Magnetic-field-induced singularity in the tunneling current through an InAs quantum dot

The tunneling transport through a GaAs/(AlGa)As/GaAs single-barrier heterostructure with self-assembled InAs quantum dots is studied experimentally at low temperatures. An anomalous increase in the tunneling current through the quantum dots is observed in magnetic fields both parallel and perpendicular to the current. This result cannot be understood in the framework of the single-electron approximation. The proposed explanation of the phenomenon is based on the modified Matveev-Larkin theory, which predicts the appearance of a singularity in the tunneling current through the zero-dimensional state in a magnetic field because of the interaction between the tunneling electron and the spin-polarized three-dimensional electron gas in the emitter. The absence of spin splitting in the experimental resonance peaks is caused by the complete spin polarization of the emitter in relatively weak magnetic fields.     

Magnetic-field-induced Fermi-edge singularity in the tunneling current through an InAs self-assembled quantum dot

The results of the investigation of tunneling transport through a GaAs/(AlGa)As/GaAs single-barrier heterostructure containing InAs self-assembled quantum dots at low temperatures are reported. An anomalous increase in the tunneling current through the quantum dots has been observed in the presence of a magnetic field both parallel and perpendicular to the current. This increase is a manifestation of a Fermi-edge singularity appearing in the current due to the interaction of a tunneling electron with the electron gas in an emitter.     

基于InAs单量子点的单光子干涉

利用分子束外延生长 InAs 单量子点样品,测量了温度为 5 K 时单量子点的荧光(PL)光谱.采用时间关联光子强度测量(HBT)验证了 PL 光谱具有单光子发射特性.单光子通过马赫曾德尔 (MZ) 干涉仪,验证了单光子自身具有干涉特性.测量了当 MZ 干涉仪两臂偏振方向的夹角改变时对应的单光子干涉及条纹可见度的变化. 关键词： 量子点单光子源 反群聚效应 马赫曾德尔干涉     

Resonant tunneling of electrons through single self-assembled InAs quantum dot studied by conductive atomic force microscopy

Resonant tunneling of electrons through a quantum level in single self-assembled InAs quantum dot (QD) embedded in thin AlAs barriers has been studied. The embedded InAs QDs are sandwiched by 1.7-nm-thick AlAs barriers, and surface InAs QDs, which are deposited on 8.3 nm-thick GaAs cap layer, are used as nano-scale electrodes. Since the surface InAs QD should be vertically aligned with a buried one, a current flowing via the buried QD can be measured with a conductive tip of an atomic force microscope (AFM) brought in contact with the surface QD-electrode. Negative differential resistance attributed to electron resonant tunneling through a quantized energy level in the buried QD is observed in the current–voltage characteristics at room temperature. The effect of Fermi level pinning around nano-scale QD-electrode on resonance voltage and the dependence of resonance voltage on the size of QD-electrodes are investigated, and it has been demonstrated that the distribution of the resonance voltages reflects the size variation of the embedded QDs.     

Optically probing the fine structure of a single Mn atom in an InAs quantum dot

We report on the optical spectroscopy of a single InAs/GaAs quantum dot doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A0 whose effective spin J=1 is significantly perturbed by the quantum dot potential and its associated strain field. The spin interaction with photocarriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of mueV, but vanishingly small for electrons.     

Optical manipulation of a single Mn spin in a CdTe quantum dot

In this paper we demonstrate optical writing of information on the spin state of a single Mn ion embedded in a CdTe/ZnTe quantum dot. As a tool for Mn spin orientation we use a spin-conserving transfer of excitation between two coupled quantum dots, one of them containing the Mn ion. Excitons created by circularly polarized light act on the Mn ion via the sp–d exchange interaction and orient its spin. The magnetic field of 1 T strongly enhances the orientation efficiency due to suppression of fast Mn spin relaxation mechanisms. Dynamics of the Mn spin under polarized excitation was measured in a time-resolved experiment, in which the intensity and polarization of excitation were modulated. Observed dynamics of the Mn spin can be described with a simple rate equation model.     

Energy-dependent tunneling in a quantum dot

We present measurements of the rates for an electron to tunnel on and off a quantum dot, obtained using a quantum point contact charge sensor. The tunnel rates show exponential dependence on drain-source bias and plunger gate voltages. The tunneling process is shown to be elastic, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements.     

Inelastic tunneling of electrons through a quantum dot with an embedded single molecular magnet

We report a theoretical analysis of electron transport through a quantum dot with an embedded biaxial single-molecule magnet (SMM) based on mapping of the many-body interaction-system onto a one-body problem by means of the non-equilibrium Green function technique. It is found that the conducting current exhibits a stepwise behavior and the nonlinear differential conductance displays additional peaks with variation of the sweeping speed and the magnitude of magnetic field. This observation can be interpreted by the interaction of electron-spin with the SMM and the quantum tunneling of magnetization. The inelastic conductance and the corresponding tunneling processes are investigated with normal as well as ferromagnetic electrodes. In the case of ferromagnetic configuration, the coupling to the SMM leads to an asymmetric tunneling magnetoresistance (TMR), which can be enhanced or suppressed greatly in certain regions. Moreover, a sudden TMR-switch with the variation of magnetic field is observed, which is seen to be caused by the inelastic tunneling.     

Few-electron molecular states and their transitions in a single InAs quantum dot molecule

We study electronic configurations in a single pair of vertically coupled self-assembled InAs quantum dots, holding just a few electrons. By comparing the experimental data of nonlinear single-electron transport spectra in a magnetic field with many-body calculations, we identify the spin and orbital configurations to confirm the formation of molecular states by filling both the quantum mechanically coupled symmetric and antisymmetric states. Filling of the antisymmetric states is less favored with increasing magnetic field, and this leads to various magnetic field induced transitions in the molecular states.     

电场调谐InAs单量子点的发光光谱

采用稳态和时间分辨发光光谱,研究了生长在p-i-n二极管结构内的InAs单量子点发光光谱的电场调谐特性.随着电场强度的增加,观察到量子点中激子发光的Stark 效应.通过选择不同波长的激光线激发量子点样品,发现随着电场强度的增加导致量子点发光强度的减弱,这是由于量子点俘获载流子概率的减小所致,而激子寿命的增加源于电场导致激子的Stark效应. 关键词： InAs单量子点 Stark效应 电子-空穴分离     

Coherent optical control of the spin of a single hole in an InAs/GaAs quantum dot

We demonstrate coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. A superposition of hole-spin states is created by fast (10-100?ps) dissociation of a spin-polarized electron-hole pair. Full control of the hole spin is achieved by combining coherent rotations about two axes: Larmor precession of the hole spin about an external Voigt geometry magnetic field, and rotation about the optical axis due to the geometric phase shift induced by a picosecond laser pulse resonant with the hole-trion transition.     

Optical pumping and a nondestructive readout of a single magnetic impurity spin in an InAs/GaAs quantum dot

We report on the resonant optical pumping of the | ± 1? spin states of a single Mn dopant in an InAs/GaAs quantum dot which is embedded in a charge tunable device. The experiment relies on a W scheme of transitions reached when a suitable longitudinal magnetic field is applied. The optical pumping is achieved via the resonant excitation of the central Λ system at the neutral exciton X(0) energy. For a specific gate voltage, the redshifted photoluminescence of the charged exciton X- is observed, which allows a nondestructive readout of the spin polarization. An arbitrary spin preparation in the | + 1? or |-1? state characterized by a polarization near or above 50% is evidenced.     

电声子相互作用对量子点分子中单电子隧穿的影响

研究了双量子点系统中的电子隧穿动力学过程，在考虑电子与声子相互作用的情况下用基于 正则变换的微扰方法解析地得到了电子动态隧穿电流的表达式. 并且详细分析电子与声子耦 合引起的退相干问题，在此基础上指出了可能的退耦机理. 关键词： 电声子相互作用 双量子点 隧穿     

Optical spectroscopy of a single InAs/GaAs quantum dot in high magnetic fields

We report on the measurements of the photoluminescence from the s-shell of a single InAs/GaAs quantum dot in magnetic fields up to 23 T. The observed multiline emission is attributed to different charge states of a single dot. Characteristic anticrossing of emission lines is explained in terms of hybridization of final states of a triply charged exciton (X⁻³).     

Efficient single photon detection by quantum dot resonant tunneling diodes

We demonstrate that the resonant tunnel current through a double-barrier structure is sensitive to the capture of single photoexcited holes by an adjacent layer of quantum dots. This phenomenon could allow the detection of single photons with low dark count rates and high quantum efficiencies. The magnitude of the sensing current may be controlled via the thickness of the tunnel barriers. Larger currents give improved signal to noise and allow sub-mus photon time resolution.     

Detection of single photons by a resonant tunneling heterostructure with a quantum dot layer

Light absorption by GaAs/AlAs heterostructures with a layer of self-assembled InAs quantum dots (QDs) at resonant tunneling through an energy-selected QD has been investigated. A high sensitivity of the current through this selected tunneling channel to the absorption of single photons with a wavelength λ ≲ 860 nm up to a temperature of 50 K is demonstrated; this sensitivity is caused by the Coulomb effect of the photoexcited holes captured by surrounding QDs on the resonance conditions. It is shown that single-photon absorption can discretely change the current through the system under study by a factor of more than 50. The captured-hole lifetimes have been measured, and a model has been developed to qualitatively describe the experimental data. It is also demonstrated that the InAs monolayer can effectively absorb photons. The properties of the heterostructure studied can be used not only to detect photons but also to design logical valves and optical memory devices.     

Quantum frequency down-conversion of single photons at 1552 nm from single InAs quantum dot

Near-infrared single photon sources in telecommunication bands, especially at 1550 nm, are required for long-distance quantum communication. Here a down-conversion quantum interface is implemented, where the single photons emitted from single In As quantum dot at 864 nm is down converted to 1552 nm by using a fiber-coupled periodically poled lithium niobate(PPLN) waveguide and a 1.95 μmm pump laser, and the frequency conversion efficiency is ~40%. The singlephoton purity of quantum dot emission is preserved during the down-conversion process, i.e., g~((2))(0), only 0.22 at 1552 nm.This present technique advances the Ⅲ-Ⅴ semiconductor quantum dots as a promising platform for long-distance quantum communication.     

Huang–Rhys side-bands in the emission line of a single InAs quantum dot

We report the first direct observation of Huang–Rhys side-bands in the photoluminescence spectrum of a single InAs/GaAs quantum dot (QD). At low temperature (10 K) the single QD spectrum has a quasi-Lorentzian profile. At higher temperatures, we observe a strong deviation from a Lorentzian profile with the appearance of asymmetric side-bands which become symmetric above 70 K. We obtain an excellent agreement with theoretical calculations done in the framework of the Huang–Rhys formalism. We conclude that the zero-phonon linewidth is the relevant parameter for the observation of the low-energy acoustic phonon side-bands.     

Suppression of spin relaxation in an InAs nanowire double quantum dot

We investigate the triplet-singlet relaxation in a double quantum dot defined by top gates in an InAs nanowire. In the Pauli spin blockade regime, the leakage current can be mainly attributed to spin relaxation. While at weak and strong interdot coupling relaxation is dominated by two individual mechanisms, the relaxation is strongly reduced at intermediate coupling and finite magnetic field. In addition we observe a characteristic bistability of the spin-nonconserving current as a function of magnetic field. We propose a model where these features are explained by the polarization of nuclear spins enabled by the interplay between hyperfine and spin-orbit mediated relaxation.