Correlated photon emission from semiconductor quantum dots grown in inverted pyramids

We demonstrate photon antibunching and deterministic single-photon operation using single InGaAs/AlGaAs semiconductor quantum dots grown on pre-patterned substrates. Additionally, we observe several types of single-photon cascades involving biexcitonic and other excitonic complexes and have modeled this behavior using numerical Monte-Carlo simulations. This method allows us to determine different non-radiative mechanisms otherwise not directly accessible via conventional spectroscopic methods.     

Spontaneous emission spectra of a four-level atom in photonic crystals driven by two coherent fields

We investigate the spontaneous emission spectra of a four-level tripod-type atom embedded in a photonic crystal and driven by two coherent fields. It is found that due to the quantum interference caused by two driving fields, the spontaneous emission spectra have different features from the case of only one driving field. The spectra are sensitively dependent on the detuning of the driving fields. A dark line occurs for some particular initial states. By appropriately adjusting the external driving fields, the spectral-line can be narrowed, enhanced or suppressed.     

磁场中带电粒子阻尼运动的分析力学表示

研究带电粒子在磁场中作阻尼运动的分析力学表示. 首先, 求解运动微分方程的Birkhoff力学逆问题, 得到带电粒子的4个Rirkhoff表示; 其次, 导出4个状态空间中Lagrange表示和对应的4个位形空间中Lagrange表示; 第三, 构造出4个Hamilton函数; 最后, 从粒子运动的分析力学表示直接得到4个第一积分, 并求出运动方程的解.     

双能带三维光子晶体中二能级原子的自发辐射

研究了双能带三维光子晶体中二能级原子自发辐射的性质.由于双能带各向异性色散关系的影响，辐射场中的局域场和传输场不能共存，导致原子上能级占据数不再具有准周期性振荡的性质，而是随时间趋于常数，或者随时间按幂函数形式衰减，或者随时间按指数形式衰减.这些性质不仅与原子上能级和能带带边的相对位置有关，而且与光子晶体上、下能带之间的能隙宽度有关.这些性质也有别于单能带各向同性光子晶体中二能级原子自发辐射的性质. 关键词： 光子晶体 二能级原子 自发辐射     

Modification of spontaneous emission from a five-level M-type atom in coupled cavity waveguide

We study the spontaneous emission spectrum of a five-level M-type atom driven by a microwave field, in which two upper levels are coupled by the same-coupled cavity waveguide reservoir to a lower level. The spectrum behavior presents a strong non-Lorentzian shape that originates from effective quantum interference in Markovian reservoir, in which the spectral line can be significantly enhanced and eliminated by adjusting the proper parameters of the system. However, for non-Markovian reservoir, it seems that the shape of emission spectrum is quite dependent on the geometry behavior of a coupled cavity waveguide.     

Classical and quantum self amplified spontaneous emission in a high gain free electron laser

We describe the quantum theory and the photon statistics of self amplified spontaneous emission (SASE) in a high gain free electron laser (FEL) using Glauber's quantum theory of coherence. We generalize a previous theory by taking into account many-mode effects and the initiation process resulting from classical shot noise, quantum noise, an injected coherent field and coherent bunching. In particular, we define the concept of quantum SASE which is appropriate when the initial quantum fluctuations dominate over the classical shot noise. We also discuss the conditions for the observation. Quantum SASE is a new quantum phenomenon in which the single electron uncertainty fluctuations of the conjugate variables position and momentum produce exponential amplification of the vacuum field.     

Spontaneous emission from an atom in a photonic crystal with two coherent bands

The dynamic and the radiative properties of an excited three-level atom embedded in an anisotropic photonic crystal with two coherent bands are investigated.The relative position of the atom in a Wigner-Seitz cell is described with a position-dependent parameter θ(r0),which is used as the coherent parameter for the two bands.The result shows that the dynamic properties of the atomic system are not only determined by atomic transition frequencies,but also affected by the gap width and the coherence of the two bands.In addition,the spontaneous emission spectrum of the atomic transition in free space is discussed.The center and the intensity of the spectrum can be obviously manipulated via the coherent parameter.     

Spontaneous emission from a microwave-driven four-level atom in an anisotropic photonic crystal

The spontaneous emission from a microwave-driven four-level atom embedded in an anisotropic photonic crystal is studied. Due to the modified density of state(DOS) in the anisotropic photonic band gap(PBG) and the coherent control induced by the coupling fields, spontaneous emission can be significantly enhanced when the position of the spontaneous emission peak gets close to the band gap edge. As a result of the closed-loop interaction between the fields and the atom,the spontaneous emission depends on the dynamically induced Autler–Townes splitting and its position relative to the PBG.Interesting phenomena, such as spectral-line suppression, enhancement and narrowing, and fluorescence quenching, appear in the spontaneous emission spectra, which are modulated by amplitudes and phases of the coherently driven fields and the effect of PBG. This theoretical study can provide us with more efficient methods to manipulate the atomic spontaneous emission.     

A charged particle in a homogeneous magnetic field accelerated by a time-periodic Aharonov–Bohm flux

We consider a nonrelativistic quantum charged particle moving on a plane under the influence of a uniform magnetic field and driven by a periodically time-dependent Aharonov–Bohm flux. We observe an acceleration effect in the case when the Aharonov–Bohm flux depends on time as a sinusoidal function whose frequency is in resonance with the cyclotron frequency. In particular, the energy of the particle increases linearly for large times. An explicit formula for the acceleration rate is derived with the aid of the quantum averaging method, and then it is checked against a numerical solution and a very good agreement is found.     

动态各向同性光子晶体中二能级原子的自发辐射

研究了动态各向同性光子晶体中二能级原子自发辐射的性质,主要讨论了光子晶体能带带边频率随时间作阶跃调制和三角函数周期调制两种情况下,原子上能级占据数随时间的演化特性.当光子晶体能带带边频率随时间作阶跃调制时,原子上能级占据数随时间的演化不仅和上能级与能带带边的相对位置δ有关,更依赖于阶跃调制发生的时刻.调制发生时刻不同,调制后原子上能级占据数随时间的演化也不同.当光子晶体能带带边频率随时间作三角函数周期调制时,二能级原子上能级占据数随时间作总体衰减的准周期振荡.通过选择调制频率和调制初相位可调控准周期振荡的频率、峰值与谷值的大小以及占据数的总体衰减速度等.     

Ultrasonic light storage in a quantum well: A photon assembly line

Surface acoustic waves are used to effectively convert photogenerated excitons in a piezoelectric semiconductor quantum well into long-lived electron–hole polarizations. The strong lateral piezoelectric fields ionize the excitons and spatially separate the resulting electrons and holes. The strongly reduced wave function overlap results in very long storage times for this polarization which propagates across the sample at the speed of sound. External and deliberate screening of the piezoelectric fields triggers radiative recombination after long delay times and at a remote location of the sample.     

Infinite chain of different deltas: A simple model for a quantum wire

We present the exact diagonalization of the Schr?dinger operator corresponding to a periodic potential with N deltas of different couplings, for arbitrary N. This basic structure can repeat itself an infinite number of times. Calculations of band structure can be performed with a high degree of accuracy for an infinite chain and of the correspondent eigenlevels in the case of a random chain. The main physical motivation is to modelate quantum wire band structure and the calculation of the associated density of states. These quantities show the fundamental properties we expect for periodic structures although for low energy the band gaps follow unpredictable patterns. In the case of random chains we find Anderson localization; we analize also the role of the eigenstates in the localization patterns and find clear signals of fractality in the conductance. In spite of the simplicity of the model many of the salient features expected in a quantum wire are well reproduced. Received 24 June 2002 Published online 29 November 2002     

Charge confinement in epitaxial metallic islands: a study at semiempirical level

Hartree-Fock calculations at semiempirical level, using the extended Debey-Hückel approximation, and tests calculations using LDA with pseudopotentials are used to describe the dispersion relationship, the density of states and the conduction charge of homoepitaxial islands formed by Cu and Ag. The islands have a three-dimensional shape with a dimension of the height and basis in the range of the bulk Fermi wavelength. The calculations indicate that at these small sizes the island steps, at variance with the ones on vicinal surfaces, act as weak and permeable barriers. In fact, the electronic charge retains bulk-like features, though faint traces of confinement can be identified.     

A dissipative particle dynamics study of the realignment of a nanodroplet of a nematic in a weak external magnetic field

We present a dissipative particle dynamics (DPD) approach for simulating the realignment of a nematic nanodroplet suspended in an isotropic fluid following a switch in the direction of an applied external magnetic field. The interaction of the mesogens with the external field is weak relative to the inter-molecular interactions. The simulations were used to investigate the way orientational equilibrium is re-established. The results reveal that the realignment process of the nanodroplet is consistent with its fluid structure. The reorientation of the nanodroplet as a whole is found to be caused by an internal structural rearrangement rather than a coherent rotation of the centres of mass of the mesogens about the centre of the nanodroplet. The switch in the field direction furthermore is found to induce a transient spatial variation in the orientational order of the long axes of the mesogens: the orientational order parameters decreases on moving from the core of the nanodroplet to the surface in contact with the isotropic environment. The results highlight differences between the time evolution of the orientation of the long molecular axes in the field and the rotations of the centres of mass of the mesogens about the centre of the nanodroplet.     

Room‐temperature terahertz emission from ZnSe‐based quantum cascade structures: A simulation study

We identified conditions for room‐temperature operation of terahertz quantum cascade lasers (THz QCLs) where variable barrier heights are used on ZnSe/Zn_1–xMg_x Se material systems. The THz QCL devices are based on three‐level two‐well design schemes. The THz QCL lasers with alternating quantum barriers with different heights were compared with THz QCL laser structures with fixed quantum barrier heights. It is found that the THz QCL device with novel design employing variable barrier heights achieved the terahertz emission of about 1.45 THz at room‐temperature (300 K), and has improved laser performance due to the suppression of thermally activated carrier leakage via higher‐energy parasitic levels. Thus, THz QCL devices employing the design with variable barrier heights may lead to future improvements of the operating temperature and performance of THz QCL lasers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A comparative study of iterative solutions to linear systems arising in quantum mechanics

This study is mainly focused on iterative solutions with simple diagonal preconditioning to two complex-valued nonsymmetric systems of linear equations arising from a computational chemistry model problem proposed by Sherry Li of NERSC. Numerical experiments show the feasibility of iterative methods to some extent when applied to the problems and reveal the competitiveness of our recently proposed Lanczos biconjugate A-orthonormalization methods to other classic and popular iterative methods. By the way, experiment results also indicate that application specific preconditioners may be mandatory and required for accelerating convergence.     

Phase-field study of free growth in a channel: from thermal to chemical solidification

The thermal and the chemical phase-field models for free growth in a two-dimensional channel are both studied in their one-sided version for which diffusion only occurs in the liquid. We compare the steady state fingers obtained in our phase-field simulations with the results of boundary integral techniques available in the literature. The excellent agreement found between both methods provides a valuable benchmark of the one-sided thin-interface phase model which makes use of an antitrapping current. Coexistence of several steady states predicted by the Green’s function calculations is also recovered. The dynamical stability of two competing modes (symmetric and asymmetric finger) is studied and the extension of their respective basins of attraction is evaluated. General implications of our results for a large class of isotropic systems are discussed.