Quantum tunneling of Bose-Einstein condensates in optical lattices under gravity

We investigate the quantum tunneling of Bose-Einstein condensates in optical lattices under gravity in the "Wannier-Stark localization" regime and "Landau-Zener tunneling" regime. Our results agree with experimental data [B. P. Anderson et al., Science 282, 1686 (1998); F. S. Cataliotti et al., Science 293, 843 (2001)]. We obtain the total decay rate which is valid over the entire range of temperatures, and show how it reduces to the appropriate results for the classical thermal activation at high temperatures, the thermally assisted tunneling at intermediate temperatures, and the pure quantum tunneling at low temperatures. We design an experimental protocol to observe this new phenomenon in further experiments.     

Oscillations of light absorption in 2D macroporous silicon structures with surface nanocoatings

We investigated the near-IR light absorption oscillations in 2D macroporous silicon structures with microporous silicon layers and CdTe, ZnO surface nanocrystals. The electro-optical effect was taken into account within the strong electric field approximation. Well-separated oscillations were observed in the spectral ranges of the surface bonds of macroporous silicon structures with surface nanocrystals. The model of the resonant electron scattering on impurity states in electric field of heterojunction “silicon-nanocoating” on macropore surface as well as realization of Wannier-Stark effect on the randomly distributed surface bonds were considered. The Wannier-Stark ladders are not broken by impurities because of the longer scattering lifetime as compared with the period of electron oscillations in an external electric field, in all spectral regions considered for macroporous silicon structures with CdTe and ZnO surface nanocrystals.     

多棱锥镜产生多光束干涉场的理论和实验研究

提出了一种使用多棱锥镜和多棱台镜产生多束相干光形成二维和三维光学格子的方法。理论分析和数值模拟了多束轴对称平面波干涉产生的二维及三维点阵结构的特性,得到了光场分布随光束数增加的关系,发现随着干涉光数目的增加,干涉场会复杂变化,当棱锥棱数足够多近似于一个圆锥时,干涉场会变为同心圆结构的贝塞尔光束的场分布。实验上使用多棱锥和多棱台镜进行了多光束干涉实验,得到了多束轴对称平面波干涉形成的光学格子,将数值模拟与实验结果进行了比较,二者完全吻合。     

Optical induction of 3D rotational symmetry refractive lattices by combined interferometric-mask method in doped LiNbO3 crystals

A novel combined interferometric–mask method for the formation of micro- and nanometric scale three-dimensional (3D) rotational symmetry quasi-crystalline refractive lattice structures in photorefractive materials is demonstrated experimentally. The method is based on micrometric scale spatial modulation of the light by amplitude mask in the radial directions and along the azimuthal angle and the use of counter-propagating beam geometry building up Gaussian standing wave, which defines the light modulation in the axial direction with half-wavelength periodicity. 3D intensity pattern can be represented as numerous mask-generated 2D quasi-periodic structures located in each anti-node of the standing wave. The formed 3D intensity distributions of the optical beams can be imparted into the photorefractive medium thus creating the micro- and sub-micrometric scale 3D refractive index volume lattices. The used optical scheme allows also the formation of 2D lattices by removing the back-reflecting mirror. 2D and 3D refractive lattices were recorded with the use of 532 nm laser beam and rotational symmetry mask in doped lithium niobate crystals and were tested by the probe beam far-field diffraction pattern imaging and direct observation by phase microscope. The formed rotational symmetry 3D refractive structures have the periods of 20–60 μm in the radial directions, 60 μm along the azimuthal angle and half-wavelength 266 nm in the axial direction.     

Wannier-Stark states in finite superlattices

Individual Wannier-Stark states are resolved in a current experiment over a wide electric-field range for a 5 and 4 period finite superlattice utilizing a hot-electron transistor. The observed field dependence of the tunneling transmission through the various states directly resembles the progressive localization of the wave functions. The basic transport through Wannier-Stark states in short-period superlattices is identified to be coherent. By tuning the Wannier-Stark state splitting with electric field into the optical phonon energy, the opening of new LO-phonon mediated transport paths is observed.     

Efficient evaluation of partition functions of inhomogeneous many-body spin systems

We present a numerical method to evaluate partition functions and associated correlation functions of inhomogeneous 2D classical spin systems and 1D quantum spin systems. The method is scalable and has a controlled error. We illustrate the algorithm by calculating the finite-temperature properties of bosonic particles in 1D optical lattices, as realized in current experiments.     

一维准周期晶格的性质及应用

准周期晶格在冷原子领域被广泛研究,它使得人们可以在一维或者二维系统里研究扩展到安德森局域的转变. 2008年, Inguscio研究组在冷原子系统里制备了一维准周期晶格,并观测到了安德森局域化现象,这极大地推动了准周期系统的理论和实验研究.后来, Bloch研究组在制备的一维和二维准周期晶格中都观测到了多体局域的现象.最近,他们还在准周期晶格中成功观测到迁移率边以及存在迁移率边的系统的多体局域现象.这些冷原子实验推动了多体局域以及迁移率边等方向的研究.准周期晶格已经成为一个平台,它对很多物理现象的影响正在被广泛研究,并可以尝试在冷原子实验中观测到这种影响.本文结合作者的一些相关工作,对一维准周期晶格一些近期的研究进行了简要综述,介绍了一些相关的重要的冷原子实验,讨论了准周期晶格的一些重要性质,以及它对一些物理现象(比如拓扑态)的影响.     

Optical Bloch oscillations in waveguide arrays

We show that optical Bloch oscillations can emerge in waveguide arrays with linearly varying propagation constants. The existence of localized modes (Wannier-Stark states) with equidistant wave-number spacing (Wannier-Stark ladder) that do not undergo diffraction is analytically proved. The evolution of arbitrary initial excitations is described, and potential applications are suggested.     

Simulating cyclotron-Bloch dynamics of a charged particle in a 2D lattice by means of cold atoms in driven quasi-1D optical lattices

Quantum dynamics of a charged particle in a two-dimensional (2D) lattice subject to magnetic and electric fields is a rather complicated interplay between cyclotron oscillations (the case of vanishing electric field) and Bloch oscillations (zero magnetic field), details of which has not yet been completely understood. In the present work we suggest to study this problem by using cold atoms in optical lattices. We introduce a one-dimensional (1D) model which can be easily realized in laboratory experiments with quasi-1D optical lattices and show that this model captures many features of the cyclotron-Bloch dynamics of the quantum particle in 2D square lattices.     

荷电膜渗透传质过程的计算机模拟：一价离子的传递

以渗透理论的方法研究一价离子通过荷电膜的传质机理,用计算机编程模拟一价带电离子通过荷电膜的过程以研究荷电膜中导电性能和荷电组分含量的关系.模拟结果表明,对于二维格栅体系在荷电组分含量为0.4～0.6时,膜导电性能存在渗透突跃现象;而对于三维格栅体系则在0.1～0.2时存在渗透突跃现象,此结果和MontoCarlo的二维、三维随机模拟结果比较接近.由于实际的荷电膜也可看作为三维格栅体系,因此,可用三维格栅体系程序对不同荷电分率膜的电导数据进行拟合,结果表明,对于实际的磺化聚苯硫醚(SPPS)/聚醚砜(PES)共混膜,当荷电组分SPPS的分率达到0.144时,共混膜即会从不良导体变为良导体,显然该值落在理论值0.1～0.2,因此,理论模拟结果与实际荷电膜传质的实验数据相吻合.     

Coherent delocalization of atomic wave packets in driven lattice potentials

Atomic wave packets loaded into a phase-modulated vertical optical-lattice potential exhibit a coherent delocalization dynamics arising from intraband transitions among Wannier-Stark levels. Wannier-Stark intraband transitions are here observed by monitoring the in situ wave-packet extent. By varying the modulation frequency, we find resonances at integer multiples of the Bloch frequency. The resonances show a Fourier-limited width for interrogation times up to 2 s. This can also be used to determine the gravity acceleration with ppm resolution.     

Bloch oscillations and Wannier-Stark ladder in the coupled LC circuits

We present a new scheme for realizing Bloch oscillations and Wannier-Stark ladder based on a lattice of coupled LC circuits. By converting the second order dynamical ODEs of the system into a first order Schrödinger-like equation, we propose an equivalent tight binding Hamiltonian to describe the circuit. We show that a synthesized electric field is produced by introducing a frequency mismatch into the resonant frequency of the adjacent LC resonators. The Wannier-Stark modes are the normal modes of the circuit and the Bloch oscillations can be observed in a coupled LC lattice. By addition of coupling capacitors between nodes of the circuit, we study the Bloch oscillation in the presence of long-range couplings. We also show that the circuit converts to a transmission line simulating synthetic electric fields in the continuum limit. The coupled LC circuit is, in some sense, amongst the simplest physical systems exhibiting Bloch oscillation and Wannier-Stark Ladder.     

Interband tunneling and wannier—stark states in superlattices with complex primitive cell

Interband electron tunneling and Wannier-Stark st ates in superlattices with a complex primitive cell were investigated. The method developed in this study allows an arbitrary number of minibands to be considered.     

基于全息成像技术构建任意的二维光晶格阵列

利用Gerchberg-Saxton算法生成任意的二维光晶格阵列的全息图,并且将全息图加载到液晶型空间光调制器上,然后将850 nm的激光照射到空间光调制器的液晶屏上,利用透镜的傅里叶变换特性,成功地显示或构建任意形状的二维光晶格阵列。将该系统应用到87Rb的冷原子实验中,成功俘获冷原子,这为接下来的单原子多量子位的量子模拟实验奠定了基础。     

Raman spectroscopy of resonance exciton tunneling in GaAs/AlAs superlattices in electric fields

Optical-resonance-Raman scattering by acoustic phonons is used to study the effect of an electric field on the state of excitons in GaAs/AlAs superlattices. When the energy of the exciting photon coincides with the energy of an exciton bound to Wannier-Stark states of a heavy hole and electron with Δn=0,±1, the acoustic Raman scattering is enhanced. Oscillations in the intensity of the Raman spectrum in the electric field are explained by resonance delocalization of the exciton ground state as it interacts with Wannier-Stark states of neighboring quantum wells or with Wannier-Stark states of a higher electron miniband. Fiz. Tverd. Tela (St. Petersburg) 40, 827–829 (May 1998)     

光控光子晶体偏光片

提出了一种新颖的调节液晶光子晶体光子带隙的方法.光子晶体波导通过往二维三角形光子晶体的介质柱之间填充液晶得到，光诱导液晶取向以改变液晶的折射指数从而改变光子晶体的光子带隙.数值模拟结果表明：通过外界光场控制所填充的向列相液晶的方向可以对这种二维三角形介质柱光子晶体的禁带结构进行调节.与电场调制方法相比，该光控液晶取向技术具有响应速度快、结构简单的优点.这种可调光子晶体可用于制作新颖的光敏偏光片.     

The Optical Properties of Fractal Nanodendrites in the Processes of Their Self-Organization

A simple method for constructing a 2D fractal cluster of nanodendrities formed in the self-organization process of nanoparticles is presented. The relationship between the structural features of dendrite-type lattices and the fractal characteristics of both studied objects and their diffraction patterns is revealed.     

Persistent clusters in lattices of coupled nonidentical chaotic systems

Two-dimensional (2D) lattices of diffusively coupled chaotic oscillators are studied. In previous work, it was shown that various cluster synchronization regimes exist when the oscillators are identical. Here, analytical and numerical studies allow us to conclude that these cluster synchronization regimes persist when the chaotic oscillators have slightly different parameters. In the analytical approach, the stability of almost-perfect synchronization regimes is proved via the Lyapunov function method for a wide class of systems, and the synchronization error is estimated. Examples include a 2D lattice of nonidentical Lorenz systems with scalar diffusive coupling. In the numerical study, it is shown that in lattices of Lorenz and Rossler systems the cluster synchronization regimes are stable and robust against up to 10%-15% parameter mismatch and against small noise.     

Ground states for lattices of ferromagnetic granules with dipolar magnetic interaction

The magnetic phase diagrams of 2D and 3D regular lattices formed by nonspherical single-domain ferromagnetic granules featuring a dipolar magnetic interaction are studied. The energy of a magnetic state of such systems is calculated using an approximate expression for the pair interaction of nonspherical granules. The character of the magnetic ground state of the system is determined by three geometric parameters: (i) the eccentricity of granules; (ii) the ratio of periods of the rectangular (2D) or tetragonal (3D) lattice; and (iii) the ratio of a lattice period to a granule size. In contrast to the case of lattices formed by point (or spherical) magnetic moments, in which the ground state is always antiferromagnetic or frustrated (for triangular lattices), the ground state of a 2D lattice composed of nonspherical granules can be ferromagnetic. The magnetic phase diagrams of the systems studied are constructed in the space of the above geometric parameters.     

Wannier-Stark resonances under strong localization conditions in natural silicon-carbide superlattices

A detailed experimental investigation is made of the electronic transport under conditions of Wannier-Stark localization of carriers in a natural superlattice of hexagonal polytypes of silicon carbide. The 4H and 6H polytypes, which possess different superlattice and miniband spectrum parameters, are employed. Direct measurements of the electronic current versus the average electric field in the active region of the sample revealed a series of regions of negative differential conductivity in fields ranging from 500 to 2100 kV/cm. Analysis of the results shows that the observed current resonances are associated with the development of the Wannier-Stark quantization process and are due to conduction mechanisms such as hopping conduction, induced between the levels of a Wannier-Stark ladder by a resonant electron-phonon interaction, and the resonant interminiband tunneling from the first into the second miniband. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 105–109 (25 July 1996)