The Kuramoto-Sivashinsky equation for the defect-deformation instability of a surface-stressed nanolayer

It is demonstrated that the evolution of the surface relief of the stressed elastic nanolayer that interacts with mobile point defects can be described using a nonlinear equation similar to the Kuramoto-Sivashinsky equation. The solution to the equation describes the threshold (with respect to the defect concentration or the mechanical stress) transition to the periodic spatially bent state of the layer with the simultaneous generation of spatially periodic defect clusters at the extrema of the spontaneously emerging surface relief. In this case, the layer deformation corresponds to the displacements in the static bending Lamb wave. The self-organizing periodic cellular deformation-defect surface structure can serve as a universal seed for the subsequent growth of nanoparticles in the processes of laser, ion, or electrochemical etching and in molecular beam epitaxy of nanostructures. Original Text ? Astro, Ltd., 2009.     

Bimodal size-distribution function in a nanoparticle ensemble formed during the laser irradiation of the surfaces of solids

A defect-deformational (DD) model of the formation of a nanoparticle ensemble on the surface of solids under multipulse laser ablation is developed. The DD theory of bimodal nanoparticle size distribution is developed, which expresses size distribution via the bimodal growth rate of DD surface gratings generated by laser irradiation. The obtained size-distribution function corresponds to the experimental data.     

Mechanism of the formation and evolution of periodic surface relief nanostructures under the scanning laser-induced inelastic photodeformation of semiconductors

A defect-deformational (DD) mechanism is proposed for the self-organization of laser-induced point defects (vacancies and interstitials) under low-threshold (far from the melting point) local (10–100 μm) light-induced heating with the scanning periodic pulsed laser irradiation of a semiconductor resulting in an inelastic deformation of micron-sized regions of Ge. A linear theory of DD instability is developed within the model of a biaxially stressed defective film. This model describes the main experimental data on the formation of two-and one-dimensional periodic nanostructures on a semiconductor surface relief.     

Quasi-hexagonal self-organization of nanoparticles upon the laser-controlled deposition of Ga atoms

A cooperative defect-deformation (DD) mechanism is developed for the nucleation of the quasi-monodisperse ensemble of nanoparticles in the low-temperature deposition of Ga atoms in the presence of laser radiation. A comparison with experiment is carried out, and a good agreement between the theoretical and experimental results has been obtained. In particular, in accordance with the DD model, it has been found that the arrangement of nanoparticles is characterized by a latent hexagonal order, and their shape is quasi-hexagonal. It has been shown that the characteristics of the deposited ensemble of nanoparticles can be controlled with the use of an anisotropic deformation.     

Direct Fabrication Methods of Surface Relief Electro-Optic Gratings in Azo-Polymer Films

Two fabrication methods of surface relief electro-optic (EO) gratings in azo-polymer films are described: surface deformation by a continuous wave laser and laser ablation by a pulse laser. In the fabrication process, the periodic intensity modulation of laser beams is formed by an interference fringe or a phase mask. The surface relief EO grating is fabricated by poling the surface relief grating (SRG) after the SRG fabrication process. The linear dependence of an applied voltage in the modulation efficiency of the first-order diffraction beam is observed. The proposed methods are effective for highly-efficient surface relief EO grating fabrication.     

Increasing the fatigue life of 12Cr1MoV steel by surface nanostructuring with a Zr<Superscript>+</Superscript> ion beam. Structure,properties, and fracture pattern

The paper presents the results of static and cyclic tensile tests and alternate cyclic bending tests of 12Cr1MoV specimens in the initial state and after surface nanostructuring with a Zr⁺ ion beam. Examination by optical and scanning electron microscopy and interference profilometry revealed differences in the formation of the deformation relief and in the character of cracking of the modified surface layer. The changes occurring in the modified surface layer were estimated by nanoindentation, X-ray analysis, and fractography. The nanostructure formed in the treated surface layer was analyzed by transmission electron microscopy. The difference in deformation is interpreted using the multiple cracking concept. The effect of substantial enhancement of fatigue strength is associated with retarded plastic deformation and fatigue crack propagation in the modified surface layer.     

Self-organization of ordered nano- and microstructures on the semiconductor surface under the action of laser radiation

Experimental results on the formation of ordered nano- and microstructures on the surface of semi-conductors under the action of laser pulses with various durations and fluences are interpreted from the unified view point of the theory of the defect-deformation (DD) instability of surface relief. A universal linear dependence of the period of the structures on the thickness of the subsurface layer enriched with mobile point defects and formed due to the laser action and occurrence of two scales of modulation of the surface relief are established and described. The structure symmetry and its evolution with an increasing laser fluence and magnitude of external anisotropic stress are described. Similarities with the formation of nanostructures under ion-beam irradiation and electrochemical etching are revealed and discussed within the framework of the DD instability theory.     

Planar-to-wavy transition of Cu–Ag nanolayered metals: a precursor mechanism to twinning

The interface-mediated plastic deformation mechanisms of a semi-coherent Cu–Ag bimetal nanolayered structure subjected to out-of-plane tension are characterized by molecular dynamics simulations. Results show that the initially planar Cu–Ag nanolayers abruptly become wavy at a critical tensile strain. This planar-to-wavy interlayer transition is facilitated by the low shear resistance of the Cu–Ag interlayer interface, which slides to accommodate the out-of-plane deformation. The process redistributes misfit dislocations along the interface to reduce the bending energy of the wavy structure. High stress concentrations subsequently develop at the summits and valleys of the wavy Cu–Ag interlayer interfaces, from which micro-twinning partials are emitted. These results demonstrate that the wavelength of the wavy Cu–Ag nanolayer structure forms a critical length scale for the localization of spatially periodic defect sources for twin nucleation. This planar-to-wavy interlayer transition mechanism is only activated in nanolayered metals with interfaces that are amenable to sliding prior to twin or dislocation emissions.     

Wave instability of a molten metal layer formed by intense laser irradiation

Wave structure of a molten metal layer flowing over the walls of a vapor-gas cavern that appears as an intense laser radiation penetrates deep into condensed media is studied theoretically taking into account surface tension, gravitation, thermocapillary effect, and nonuniform evaporation from the free surface of the melt. A long-wavelength evolution equation describing the evolution of nonlinear waves on the free surface of a plane molten layer is derived. The spatially periodic running solution to this equation is obtained, and the main characteristics (amplitude and period) of the nonlinear wave structures are determined.     

Nonlinear regime of the excitation of a surface electromagnetic wave on the silicon surface by an intense femtosecond laser pulse

A surface electromagnetic wave has been excited on an atomically smooth silicon surface by an intense infrared femtosecond laser pulse as a result of its self-diffraction on a microscale short-lived optical inhomogeneity of the excitation region rather than on the roughness of its surface relief. This wave has been visualized in the form of the pattern of its interference with the same incident infrared ultrashort pulse, which corresponds to the instantaneous surface dielectric constant grating (reflection), as well as the resulting surface relief grating, using time-resolved far-field optical reflection microscopy.     

Computer simulation of water vapor nucleation on charged nanoparticles

The Monte Carlo method is applied to the study of the formation of condensed-phase nuclei from water vapor on electrically charged silver iodide nanocrystals. This study is a continuation of the investigations carried out earlier in [1] with electrically neutral nucleation centers. Nanoparticles with a size of up to 4 nm and flat nanoparticles with a size of up to 10 nm are investigated. The free energy, entropy, and the work of formation of nuclei with a size of up to 6729 molecules are calculated at the atomic level by the bicanonical statistical ensemble (BSE) method at a temperature of 260 K. Thermodynamic stability of nuclei is investigated depending on the size, shape, and charge of nanocrystal nucleation centers, as well as depending on the presence of crystal defects and the degree of spatial localization of charge on the surface of nanoparticles. The excess charge has a crucial effect on the work of formation of a nucleus only in the case of strong spatial localization of the latter near a point crystal defect; however, this effect is restricted to a relatively small size of the nuclei and therefore cannot substantially enhance the ice-forming activity of nanoparticles. A nucleus that grows on the surface of a nanoparticle evolves through three stages that differ in molecule retention mechanism and thermodynamic stability. The charge of a nanoparticle has a small effect on these factors. The leading factor that determines the ice-forming activity of ion nanocrystals is their intrinsic electric field due to the nonuniform distribution of charge within a unit cell of the crystal lattice.     

Formation and evolution of nanostructures on semiconductor surfaces in the process of laser-induced inelastic deformation

A hierarchy of formed Ge surface relief nanostructures, including a disordered nanocluster structure, a two-dimensional lattice, and a one-dimensional lattice, is observed as the laser irradiation dose is increased. It is described in the framework of the defect deformation mechanism.     

Exact and approximate analysis of surface acoustic waves in an infinite elastic plate with a thin metal layer

For an accurate approximation of the effect of a thin layer over a finite substrate, we consider the displacements are continuous across the interface, while the stress components are obtained from derivatives of displacements. As a result, the stress boundary conditions are transformed to a relationship between stresses in the vicinity of the interface of two layers and density of the metal layer. Through this approximation, we eventually have four equations to solve for the surface acoustic wave dispersion relation of the two-layer structure. The approximate and accurate results are compared with good agreement for small thickness of the metal layer. These results are intended for periodic structures with separate solutions for electroded and unelectroded regions, which can be connected by the continuity boundary conditions for the analysis of the complete structure of typical surface acoustic wave resonators.     

Modeling of the polarization behavior of elliptical surface-relief VCSELs

We undertake a theoretical study of the polarization behavior and mode structure of Vertical-Cavity Surface-Emitting Lasers (VCSELS) with an elliptical surface relief. The aim of the relief is twofold, first to enhance the single mode operation by introducing spatially distributed losses, and second, to enhance polarization stability by making these losses anisotropic. First, we use an analytical variational approach, for the two fundamental VCSEL modes within a Gaussian and weak guidance approximation. As a second approach we use a more refined numerical technique based on vectorial plane wave decomposition with periodic boundary conditions, which we formulate to include material anisotropy. Our studies show that whereas the polarization direction is governed by the orientation of the index ellipsoid, the selection of the lasing LP mode is mainly determined by the anisotropy in the mirror losses introduced by the elliptical surface relief.     

Thermocapillary mechanism of convection in nanofluids due to the absorption of spatially periodic laser radiation

The problem of origination of regular convection in the layer of nanofluid with one free surface by the absorption of light waves with spatially periodic intensity distribution in the plane of the layer caused by the temperature dependence of the surface tension coefficient is studied. The profiles of velocity and temperature in the volume of medium are determined. It is shown that the response of the system on the influence of a light wave is maximal other conditions being equal, when the period of interference pattern is about twice as large as the layer thickness.     

Analysis of the relief of the GaAs surface formed upon exposure to diffraction-modulated laser radiation

A relief of the GaAs surface irradiated by a laser through diffraction masks having different profiles is investigated. The incident beam energy density does not exceed the threshold value. Clusters of point defects are formed in the crystal surface layer upon exposure to diffraction-modulated laser radiation. With the help of theoretical calculations, it is established that clusterization in the surface layer results from redistribution of point defects in the periodic field of thermoelastic stresses. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 86–89, November, 2007.     

Multiscale dislocation dynamics simulations of shock-induced plasticity in small volumes

Multiscale dislocation dynamics plasticity (MDDP) was used to investigate shock-induced deformation in monocrystalline copper. In order to enhance the numerical simulations, a periodic boundary condition was implemented in the continuum finite element (FE) scale so that the uniaxial compression of shocks could be attained. Additionally, lattice rotation was accounted for by modifying the dislocation dynamics (DD) code to update the dislocations’ slip systems. The dislocation microstructures were examined in detail and a mechanism of microband formation is proposed for single- and multiple-slip deformation. The simulation results show that lattice rotation enhances microband formation in single slip by locally reorienting the slip plane. It is also illustrated that both confined and periodic boundary conditions can be used to achieve uniaxial compression; however, a periodic boundary condition yields a disturbed wave profile due to edge effects. Moreover, the boundary conditions and the loading rise time show no significant effects on shock–dislocations interaction and the resulting microstructures. MDDP results of high strain rate calculations are also compared with the predictions of the Armstrong–Zerilli model of dislocation generation and movement. This work confirms that the effect of resident dislocations on the strain rate can be neglected when a homogeneous nucleation mechanism is included.     

Spatially periodic modulation of the level population upon saturation for centers with weak and strong electron-phonon interactions

Simple relationships are derived for describing a spatially periodic modulation of the difference between the diagonal elements of the density matrix for individual orientation groups of quantum systems in anisotropic crystals and for centers of the crystal as a whole. It is revealed that the modulation depth of the level populations upon saturation for centers with a strong electron-phonon interaction in the limit is two times greater than that for centers with pure electronic optical transitions. The effect of doubling of the wave number of the periodic structures induced by excited centers is described using crystals with a rutile structure and a four-fold optic axis as an example.     

Generation of the second harmonic of femtosecond pulses on reflection from a metal surface: amplification due to periodic modulation of the relief

It is observed experimentally that the optical second-harmonic generation signal produced when femtosecond laser pulses are reflected from a metal surface increases more than 100-fold when a specially chosen periodic relief that gives quasiresonance excitation of a surface electro-magnetic wave is depoited on the surface. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 155–159 (10 February 1996)     

Nucleation of Pd dimers at defect sites of the MgO(100) surface

Point defects on oxide surfaces are presumed to be preferential nucleation sites for supported metal clusters. Under typical growth conditions, dimers constitute the first step in island nucleation. First-principles calculations on the formation of Pd dimers on regular and defect sites of the MgO(100) surface show that nucleation occurs with large dimer binding energies at divacancies and charged oxygen vacancies (F+ centers), while it is less favorable on steps and neutral F centers. The extensive database of defect trapping/attachment properties gives a firm basis to rationalize recent atomic-force microscopy findings and provides guidelines valid, in general, for ionic substrates.