Self-assembled quantum dots have remarkable optical, electronic and spintronic properties that make them leading candidates for quantum information technologies. Their characterization requires rapid and local determination of both charge and spin degrees of freedom. We present a way to probe the capacitance of small ensembles of quantum dots at microwave frequencies. The technique employs a capacitance sensor based on a microwave microstrip resonator with sensitivity ~10^(-19) F/rt(Hz), high enough to probe single electrons. The integration of this design in a scanning microscope will provide an important tool for investigating single charge and spin dynamics in self-assembled quantum dot systems.
Magnetic semiconductor quantum dots with a few carriers represent an interesting model system where ferromagnetic interactions can be tuned by voltage. By designing the geometry of a doped quantum dot, one can tailor the anisotropic quantum states of magnetic polarons. The strong anisotropy of magnetic polaron states in disk-like quantum dots with holes comes from the spin splitting in the valence band. The binding energy and spontaneous magnetization of quantum dots oscillate with the number of particles and reflect the shell structure. Due to the Coulomb interaction, the maximum binding ener…
Measuring single-electron charge is one of the most fundamental quantum technologies. Charge sensing, which is an ingredient for the measurement of single spins or single photons, has been already developed for semiconductor gate-defined quantum dots, leading to intensive studies on the physics and the applications of single-electron charge, single-electron spin and photon-electron quantum interface. However, the technology has not yet been realized for self-assembled quantum dots despite their fascinating quantum transport phenomena and outstanding optical functionalities. In this paper, we r…
Effects of charged defects on the electronic and optical properties of self-assembled quantum dots
研究了点电荷缺陷对应变In $_{0.6}$ Ga $_{0.4}$ As/GaAs和无应变GaAs/Al $_{0.3}$ Ga $_{0.7}$ As量子点的单粒子电子结构、发射能量、精细结构分裂和激子跃迁振子强度的影响.我们发现，带电缺陷显着修改单粒子的电子结构和激子光谱在应变和无应变的结构。然而，激子精细结构分裂，偏振各向异性和偏振方向在应变量子点几乎不受影响，而观察到显着的变化为无应变量子点。
We demonstrate full charge control, narrow optical linewidths, and optical spin pumping on single self-assembled InGaAs quantum dots embedded in a $162.5\,\text{nm}$ thin diode structure. The quantum dots are just $88\,\text{nm}$ from the top GaAs surface. We design and realize a p-i-n-i-n diode that allows single-electron charging of the quantum dots at close-to-zero applied bias. In operation, the current flow through the device is extremely small resulting in low noise. In resonance fluorescence, we measure optical linewidths below $2\,μ\text{eV}$, just a factor of two above the transform l…
We study theoretically the spin-flip relaxation processes for a single electron in a self-assembled InAs/GaAs quantum dot, using an 8-band kp theory in the envelope function approximation. We show that the dominating channel of spin relaxation is spin admixture induced by symmetry-breaking shear strain, which can be mapped onto two effective spin-phonon terms in a conduction band (effective mass) Hamiltonian that have a similar structure and interfere constructively. Unlike the Dresselhaus coupling that dominates spin relaxation in larger, unstrained dots, the shear strain contribution cannot …
Investigation of in-plane nuclear field formation in single self-assembled quantum dots
研究了In $_{0.75}$ Al $_{0.25}$ As/Al $_{0.3}$ Ga $_{0.7}$ As量子点中面内核场的形成机制。在同一个量子点样品中生长的多个单量子点在临界横向磁场处出现了宽度非常大的Hanle曲线和滞后行为。为了解释Hanle曲线中的异常现象，即垂直于光注入电子自旋极化方向的大核场的形成，我们在现有的动态核自旋极化唯象模型的基础上提出了一个新的模型.该模型包括核四极相互作用的影响和符号反转之间的面内和面外g-因子，模型计算成功地再现了所观察到的异常的汉勒曲线的特征。
The control over the spectral broadening of an ensemble of emitters, mainly attributable to the size and shape dispersion and the homogenous broadening mechanisms, is crucial to several applications of quantum dots. We present a convenient self-assembly approach to deliver strain-free GaAs quantum dots with size distribution below 15%, due to the control of the growth parameters during the preliminary formation of the Ga droplets. This results in an ensemble photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the absence of a wetting layer promoting dot-dot coupling all…
We investigate the electron and hole spin relaxation in an ensemble of self-assembled InAs/In$_{0.53}$Al$_{0.24}$Ga$_{0.23}$As/InP quantum dots with emission wavelengths around $1.5$~$μ$m by pump-probe Faraday rotation spectroscopy. Electron spin dephasing due to the randomly oriented nuclear Overhauser fields is observed. At low temperatures we find a sub-microsecond longitudinal electron spin relaxation time $T_1$ which unexpectedly strongly depends on temperature. At high temperatures the electron spin relaxation time is limited by optical phonon scattering through spin-orbit interaction de…
Quantum dots are arguably one of the best platforms for optically accessible spin based qubits. The paramount demand of extended qubit storage time can be met by using quantum-dot-confined dark exciton: a longlived electron-hole pair with parallel spins. Despite its name the dark exciton reveals weak luminescence that can be directly measured. The origins of this optical activity remain largely unexplored. In this work, using the atomistic tight-binding method combined with configuration-interaction approach, we demonstrate that atomic-scale randomness strongly affects oscillator strength of d…
The optical spectrum of a quantum dot is typically dominated by the fundamental transition between the lowest-energy configurations. However, the radiative Auger process can result in additional red-shifted emission lines. The origin of these lines is a combination of Coulomb interaction and symmetry-breaking in the quantum dot. In this paper, we present measurements of such radiative Auger lines for a range of InGaAs/GaAs self-assembled quantum dots and use a tight-binding model with a configuration interaction approach to explain their appearance. Introducing a composition fluctuation cluste…
Some electronic and optical properties of self-assembled quantum dots: asymmetries in a lens domain
具有透镜畴的自组装量子点具有旋转对称性，但当电子垂直于其圆形基底运动时，其本质上是不对称的，{ \it i. e.}沿着旋转轴。为了表征这种不对称性，沿旋转轴的正或负方向施加外部电场。我们报告了不同的斯塔克位移出现在光谱中作为一个函数的场强度为不同的透镜域。结果表明，对于一个平坦的透镜域的不对称性的影响减少，但即使是非常平坦的透镜，他们不能近似的柱面域。最后，我们研究了它的介电常数和电吸收等光学性质。能谱的特征显示了这些量。强调了只要调节特定能级跃迁的大小和方向场，就应考虑适当的透镜域的重要性。
