Polarization spectroscopy of positive and negative trions in an InAs quantum dot

Using polarization-sensitive photoluminescence and photoluminescence excitation spectroscopy, we study single InAs/GaAs self-assembled quantum dots. The dots were embedded in an n-type, Schottky diode structure allowing for control of the charge state. We present here the exciton, singly charged exciton (positive and negative trions), and the twice negatively charged exciton. For non-resonant excitation below the wetting layer, we observed a large degree of polarization memory from the radiative recombination of both the positive and negative trions. In excitation spectra, through the p-shell, we have found several sharp resonances in the emission from the s-shell recombination of the dot in all charged states. Some of these excitation resonances exhibit strong coulomb shifts upon addition of charges into the quantum dot. One particular resonance of the negatively charged trion was found to exhibit a fine structure doublet under circular polarization. This observation is explained in terms of resonant absorption into the triplet states of the negative trion.     

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.     

Coherent spin precession of electrons and excitons in charge tunable InP quantum dots

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.     

Polarized fine structure in the photoluminescence excitation spectrum of a negatively charged quantum dot

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.     

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.     

Fast initialization of the spin state of an electron in a quantum dot in the Voigt configuration

We consider the initialization of the spin state of a single electron trapped in a self-assembled quantum dot via optical pumping of a trion level. We show that with a magnetic field applied perpendicular to the growth direction of the dot, a near-unity fidelity can be obtained in a time equal to a few times the inverse of the spin-conserving trion relaxation rate. This method is several orders of magnitude faster than with the field aligned parallel, since this configuration must rely on a slow hole spin-flip mechanism. This increase in speed does result in a limit on the maximum obtainable fidelity, but we show that for InAs dots, the error is very small.     

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.     

Optically induced hybridization of a quantum dot state with a filled continuum

We present an optical signature of a hybridization between a localized quantum dot state and a filled continuum. Radiative recombination of the negatively charged trion in a single quantum dot leaves behind a single electron. We show that in two regions of vertical electric field, the electron hybridizes with a continuum through a tunneling interaction. The hybridization manifests itself through an unusual voltage dependence of the emission energy and a non-Lorentzian line shape, features which we reproduce with a theory based on the Anderson Hamiltonian.     

Nuclear spin nanomagnet in an optically excited quantum dot

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron. As a result the optically selected single quantum dot represents a tiny magnet with the ferromagnetic ordering of nuclear spins-the nuclear spin nanomagnet.     

Selective optical control of electron spin coherence in singly charged GaAs-Al0.3Ga0.7As quantum dots

Coherent transient excitation of the spin ground states in singly charged quantum dots creates optically coupled and decoupled states of the electron spin. We demonstrate selective excitation from the spin ground states to the trion state through phase sensitive control of the spin coherence via these three states, leading to partial rotations of the spin vector. This progress lays the ground work for achieving complete ultrafast spin rotations.     

Theory of fast optical spin rotation in a quantum dot based on geometric phases and trapped states

A method is proposed for the optical rotation of the spin of an electron in a quantum dot using excited trion states to implement operations significantly faster than those of most existing proposals. Key ingredients are the geometric phase induced by 2pi hyperbolic secant pulses, use of coherently trapped states and use of naturally dark states. Our proposal covers a variety of quantum dots by addressing different parameter regimes. Numerical simulations with typical parameters for InAs self-assembled quantum dots, including their dissipative dynamics, give fidelities of the operations in excess of 99%.     

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.     

量子点中极化子的内部激发态性质

采用Huybrechts线性组合算符和幺正变换方法研究了抛物量子点中的强、弱耦合极化子的激发态性质。分别导出强、弱耦合情况下,抛物量子点中的极化子的第一内部激发态能量、激发能量、共振频率与量子点的有效受限长度和电子-声子耦合强度的关系。数值计算结果表明,量子点中弱耦合和强耦合极化子的内部激发态能量、激发能量和共振频率都随量子点的有效受限长度的减小而迅速增大。弱耦合极化子的第一内部激发态能量随电子-声子耦合强度的增加而减少;而强耦合极化子的振动频率随量子点的有效受限长度的减小而迅速增加。弱耦合极化子的第一内部激发态能量、激发能量和共振频率随电子-声子耦合强度的增加而减小。     

Origin of the oscillator strength of the triplet state of a trion in a magnetic field

The dynamics of the spin-triplet trion state, under high magnetic field in a GaAs/AlGaAs quantum well, are studied using time resolved spectroscopy. The oscillator strength of the triplet transition is shown to rise with increasing electron density, in good agreement with a theoretical model where the trion interacts with excess electrons in the quantum well. This analysis suggests that the spin-triplet trion state, which is expected to be an optically "dark" state, is experimentally observable due to the interactions with the excess electrons, demonstrating that X- cannot be regarded as an isolated three particle complex.     

Microwave control of transport through a chaotic mesoscopic dot

We establish analogy between a microwave ionization of Rydberg atoms and a charge transport through a chaotic quantum dot induced by a monochromatic field in a regime with a potential barrier between dot contacts. We show that the quantum coherence leads to dynamical localization of electron excitation in energy so that only a finite number of photons is absorbed inside the dot. The theory developed determines the dependence of localization length on dot and microwave parameters showing that the microwave power can switch the dot between metallic and insulating regimes. ultiphoton ionization and excitation to highly excited states (e.g., Rydberg states)     

Excitations in doped quantum dot insisted by discontinuous reversals of static electric field: Interplay between pulse and dopant site

We explore the excitation profile of a repulsive impurity doped quantum dot. The quantum dot is subject to a discontinuously reversing static electric field. The dopant impurity potential chosen assumes Gaussian form. The investigation reveals the key role played by the dopant location and the number of pulses offered by the external field to the dot in controlling the excitation rate. Time-dependent Hellman-Feynman theorem has been invoked to understand the extent of energy transfer between field direction and the direction where no field is applied. The combined transition rate from ground to other excited states is also determined to support the findings.     

Second-Order Correlation Function of the Photon Emission from a Single Quantum Dot

The photon correlation of photon emission from a single quantum dot with cw excitation and pulsed excitation is investigated in details. To calculate the second-order correlation function for optical pumping, we deduce rate equations with a simplified two-level model under cw excitation and present the master equation approach in the interaction picture to the study of evolution of a three-level system under pulsed excitation. In addition, we report photon correlation measurements on a single self-assembled In0.5Ga0.5As quantum dot, which show strong antibunching behaviour under both the conditions of cw and pulsed excitations. The calculated results are in agreement with the experimental measurements.     

Electron spin dynamics in a self-assembled semiconductor quantum dot: the limit of low magnetic fields

Using the trion as an optical probe, we uncover novel electron spin dynamics in CdSe/ZnSe Stranski-Krastanov quantum dots. The longitudinal spin lifetime obeys an inverse power law associated with recharging processes in the dot ensemble. No hint at spin-orbit mediated spin relaxation is found. At very weak magnetic fields (< 50 mT), electron spin dynamics related to the hyperfine interaction with the lattice nuclei is uncovered. A strong Knight field gives rise to nuclear ordering and formation of dynamical polarization on a 100-micros time scale under continuous electron spin pumping. The associated spin transients are temperature robust and can be observed up to 100 K.     

All-optical trion generation in single-walled carbon nanotubes

We present evidence of all-optical trion generation and emission in pristine single-walled carbon nanotubes (SWCNTs). Luminescence spectra, recorded on individual SWCNTs over a large cw excitation intensity range, show trion emission peaks redshifted with respect to the bright exciton peak. Clear chirality dependence is observed for 22 separate SWCNT species, allowing for determination of electron-hole exchange interaction and trion binding energy contributions. Luminescence data together with ultrafast pump-probe experiments on chirality-sorted bulk samples suggest that exciton-exciton annihilation processes generate dissociated carriers that allow for trion creation upon a subsequent photon absorption event.     

磁场和库仑场对抛物量子点中弱耦合极化子激发态性质的影响

采用线性组合算符和幺正变换方法研究了磁场和库仑场对抛物量子点中极化子激发态性质的影响。导出了抛物量子点中弱耦合束缚磁极化子的振动频率、第一内部激发态能量、激发能量与量子点的有效受限长度、库仑束缚势和磁场的回旋频率之间的变化关系。通过数值计算,结果表明:抛物量子点中弱耦合束缚极化子的振动频率、第一内部激发态能量、激发能量均随量子点的有效受限长度减小而迅速增大。随库仑束缚势增大而增大。随磁场的回旋频率的增加而增大。