半导体量子点中浸润层跃迁与纯失相对Rabi振荡退相干的影响

理论上分析了半导体量子点中浸润层跃迁(包括泄漏和俘获两个过程)对Rabi振荡退相干的影响,含浸润层跃迁的粒子数运动方程可以很好地拟合实验结果.同时还对比分析了纯失相(pure dephasing)对Rabi振荡退相干的影响.分析表明,可以用简单的纯失相强度相关衰减因子来等效地分析复杂的多能级跃迁体系的退相干特性.     

初态与纯失相对半导体量子点V型三能级系统拉比振荡的影响

相干光学操控半导体量子点(SQDs)激子态在量子信息、量子计算中具有很重要的应用。通过对影响量子点(QDs)拉比振荡因素的分析,可获得导致量子点体系退相干的物理机制,这为全光操控半导体量子点体系提供了理论依据。线偏振脉冲激光激发自组织InGaAsQDs,运用系统粒子数主方程并结合光学布洛赫矢量推导了单脉冲激发下V型三能级系统激子动力学方程,利用此方程讨论了初始状态及纯失相对拉比振荡的影响。研究表明,可以通过改变系统初态条件和激发场的偏振角对该体系激子拉比振荡的振幅和频率进行调控,可以用纯失相强度相关衰减因子等效地分析V型三能级体系的退相干特性,从而使相关计算得到了简化。     

激子自旋弛豫对简并量子点发射光子对纠缠度的影响

利用粒子数运动方程和量子回归理论，计算了单个半导体量子点双激子体系脉冲激发下粒子在各能级间辐射跃迁的二阶交叉相关函数以及系统发射光子对的偏振密度矩阵.分析了激子态能级简并量子点体系发射光子对偏振纠缠特性，讨论了纠缠度随激子态间自旋弛豫的变化关系.研究表明，激子自旋弛豫会破坏该系统发射光子对的纠缠度. 关键词： 纠缠光子对 半导体量子点 二阶相关函数     

两个正交子能级的V型三能级系统的粒子数振荡特性

研究了线偏振脉冲光场激发下,半导体量子点中两个正交子能级上的粒子数振荡特性.利用旋转波近似方法给出了共振激发和无衰减时粒子数运动方程的解析解.引入了该系统的等效跃迁偶极矩和等效脉冲面积，并给出了其关系表达式.理论分析表明，该体系中的子能级上粒子数振荡的振幅和频率都可以通过改变激发场的偏振角和系统的初态条件进行调控. 关键词： Rabi振荡 旋转波近似 三能级系统     

亚单层InGaAs量子点-量子阱异质结构的时间分辨光致发光谱

测定了亚单层InGaAs/GaAs量子点-量子阱异质结构在5K下的时间分辨光致发光谱.亚单层量 子点的辐射寿命在500 ps 至 800 ps之间，随量子点尺寸的增大而增大，与量子点中激子的 较小的横向限制能以及激子从小量子点向大量子点的隧穿转移有关.光致发光上升时间强烈 依赖于激发强度密度.在弱激发强度密度下，上升时间为 35 ps，纵光学声子发射为主要的 载流子俘获机理.在强激发强度密度下，上升时间随激发强度密度的增加而减小，俄歇过程 为主要的载流子俘获机理.该结果对理解亚单层量子点器件的工作特性非常有用. 关键词： 亚单层 量子点-量子阱 时间分辨光致发光谱     

不对称双量子阱中电子布居转移的相干控制研究

采用龙格-库塔算法对激光与半导体双量子阱系统相互作用的密度矩阵方程进行数值求解,研究了不对称半导体双量子阱系统中电子布居转移的相干控制。研究表明,调节激光场强和脉冲时间延迟可控制两个下能级间的粒子数转移;利用激光场相对相位可精确地控制布居转移几率。这些结果在相干叠加态制备、量子纠缠和量子信息处理等领域具有潜在的应用价值。     

室温下CdSe胶体量子点超快自旋动力学

利用时间分辨法拉第旋转光谱技术研究了室温下CdSe胶体量子点的自旋相干特性.获得了不同磁场下的自旋退相干时间,并分析了自旋退相干的物理机理.零磁场时量子点激子自旋退相干时间为102 ps,主要受电子与核自旋之间的超精细相互作用所影响.当外加横向磁场强度为250 mT时,激子自旋退相干时间为294 ps;增大磁场强度,自旋退相干时间逐渐减小.在较强磁场环境中(≥250mT),量子点激子自旋动力学由非均匀退相干机制所主导.     

LA声子对激子qubit纯退相干的影响

在一个抛物量子点中,以激子的真空态和基态作为量子比特(qubit),采用求密度矩阵元的方法,计算了由形变势下声学声子引发的激子量子比特纯退相干.找到了激子量子比特纯退相干因子对时间、温度和量子点受限长度的依赖关系.研究发现,激子量子比特的退相干因子在2.5ps的时间范围内随时间的增加而迅速增加,其纯退相干时间在ps量级;在温度即使为绝对温度0K时由LA声子引发的退相干依然存在,在温度大于3K后退相干因子随温度的增大而开始迅速增大;并同时发现量子点受限长度对退相干因子有重要影响,激子越受限退相干越快.研究结果表明,对激子量子比特使用适当大小量子点,且保持环境低温,并采用低能超快光学操作可以有效地抑制声子对激子量子比特纯退相干的影响. 关键词： 量子点 量子信息 量子比特     

V型原子系统中相干布居俘获的相干相位调制研究

在延迟脉冲激光场作用下的V型原子系统中，实现了对相干布居俘获，电磁感应透明的相干相位调制，选择适当的能级其调制频率可以达到飞秒量级.利用数值和解析分析得到了延迟脉冲和叠加态粒子数布居的关系，以及粒子布居受光场相位调制的特点和变化规律.利用这种机理可以实现飞秒量级的量子开关作用. 关键词： 相干相位调制 相干布居俘获 电磁感应透明 量子开关     

神经网络法研究多溴代二苯胺热力学性质

通过时间分辨光谱技术详细地研究了ZnCuInS/ZnSe/ZnS量子点的激子弛豫动力学. 基于速率分布模型,波长依赖的发射动力学表明本征激子、界面缺陷态中的激子、给-受体对态中的激子都会参与量子点的发光过程,整个发光过程主要依赖于给-受体对态发射. 瞬态吸收数据表明激发后本征激子和界面缺陷物种可能会同时出现,在高激发光强下,光强依赖的俄歇复合过程也存在于量子点中     

Coherent optical manipulations of the fundamental state in a single quantum dot

By means of an original all-optical experimental technique using microphotoluminescence in a waveguiding geometry, resonant coherent manipulation of quantum states in a single quantum dot becomes possible now. Resonant Rabi oscillation of the fundamental exciton state in a single quantum dot has been realized. We present the results obtained on two different kinds of samples: InAs/GaAs self-assembled quantum dots and naturally formed GaAs quantum dots by thickness fluctuations.     

Optical Bloch Equations Modified with Phonon-Induced Intensity-Dependent Dephasing

We extend the exciton population equations of a two-level quantum dot system with weak excitation to the ones with strong excitations, in which, the phonon-induced intensity-dependent dephasing time and decay rate are involved. The straightforward calculated populations from the modified population equations demonstrate the damping behavior of Rabi oscillation as the external field increasing. The effect of the intensity-dependent dephasing time and the intensity-dependent decay rate are also discussed.     

Rabi oscillation damped by exciton leakage and Auger capture in quantum dots

The decoherence of Rabi oscillation (RO) caused by biexciton, population leakage to the wetting layer (WL), and Auger capture in semiconductor quantum dots is theoretically analyzed with multilevel optical Bloch equations. The corresponding effects on the quality factor of RO are also discussed. We have found that the biexciton effect is relatively trifling, as the pulse duration is longer than 5 ps. The population leakage to the WL leads to a decrease of the RO average even though the damping rate is similar to that observed in the experiment. Auger capture in quantum dots results in RO damping that is consistent with the experimental data, which implies that Auger capture is an important decoherence process in quantum dots.     

Optical Bloch Equations Modified with Phonon-Induced Intensity-Dependent Dephasing

We extend the exciton population equations of a two-level quantum dot system with weak excitation to the ones with strong excitations, in which, the phonon-induced intensity-dependent dephasing time and decay rate are involved. The straightforward calculated populations from the modified population equations demonstrate the damping behavior of Rabi oscillation as the external field increasing. The effect of the intensity-dependent dephasing time and the intensity-dependent decay rate are also discussed.     

Optical control of excitons in a pair of quantum dots coupled by the dipole-dipole interaction

We demonstrate coherent nonlinear-optical control of excitons in a pair of quantum dots (QDs) coupled via dipolar interaction. The single-exciton population in the first QD is controlled by resonant picosecond excitation, giving rise to Rabi oscillations. As a result, the exciton transition in the second QD is spectrally shifted and concomitant Rabi oscillations are observed. We identify coupling between permanent excitonic dipole moments as the dominant interaction mechanism, whereas quasiresonant (F?rster) energy transfer is weak. Such control schemes based on dipolar interaction are a prerequisite for realizing scalable quantum logic gates.     

Resonantly driven exciton Rabi oscillation in single quantum dots emitting at 1300 nm

We report on the resonance fluorescence(RF) from single In As quantum dots(QDs) emitting at the telecom band of 1300 nm. The InAs/GaAs QDs are embedded in a planar optical microcavity and the RF is measured by an orthogonal excitation-detection geometry for deeply suppressing the residual laser scattering. An ultra-weak He–Ne laser is necessary to be used as a gate laser for obtaining RF. Rabi oscillation with more than one period is observed through the picosecond(ps) pulsed laser excitation. The resonant control of exciton opens up new possibilities for realizing the on-demand single photon emission and quantum manipulation of solid-state qubits at telecom band.     

Interplay of Rabi oscillations and quantum interference in semiconductor quantum dots

We investigated the manifestation of Rabi oscillation in the coherent dynamics of excitons in self-assembled semiconductor quantum dots. The Rabi oscillation phenomenon was directly observed as a function of the input pulse area. Furthermore, by performing wave packet interferometry in the nonlinear excitation regime, we discover a new type of quantum interference phenomenon, resulting from the interplay between Rabi oscillation and quantum interference.     

Light–matter coupling in nanostructures without an inversion center

A theoretical analysis of the electronic interaction with an intense electromagnetic field in a two-level asymmetrical quantum dot is presented. As a consequence of a strong light–matter coupling in such a system, dipole radiation at the Rabi frequency turns out to be possible. Since the Rabi frequency is controlled by the strength of the coupling electromagnetic field, the effect can serve to provide frequency-tuned parametric amplification and generation of electromagnetic waves. The manifestation of the effect is discussed for group III nitride quantum dots. Terahertz emission from arrays of such quantum dots is shown to be experimentally observable.     

Robust quantum dot exciton generation via adiabatic passage with frequency-swept optical pulses

The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust population inversion in a single quantum dot using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept through the resonance. This observation in photoluminescence experiments is made possible by introducing a novel optical detection scheme for the resonant electron hole pair (exciton) generation.     

Absorption Spectra and Refractive Index Changes of an Exciton in a Core/Shell Quantum Dot

The absorption spectra and the refractive index changes are calculated theoretically for an exciton in a core/shell quantum dot. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The dimensionality effect of exciton quantum dots on the optical absorptions has been studied. It has been found that in the same regime, the optical absorption intensities of excitons are much smaller for the core/shell quantum dots than for the two-dimensional quantum rings. The linear and the nonlinear optical absorption coefficients and refractive index changes have been examined with the change of the confinement potential. The results show that the optical absorptions and the refractive index changes are strongly affected by the repulsive core of core/shell quantum dots. Moreover, the calculated results also reveal that as the inner radius increases, the peak values of the absorption coefficients and the refractive index changes of an exciton will show the optical Aharonov–Bohm oscillation in core/shell quantum dots.