Defect-deformational surface layer instability as a universal mechanism for forming lattices and nanodot ensembles under the effect of ion and laser beams on solid bodies

The defect-deformational (DD) model of surface nanostructure formation under the effect of ion and laser beams is extended to an anisotropic case. The DD theory describes from a unified point of view simultaneous bifurcations in the dependences of grating orientation and period on the ion beam incidence angle, grating rotation, and transition to the formation of a nanodot structure, all of which have been recently observed during ion sputtering of Si. A nonlinear two-dimensional Kuramoto-Sivashinsky DD-equation (DDKS) that describes the formation of lamella-like structures and ordered hexagonal ensembles of nanodots upon normal incidence of ion and laser beams on solid bodies is obtained.     

Parameter optimization of microcantilevers optically excited by a semiconductor laser with optical feedback

A novel approach of optically exciting and detecting the vibration of a microcantilever by a sinusoidally driven semiconductor laser subject to optical feedback is proposed. A theoretical model for the approach is established, and the working parameters are optimized. In our experiments, an Al-coated microcantilever is designed with the optimized working parameters, and the microcantilever is optically excited by a sinusoidally driven semiconductor laser. We have demonstrated that there exist periodic dips of the optical power of the semiconductor laser, which is caused by the resonant vibration of the microcantilever. The optical power of the semiconductor laser is also analyzed theoretically by using the Lang–Kobayashi (L–K) equations. The experimental result is consistent with the result of theoretical analysis.     

Formation of nanometer ordered defect-deformational structures induced by energy fluxes in solids

A defect-deformational theory is constructed for the hierarchic formation of nanometer clusters in periodic structures of point defects under the action of an energy flux. The theoretical results obtained are compared with experimental results on the formation of nanometer pore lattices in metals under the action of particle beams and the laser-induced structural modification of semiconductor surfaces. Good agreement between the theory and experiment is obtained. Fiz. Tverd. Tela (St. Petersburg) 39, 2029–2035 (October 1997)     

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.     

Defect-induced instability of the surface layer involving static coupled Lamb and Rayleigh waves as a universal mechanism of the formation of an ensemble of nanodot nucleation centers

It is demonstrated that a stressed flat surface nanolayer saturated with mobile point defects exhibits a threshold (with respect to the defect concentration or mechanical stress) transition to a periodic spatially bent state with a simultaneous formation of the spatially periodic defect pile-ups at the extrema of the spontaneously emerging surface relief. In this case, the layer deformation corresponds to the displacements in a static bending Lamb wave and the deformation of the underlying elastic continuum corresponds to the displacements in the static Rayleigh wave. For the first time, we demonstrate that the analysis simultaneously involving the nonlocal character of the defect interaction with the lattice atoms and both (normal and lateral) defect-induced forces that cause the bending of the surface layer yields two maxima on the curve of the instability growth rate versus the period of the generated relief. This corresponds to the experimentally observed two scales of the surface relief modulation upon the laser and ion irradiation of semiconductors. Based on the results obtained, we propose a cooperative defect-deformational (DD) mechanism for the formation of an ensemble of the nanoparticle nucleation centers above the critical levels of the stress or the defect concentration. An approach to the calculation of a bimodal distribution function of the nanoparticle nucleation centers with respect to their size is adequately developed to the DD mechanism of nucleation which represents the distribution function in terms of the growth rate. The calculated results are compared with the experimental data for the molecular beam epitaxy of the nanodot ensemble and the pulsed laser nanostructuring of a solid surface.     

半导体光折变介质中光束传输的自囚禁及周期调制

推导了光束在半导体光折变介质的光子晶格中演化的二能级形式, 给出了光束在其中传播时二能级方程的经典正则形式. 解析计算出了经典正则方程的不动点并对其稳定性作了分析, 计算出了拓扑结构变化的临界值. 根据二能级方程的经典正则形式做出了空间相图, 进一步分析了半导体光折变介质中光束传输的自囚禁现象, 发现有两种形式的自囚禁: 1)能级中的布居数差和相对相位都在平衡点附近振动; 2)能级中的布居数差在平衡点附近振动, 而相对相位单调变化. 分别从高频、低频、中频三个方面研究了外加周期调制对自囚禁的影响, 发现在高频调制中发生自囚禁现象的相变参数能够被周期场非常有效的调制, 使得光束在半导体光折变介质中传输时, 在非线性效应影响较小时也能够发生自囚禁. 关键词： 半导体光折变介质 自囚禁 二能级形式 周期调制     

Model of periodic superstructure formation induced by mobile defects on a semiconductor surface

A model for forming a periodic superstructure on a semiconductor surface with mobile defects is proposed. It is shown that a self-organized system of such defects could lead to considerable modulation of incommensurate phase parameters in near-surface layers under the conditions of defect interaction with an incommensurate displacement wave. This type of defect system could induce a substantial energy decrease for the incommensurate phase and its additional stabilization on the one hand, and an increase in modulation amplitude that might be accompanied by a soliton-like nonsinusoidal incommensurate displacement wave on the other. The model allows us to explain the experimental results regarding the periodic superstructure discovered by G.V. Benemanskaya et al. on the surface of a GaN semiconductor. In particular, the model enables us to explain the nature of such superstructures based on the self-organization of mobile neutral triad-type defects, composed of negatively charged (3−) vacancies of the host lattice centers of Ga³⁺ and surface impurity ions of Cs⁺ and Ba²⁺, under conditions of triad interaction with an incommensurate displacement wave.     

Characteristic responses of a semiconductor gas sensor depending on the frequency of a periodic temperature change

A novel gas-sensing system based on a dynamic nonlinear response is reported to enhance the selectivity toward sample gases using a single detector. A periodic temperature change was applied to a semiconductor gas sensor and the resulting conductance of the sensor was evaluated by fast Fourier transformation (FFT). The dynamic nonlinear response to the sample gases was further characterized depending on the frequency of the temperature change. The characteristic sensor response under the application of a temperature change was theoretically simulated by considering the kinetics of gas molecules on the semiconductor surface.     

Laser-induced generation of surface periodic structures in media with nonlinear diffusion

A model of fast formation of high-contrast periodic structure appearing on a semiconductor surface under action of laser radiation is proposed. The process of growing a surface structure due to the interaction surface plasmon- polaritons excited on nonequilibrium electrons with incident laser radiation are considered in the framework of a medium with nonlinear diffusion of nonequilibrium carriers (defects). A resonance effect of superfast pico- and subpicosecond amplification of the plasmon-polariton structure generated on the surface, the realization of which can result in a high-contrast defect lattice.     

Theoretical modeling of the dynamics of a semiconductor laser subject to double-reflector optical feedback

We theoretically model the dynamics of semiconductor lasers subject to the double-reflector feedback. The proposed model is a new modification of the time-delay rate equations of semiconductor lasers under the optical feedback to account for this type of the double-reflector feedback. We examine the influence of adding the second reflector to dynamical states induced by the single-reflector feedback: periodic oscillations, period doubling, and chaos. Regimes of both short and long external cavities are considered. The present analyses are done using the bifurcation diagram, temporal trajectory, phase portrait, and fast Fourier transform of the laser intensity. We show that adding the second reflector attracts the periodic and perioddoubling oscillations, and chaos induced by the first reflector to a route-to-continuous-wave operation. During this operation, the periodic-oscillation frequency increases with strengthening the optical feedback. We show that the chaos induced by the double-reflector feedback is more irregular than that induced by the single-reflector feedback. The power spectrum of this chaos state does not reflect information on the geometry of the optical system, which then has potential for use in chaotic (secure) optical data encryption.     

Thermal broadband tunable Terahertz metamaterials

A thermally tunable metamaterial comprising a periodic array of metallic split-ring resonators with an embedded semiconductor has been proposed. The resonance frequencies of the metamaterial are demonstrated to be continuously tuned in the terahertz regime by increasing the temperature. The tunability is attributed to the temperature-dependent permittivity of the embedded semiconductor and well analyzed by an equivalent capacitor-inductor model. The proposed designs ensure broadband thermally tunable terahertz devices.     

Analysis of the stability of states of semiconductor superlattice in the presence of tilted magnetic field

A method to calculate the spectrum of the Lyapunov exponents for a periodic semiconductor nanostructure (superlattice) described in the framework of a semiclassical approach is proposed. The analysis of the stability of a stationary state in such a system is performed for autonomous dynamics and in the presence of a tilted magnetic field. The method of the Lyapunov exponents is used to study the effect of the tilted magnetic field on the stability of the stationary state and the characteristics of subterahertz oscillation regimes.     

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.     

Spiking in an activator-inhibitor model for elements with S-shaped negative differential conductivity

Self-organized spatio-temporal dynamics of electrical transport is described by a simple noncubic activator-inhibitor system derived for layered semiconductor structures. The model exhibits a novel mode of self-sustained oscillations due to current filaments switching on and off (spiking) which may be periodic or chaotic. Additionally, we obtain complex multifilamentary spatio-temporal patterns. As such phenomena have been observed in various devices exhibiting S-shaped negative differential conductivity, our model is suggested to describe a generic mechanism.     

Wavefront depinning in semiconductor superlattices due to discrete-mapping failure

We investigate the wavefronts depinning in current biased, infinitely long semiconductor superlattice systems by the method of discrete mapping and show that the wavefront depinning corresponds to the discrete mapping failure. For parameter values near the lower critical current in both discrete drift model （DD model） and discrete drift-diffusion model （DDD model）, the mapping failure is determined by the important mapping step from the bottom of branch to branch α. For the upper critical parameters in DDD model, the key mapping step is from branch γ to the top of the corresponding branch α and we may need several active wells to describe the wavefronts.     

Excitability of periodic and chaotic attractors in semiconductor lasers with optoelectronic feedback

The dynamics of a semiconductor laser with AC-coupled nonlinear optoelectronic feedback has been experimentally studied. A period doubling sequence of small periodic and chaotic attractors is observed, each of them displaying excitable features. This scenario is found also in a simplified physical model of the system, thus extending the concept of excitability, usually associated to fixed points, also to the case of higher-dimensional attractors.     

集成双波导半导体光放大器光开关实现波长转换的理论研究

建立了基于集成双波导半导体光放大器的光开关(ITG-SOA-Switch)的理论分析模型.与半导体光放大器(SOA)的特性相比较表明，由于ITG-SOA-Switch合并了多种物理效应，故其静态增益饱和曲线在饱和功率点附近具有大幅度陡峭下降的独特性质.理论分析和10 Gbit/s波长转换模拟结果显示，恰当地选择输入抽运光的功率范围，ITG-SOA-Switch波长转换器输出转换光的消光比特性较之输入抽运光会有显著的改善. 关键词： 波长转换 半导体光放大器 集成双波导半导体光放大器 光开关     

Anomalous increase of thermopower in epitaxial graphene

Thermoelectric effect in epitaxial graphene formed on the surface of a semiconductor is considered in the context of the Davydov model. The approach based on the Kubo formula for the conductivity and differential thermopower is used. It is shown that near the edges of the semiconductor bandgap, the thermopower of epitaxial graphene increases by more than four times as compared to the thermopower near the Dirac point. A possible explanation of this effect is given.     

Defect-deformational Kuramoto-Sivashinsky equation and formation of surface nano- and microstructures under the laser and ion-beam irradiation

Systematic numerical investigation of recently derived isotropic 2D defect-deformational (DD) Kuramoto-Sivashinsky (KS) equation that desribes the surface relief of the stressed nanolayer with nonequilibrium mobile point defects is carried out. It is demonstrated that the DDKS equation describes the formation of the surface DD structures of three main types: lamellar-like structures, and disordered and hexagonally ordered ensembles of nanoparticles. The generalization of the DDKS equation to the anisotropic diffusion of surface defects is performed and the numerical solution to the derived anisotropic DDKS equation is obtained to describe the formation of 1D surface DD ripples. The calculated results are compared with the experimental data on the formation of surface nano- and microstructures on solid surface under the laser and ion-beam irradiation.