称GaAs/Al0.3Ga0.7As双量子阱中激子体系束缚能,研究了体系束缚能随阱宽和垒宽的变化情况.发现双量子阱中激子体系束缚能随阱宽变化同单量子阱情况类似,但束缚能的峰值出现在阱宽为10(A)左右,峰值位置小于单阱的情况;束缚能随垒宽的增加有一极小值,这与波函数向垒中的渗透有关.     

Study of single event effect for the InP-based HEMT with In0.53Ga0.47As/In0.3Ga0.7As/In0.7Ga0.3As composite channel

In this study, the single event effects in In_0.53Ga_0.47As/In_0.3Ga_0.7As/In_0.7Ga_0.3As composite channel InP-based HEMT are investigated using TCAD simulation for the first time. Due to the higher conduction band difference between bottom In_0.7Ga_0.3As channel and InAlAs buffer, the electrons in the buffer layer induced by ions strike cannot enter the channel, led to reduce the peak concentration in the composite channel and significantly weakened the drain current for composite channel device. Meanwhile, higher barrier height under the gate for composite channel InP-based HEMT is formed after particle strike, and further attenuate the drain current. Therefore, the single event effects can be effectively reduced by designing channel structure. In addition, drain voltage and incident position also show significant impact drain current. With the increase in the drain voltage, the drain current increase and the most sensitive incident position is the gate electrode for the device.     

Fast spin state initialization in a singly charged InAs-GaAs quantum dot by optical cooling

Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error correction. Here, we demonstrate fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin. The electron spin is successfully cooled from 5 to 0.06 K at a magnetic field of 0.88 T applied in Voigt geometry. The spin cooling rate is of order 10(9) s-1, which is set by the spontaneous decay rate of the excited state.     

Coherent oscillations of holes in GaAs0.86P0.14/Al0.7Ga0.3As surface quantum well

We show that in a GaAs_0.86P_0.14/Al_0.7Ga_0.3As near-surface quantum well, there is coherent oscillation of holes observed in time-resolved reflectivity signal when the top barrier of the quantum well is sufficiently thin. The quantum well states interact with the surface states under the influence of the surface electric field. The time period of the observed oscillation is 120±10 fs.     

Variational calculations of ionized-donor-bound excitons in GaAs-Al x Ga 1 - x As quantum wells

Using a two-parameter wave function, we calculate variationally the binding energy of an exciton bound to an ionized donor impurity (D^+,X) in GaAs-Al_xGa_1-xAs quantum wells for the values of the well width from 10 to 300 ?, when the dopant is located in the center of the well and at the edge of the well. The theoretical results confirm that the previous experimental speculation proposed by Reynolds et al. [Phys. Rev. B 40, 6210 (1989)] is the binding energy of D^+,X for the dopant at the edge of the well. In addition, we also calculate the center-of-mass wave function of the exciton and the average interparticle distances. The results are discussed in detail. Received 17 July 2000 and Received in final form 13 November 2000