Self-organized coherence in fiber laser arrays

We report the production of stable, coherent, and same-phase states in arrays of fiber lasers. Provided that proper interactions between the lasers are present, arrays will spontaneously self-organize into stable coherent same-phase states. There is no need for active control. Power scaling, power spectra, spatial interference fringes, and temporal data all support this conclusion.     

表面纳米结构的自组装生长

无论是对低维基本物理以及其中新奇量子现象的探索与认识,还是微电子工业水平的持续发展,都迫切地需要掌握一种能够精确、可靠地操控表面纳米结构的方法.自组织生长,即粒子聚集时由于介观尺度力场或受限运动作用而导致的自发有序现象,在原子尺度上可以实现对纳米结构的精确控制,而在介观尺度上又可以调节这些微观结构单元的组织构型.文章结合作者近年来在表面纳米结构生长与物理性质研究方面所做过的工作,从自组织生长的原理出发,介绍了对金属纳米线、有序分子薄膜以及合金量子点阵列生长进行人工操控的方法.     

表面纳米结构的自组装生长

无论是对低维基本物理以及其中新奇量子现象的探索与认识,还是微电子工业水平的持续发展,都迫切地需要掌握一种能够精确、可靠地操控表面纳米结构的方法.自组织生长,即粒子聚集时由于介观尺度力场或受限运动作用而导致的自发有序现象,在原子尺度上可以实现对纳米结构的精确控制,而在介观尺度上又可以调节这些微观结构单元的组织构型.文章结合作者近年来在表面纳米结构生长与物理性质研究方面所做过的工作,从自组织生长的原理出发,介绍了对金属纳米线、有序分子薄膜以及合金量子点阵列生长进行人工操控的方法.     

Self-organized silicon microcolumn arrays generated by pulsed laser irradiation

Cumulative nanosecond pulsed excimer laser irradiation of silicon produces an array of high-aspect-ratio microcolumns that protrude well above the initial surface. The growth of these microcolumns is strongly affected by the gas environment, being enhanced in air or in other oxygen-containing atmosphere. An array of very large and complex conical structures that also protrude above the surface is formed if the irradiation is performed in sulfur hexafluoride (SF₆). Kinetics studies of microcolumn growth show that: (i) A certain number of pulses is required to initiate growth of microcolumns; (ii) column nucleation is inhomogeneous, taking place always at the edges of deep grooves or pits; (iii) growth is fast with the earlier pulses but slows down to a halt when the columns reach a certain length. These studies show that columns nucleate and grow by continuous influx of silicon with each laser pulse. It is proposed that the axial growth of microcolumns and cones is due to the deposition of atoms or clusters at their tips. The column/cone tips are melted during irradiation and act as preferred sites for deposition, resulting in a very high axial growth rate. The contribution of etching and ablation to the flux of silicon-rich vapor produced during irradiation is discussed. The mechanism of columnar growth is compared with the vapor-liquid-solid method to grow silicon whiskers.     

Magnetic moment of square SIS Josephson arrays: Self-organized criticality

The temperature-and magnetic-field dependences of the magnetic moment of square Josephson arrays with SIS-type junctions are studied experimentally. Two temperature regions are observed with different types of magnetization curves. Magnetic flux avalanches are detected in the low-temperature region. Statistical analysis of avalanche amplitudes A shows that their size distribution varies in accordance with the power law P∝Aⁿ with crossover, when exponent n varies from n=?0.7 for small avalanches to n=?6 for large avalanches, while the frequency spectrum varies in accordance with the law 1/f^α. Such behavior is interpreted as a manifestation of self-organized criticality.     

Self-organized growth of quantum dot-tunnel barrier systems

Surface selective epitaxial growth on patterned substrates is used to fabricate quantum dot-tunnel barrier systems, which can be used as single-electron transistor devices. In the centre of a pre-patterned constriction a self-assembled GaAs quantum dot embedded in barrier material is formed during the molecular beam epitaxial growth of an Al_0.33Ga_0.67As/GaAs heterostructure. The quantum dot is connected via self-aligned tunnelling barriers to source and drain electrodes. In-plane-gate electrodes are also realized within the epitaxial growth process. The paper describes the fabrication process of the device and the characterization of the direct grown quantum dot-tunnel barrier system using scanning-electron microscopy, atomic-force microscopy and transport spectroscopy.     

Self-organized dynamics on a curved growth interface

We experimentally address long-time dynamics of an artificially curved growth interface in directional solidification. Repetitive cell nucleations are found to appear in a disordered way, but to eventually organize themselves in a coherent way, for long times. This behavior is recovered by simulation of a nonlinear advection-diffusion model for phase dynamics. The existence of a periodic attractor is supported by the derivation of a Lyapunov functional for this model.     

Self-organized arrays of submicrometer particle of organic dye and its near-field optical study

Organic dye particles of micrometer and submicrometer diameters were prepared by a wetting/dewetting procedure on a hydrophilic glass surface and a self-organized one- or two-dimensional registration was observed. To analyze the molecular assembly in these particles the near-field-excited near-field fluorescence from single particles were detected, while the majority of particles with diameters around 2 μm or less did not show fluorescence. Far-field fluorescence, in contrast, was observed for every particle, and the intensity depended on the excitation polarization when a polarized evanescent field was used for excitation, indicating that the molecules’ transition moment within dye particles orient parallel to the substrate surface. These two observations suggest that the near-field at the tip of the probe was polarized parallel to the probe axis. Another observation, that neighboring particles show similar molecular orientations, suggests that the dewetting process contributed to the alignment of the molecular directions in adjacent particles, which further proves that the dye particles were formed by a self-organizing mechanism.     

Self-organized epitaxial growth on spontaneously nano-patterned templates

Self-ordering at crystal surfaces has been the subject of intense efforts during the last ten years, since it has been recognized as a promising way for growing uniform nanostructures with regular sizes and spacings in the 1–100 nm range. In this article we give an overview of the self-organized nanostructures growth on spontaneously nano-patterned templates. A great variety of surfaces exhibits a nano-scale order at thermal equilibrium, including adsorbate-induced reconstruction, surface dislocations networks, vicinal surfaces or more complex systems. Continuum models have been proposed where long-range elastic interactions are responsible for spontaneous periodic domain formation. Today the comparison between experiments such as Grazing Incidence X-Ray Diffraction experiments and calculations has lead to a great improvement of our fundamental understanding of the physics of self-ordering at crystal surfaces. Then, epitaxial growth on self-ordered surfaces leads to nanostructures organized growth. The present knowledge of modelization of such an heterogeneous growth using multi-scaled calculations is discussed. Such a high quality of both long-range and local ordered growth opens up the possibility of making measurements of physical properties of such nanostructures by macroscopic integration techniques. To cite this article: S. Rousset et al., C. R. Physique 6 (2005).     

Self-organized growth of nanopucks on Pb quantum islands

Electronic Moirè patterns found on lead (Pb) quantum islands can serve as a template to grow self-organized cluster (nanopucks) arrays of various materials. These patterns can be divided into fcc- and hcp-stacked areas, which exhibit different binding strengths to the deposited adatoms. For Ag adatoms, the binding energy can differ substantially and the confined nucleation thus occurs in the fcc sites. Both the size distribution and spatial arrangement of the Ag nanopucks are analyzed and found to be commensurate with the characteristics of the template island, which exhibits a bilayer oscillatory behavior.     

Self-organized superlattice formation during crystal growth from continuous beam fluxes

Alloy superlattice structures consisting of alternating Si-rich and C-rich layers form spontaneously during gas-source molecular beam epitaxy of Si(1-y)C(y) on Si(001) from constant Si2H6 and CH3SiH3 precursor fluxes at T(s)=725-750 degrees C. The self-organized patterning is due to a complex interaction among competing surface reactions. During growth of the initial Si-rich layer, strain-driven C segregation to the subsurface results in charge transfer from surface Si atom dangling bonds to C backbonds. This decreases the Si2H6 sticking probability, and, hence, the instantaneous deposition rate, thereby enhancing C segregation. The Si-rich layer continues until a critical C coverage is reached allowing nucleation of a C-rich layer which grows until the excess subsurface C is depleted. The process then repeats with periods tunable through the choice of T(s) and y(avg).     

Self-organized quantum-wire lattice via step flow growth of a short-period superlattice

We develop a theoretical model for step flow growth of multilayer films, taking into account the interlayer step-step interaction induced by misfit strain. We apply the model to simulate the growth of strain-compensated short-period superlattices. Step-bunch ordering improves in successive layers, leading to self-organized growth of a lattice of quantum wires. This quantum-wire array has some similarities to the "lateral composition modulation" observed experimentally in short-period superlattices.     

Off-lattice Eden-C cluster growth model

A non-trivial cluster growth model, equivalent to the lattice-free Eden-C model, is proposed. The model is constructed by randomly adding contiguous circles without overlapping. Large-scale computer simulations show the interior density is constant at 0.650, while the boundary is fractal, with a thickness proportional to the 0.396 power of the mean radius.     

Catalyst-free growth of single-crystalline alumina nanowire arrays

Alumina nanowires were synthesized on a large-area silicon substrate via simple thermal evaporation of a mixture of aluminum and alumina powders. The microstructure of the as-grown alumina nanowires was characterized using X-ray diffraction and transmission electron microscopy techniques. These nanowires usually have a straight morphology and are single crystalline with the wire axis parallel to the (001) direction. Arrays of the alumina nanowires were also observed grown on the surface of many large particles. The catalyst-free growth of the alumina nanowires was explained in the framework of vapor–solid growth. PACS 81.07.-b; 81.16.-c; 81.07.Bc     

Self-organized growth of nanoparticles on a surface patterned by a buried dislocation network

The self-organized growth of Co nanoparticles is achieved at room temperature on an inhomogenously strained Ag(001) surface arising from an underlying square misfit dislocation network of 10 nm periodicity buried at the interface between a 5 nm-thick Ag film and a MgO(001) substrate. This is revealed by in situ grazing-incidence small-angle x-ray scattering. Simulations of the data performed in the distorted wave Born approximation framework demonstrate that the Co clusters grow above the dislocation crossing lines. This is confirmed by molecular dynamic simulations indicating preferential Co adsorption on tensile sites.     

Alignment-controlled hydrothermal growth of well-aligned ZnO nanorod arrays

ZnO nanorod arrays (ZNAs) were prepared via a two-step seeding and solution hydrothermal growth process. Effects of preparing parameters such as seed layer, colloid concentration, substrate and precursor concentration, on the alignment control of ZNAs were systematically investigated. The deviation angle of ZnO nanorods was measured to evaluate the alignment of arrays. Results show that seed layer not only controls the vertical orientation of ZNAs, but also the compactness of ZNAs. Altering colloid concentration and substrate can influence the microstructure of ZnO seed layer and affect the ordered alignment of ZNAs. The precursor concentration has an insignificant effect on the alignment of ZNAs but has great impact on the morphology of ZNAs. Alignment-controlled and well-aligned ZnO nanorods with different diameter and aspect ratio can be obtained by properly controlling the preparing parameters. A growth mechanism was proposed for the growth of ZnO nanorods.     

两步法制备空间取向高度一致的ZnO纳米棒阵列

采用两步法,即先用磁控溅射在Si(100)表面生长一层ZnO籽晶层、再利用液相法制备空间取向高度一致的ZnO纳米棒阵列.用扫描电子显微镜、X射线衍射、高分辨透射电子显微镜和选区电子衍射对样品形貌和结构特征进行了表征.结果表明,ZnO纳米棒具有垂直于衬底沿c轴择优生长和空间取向高度一致的特性和比较大的长径比,X射线衍射的(XRD)(0002)峰半高宽只有0.06°,选区电子衍射也显示了优异的单晶特性.光致发光谱表明ZnO纳米棒具有非常强的紫外本征发光和非常弱的杂质或缺陷发光特性. 关键词： ZnO纳米棒阵列 ZnO籽晶层 两步法 液相生长     

Cyclic cluster growth in ferroelectric perovskites

We prove that point defect clustering in perovskite ferroelectrics and the subsequent loss in switchable polarization can occur only if the depolarizing fields are locally unscreened during certain fractions of time in each of the multiple bipolar electric field cycles.