单量子点微腔激光器的研究

研究一个三能级量子点耦合到单模光学腔组成的系统,该单量子点激光器的泵浦阈值较小,激光能较快地达到稳定输出,当量子点和腔模耦合强度增强时,其泵浦阈值更小,激光达到稳定输出的速度更快。     

消逝场耦合圆柱形微腔中回音壁模式结构的实验研究

采用石英圆柱形微腔与锥形光纤通过消逝场耦合的方式,在实验上观察到了不同半径的柱形微腔中清晰的回音壁模式,耦合效率接近10%. 利用柱形微腔回音壁模式位置和间隔的解析近似公式,对实验所得的透射光谱做了模式的精确标定,共振波长的实验值和标定值间的误差小于0.05nm. 引入回音壁模式的“模场半径”概念,由“模场半径”计算出的模式间隔和实验测量值之间符合很好. 利用透射光谱中的共振波长和理论标定波长间的差值,得到了实验光谱范围内(1295—1320nm)石英光纤的色散曲线,折射率精度达10^-5. 关键词： 圆柱形微腔 回音壁模式 模式标定 模场半径     

量子微腔中运动原子的辐射压力

利用全量子理论研究了量子微腔中运动原子的辐射压力. 从量子微腔与运动原子相互作用模型出发, 利用代数动力学方法对系统的哈密顿量进行规范变换, 推导出系统的时间演化算符和原子内态约化密度算符的表达式, 在此基础上给出辐射压力的解析解, 并讨论了驻波场和行波场中运动二能级原子和三能级原子的辐射压力, 数值结果与实验符合. 关键词： 量子微腔 运动原子 代数动力学 辐射压力     

非线性双光束共振时的量子非破坏测量

运用全量子方法建立双光束Λ型三能级原子相互作用系统的理论模型，讨论了该系统的动力学行为和光学量子非破坏测量特性.研究表明在非线性双光束共振状态下，可以实现光学量子非破坏测量. 关键词：     

两模腔场谱间的量子干涉

研究了高Q腔中单个二能级原子与两模二项式光场依赖强度耦合相互作用系统的腔场谱,给出了弱初始场条件下的数值结果,讨论了两模光场之间的量子干涉对腔场谱结构的影响. 发现当两模光场的频率差Δ>g（g为原子与腔场间的耦合常数）时,两模光场间的干涉效应对谱结构没有影响,系统的腔肠谱只是两模腔肠谱的简单叠加;当Δ≤g时两模腔场谱间的干涉比较明显. 在强初始场条件下,量子干涉效应可忽略. 关键词： 腔场谱 量子干涉 两模二项式光场     

经典光场相干控制金属纳米线表面等离子体传输

利用全量子力学的方法从理论上研究了利用经典光场相干控制半导体量子点-金属纳米线复合体系中表面等离子体传输特性. 计算中假设金属纳米线表面等离子体具有线性色散关系,半导体量子点具有梯形三能级结构. 分析表明:通过加或不加经典光场,可以控制表面等离子体被反射还是透射;加上光场后,反射峰、透射峰的位置以及两反射峰之间的距离都可以由经典光场控制. 关键词： 表面等离子体 量子点 散射     

微腔中单量子点的受激辐射行为

研究了微腔中单模光场与一个三能级量子点相互作用系统.利用系统主方程作数值模拟计算微腔中单量子点的净受激辐射率随量子点和腔模耦合强度的变化,同时研究激光能级间衰减率γ23对净受激辐射率的影响.发现净受激辐射率随泵浦强度增大而呈现较快趋于饱和的曲线,这些饱和曲线随着耦合强度的增加又较快地趋于一个极值.而激光能级间衰减率γ23的增大使该激光的净受激辐射率的曲线组表现出相反的规律.     

微腔增强发射的半导体量子点单光子源

单光子源是实现量子密匙分配、线性光学量子计算的基本单元。作者回顾了单光子源在量子信息科学发展中的作用,讨论了光子的统计特性,分析了具有类似原子二能级结构的半导体量子点作为单光子发射源的特点,介绍了微腔与二能级系统的耦合以及微腔量子电动力学基本原理。在弱耦合区,Purcell效应导致微腔中量子点激子复合寿命降低,因此可用微腔来改善量子点单光子发射效率。文章总结了近年来在半导体微腔增强量子点单光子发射领域的进展,探讨了分布式布拉格反射微腔、柱状微腔和光子晶体微腔等结构对改善半导体量子点单光子发射和收集效率、光子极化以及光子全同性等方面的作用,并对未来半导体量子点单光子源的发展进行了展望。     

Λ型和V型三能级原子与耦合腔相互作用系统中的纠缠特性

研究了由一个Λ型三能级原子、一个V型三能级原子和光纤连接的双模腔构成的系统,给出了系统态矢的演化. 采用部分转置密度矩阵的负本征值来描述两个子系统间的纠缠,利用数值计算方法研究了原子与原子之间和腔场与腔场之间的纠缠特性. 讨论了光纤模与腔场间的耦合强度对纠缠特性的影响. 研究结果表明:随光纤模与腔场间的耦合强度增强,原子间的纠缠和腔场间的纠缠均增强. 关键词： 量子光学 原子-腔-光纤复合系统 三能级原子 量子纠缠     

微腔中单量子点的激光输出特性研究

研究一个四能级量子点耦合到单模光学腔中的量子系统,利用系统的主方程作数值模拟计算微腔中单量子点的激光输出强度随非相干泵浦的变化关系.结果显示量子点在泵浦作用下激光的输出有一个阈值;且量子点和腔模耦合强度增强时,产生激光的阈值明显减小,输出激光的峰值却增大.当泵浦作用继续增强到一定程度,因激光能级间的相干性被过强的非相干泵浦所破坏,单量子点激光输出变为零----出现了淬灭现象.     

Coupling to a microdisk cavity containing a three-level quantum-dot with two orthogonal modes

The dynamics of a composite system containing two orthogonal degenerate whispering-gallery cavity modes coupling to a quantum dot (QD) is presented by a full quantum approach. The energy levels of the quantum dot are modeled as a V-type three-level system, which consist of the ground state, right- and left-polarized excitons. The counterclockwise mode a and the clockwise mode b are coupled with the transitions corresponding to the right- and left-polarized excitons with coupling rates g_R and g_L, respectively. An exact solution is proposed in a real-space approach. We majorly discuss the effects of the backscattering rate β on the spectra of the transmission and reflection in a strong coupling regime. A new insight is that one can overcome the excitons' fine structure splitting of a real QD with appropriate backscattering rate β by fine designing the cavity, which would be possible for applications to produce the degenerate entangled photon pairs in a real QD system.     

Controllable four-wave mixing based on quantum dot-cavity coupling system

We theoretically study the four-wave mixing(FWM) response in a quantum dot-cavity coupling system, where a two-level quantum dot(QD) is placed in an optical cavity while the cavity mode is coupled to the nanomechanical resonator via radiation pressure. The influences of the QD-cavity coupling strength, the Rabi coupling strength of the QD, and the power of the pump light on the FWM intensity are mainly considered. The numerical results show that the FWM intensity in this hybrid system can be significantly enhanced by increasing the QD-cavity coupling strength. In addition, the FWM intensity can be effectively modulated by the Rabi coupling strength and the pump power. Furthermore, the effects of the cavity decay rate and the cavitypump detuning on the FWM signal are also explored. The obtained results may have potential applications in the fields of quantum optics and quantum information science.     

Another model for a multiexcitonic quantum dot in an optical microcavity

Very recently, a multiexcitonic quantum dot in an optical microcavity have been theoretically studied [Herbert Vincka, Boris A. Rodriguez, and Augusto Gonzalez, Physica E, 2006, 35: 99–102]. However, due to the inevitable damping losses through the microcavity, in this work, we will present a more precise and sound model in the Lindblad form master equation to investigate the photonic properties of a single quantum dot (QD) in an optical microcavity system, in which the QD may confine the multiexcitons and be in resonant interaction with a single photonic mode of an optical microcavity. The excitation energies, and the properties of the emission photon from the QD microcavity are computed as functions of the exciton-photon coupling strength, detuning, and pump rate. We further compare our results with their results, and find that the calculated intensity of the emitted photon and the spectra crucially depend on the exciton-photon coupling strength g, the photon detuning, and the number of excitons in the QD. Finally, we will give a physical mechanism of the dressed-state picture for the strong coupling between the single mode of an optical microcavity and the QD emitters to explain the details of the emission photon spectra. Our study establishes useful guidelines for the experimental study of such multiexcitonic quantum dot in an optical microcavity system.      

Nanoscale solid-state single photon router

A novel scheme for a solid-state single-photon router based on a single quantum dot (QD) coupled to a nanomechanical resonator (NR) is proposed and analyzed theoretically. It relies on the coherent coupling between the quantum dot and the NR. We demonstrate that when a single-photon signal is tuned on resonance with the exciton in the QD, one can use a strong pump field to choose to what output port of this signal field is delivered, which is based on the analogue of electromagnetically induced transparency (EIT) effect which we refer it as phonon induced transparency (PIT) in this coupled system. The path between the reflection output port and the transmission output port can be achieved by simply turning off and on the pump field. The numerical results also indicate that this router can operate efficiently in the optical regime and at ultralow pump power as well as short switching time (～ns). This nanoscale router presented here will offer potential applications in scalable solid-state quantum networks and quantum information.     

Coupling effects in a photonic crystal microcavity with embedded semiconductor quantum dot

The present work investigates the effects of relevant parameters of InAs/GaAs quantum dot and photonic crystal slab-based microcavity on the QD–cavity coupling characteristics, in detail. We employ variational approach to find exciton state in QD and to find cavity modes we use the open source GME code. Calculations have performed in linear regime where excitons behave as bosons which correspond to the limit of low excitation. The dynamics of the system are studied using the first order correlation function (G⁽¹⁾(t,τ)). We will show how G⁽¹⁾ varies with time in both strong and weak coupling regimes. Our results indicate that the achieving of strong coupling regime is affected by the size of the quantum dot and how to engineer the photonic crystal microcavity to maximize the ratio of quality factor and mode volume.     

Ab Initio Simulation of Charge Transfer at the Semiconductor Quantum Dot/TiO2 Interface in Quantum Dot‐Sensitized Solar Cells

Quantum dot‐sensitized solar cells (QDSSCs) have emerged as a promising solar architecture for next‐generation solar cells. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO₂ acceptor with size quantization properties of QDs that allows for the modulation of band energies to control photoresponse and photoconversion efficiency of solar cells. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs with different sizes on the TiO₂ substrate are simulated using a rigorous ab initio density functional method. This method capitalizes on localized orbital basis set, which is computationally less intensive. Quite intriguingly, a remarkable set of electron bridging states between QDs and TiO₂ occurring via the strong bonding between the conduction bands of QDs and TiO₂ is revealed. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO₂ acceptor, and may be a general feature for strongly coupled donor/acceptor systems. All the QDs/TiO₂ systems exhibit type II band alignments, with conduction band offsets that increase with the decrease in QD size. This facilitates the charge transfer from QDs donors to TiO₂ acceptors and explains the dependence of the increased charge transfer rate with the decreased QD size.     

Exciton states in strongly coupled asymmetric semimagnetic double quantum dots

Exciton states in a pair of strongly coupled artificial asymmetric quantum dots (QDs) have been studied in magnetic fields up to B = 8T by means of photoluminescence spectroscopy. The QD molecules have been fabricated using a selective interdiffusion technique applied to asymmetric CdTe/(Cd,Mg,Mn)Te double quantum wells. The lateral confinement potential within the plane induced by the diffusion gives rise to effective zero-dimensional exciton localization. Incorporation of the Mn ions in only one dot results in a pair of QDs with a markedly different spin splitting. In contrast to a positive value of the exciton Lande g factor in nonmagnetic (Cd,Mg)Te-based single QDs, the ground exciton transition in the nonmagnetic QD demonstrates nearly zero g factor, thus, indicating a strong electron coupling between the dots. A new low-energy band with a strong red shift appears at high B signifying formation of the indirect exciton in accordance with our calculations. The text was submitted by the authors in English.     

Nuclear-induced time evolution of entanglement of two-electron spins in anisotropically coupled quantum dot

We study the time evolution of entanglement of two spins in an anisotropically coupled quantum dot interacting with the unpolarised nuclear spins environment. We assume that the exchange coupling strength in the z direction J _z is different from the lateral one J _l . We observe that the entanglement decays as a result of the coupling to the nuclear environment and reaches a saturation value, which depends on the value of the exchange interaction difference J = ‖J _l ? J _z ‖ between the two spins and the strength of the applied external magnetic field. We find that the entanglement exhibits a critical behaviour controlled by the competition between the exchange interaction J and the external magnetic field. The entanglement shows a quasi-symmetric behaviour above and below a critical value of the exchange interaction. It becomes more symmetric as the external magnetic field increases. The entanglement reaches a large saturation value, close to unity, when the exchange interaction is far above or below its critical value and a small one as it closely approaches the critical value. Furthermore, we find that the decay rate profile of entanglement is linear when the exchange interaction is much higher or lower than the critical value but converts to a power law and finally to a Gaussian as the critical value is approached from both directions. The dynamics of entanglement is found to be independent of the exchange interaction for an isotropically coupled quantum dot.