Analysis of the linear stability of stationary solutions in a circular laser containing a saturable absorber “LSA” for inhomogeneous broadening

In this present paper, the optical bistability in a circular laser containing a saturable absorber LSA for inhomogeneous broadening is studied. A simple mathematical model was developed to describe the action of this type of laser in a circular resonator under two conditions, when the spontaneous emission is neglected or included in the cavity. For both of these two cases, the coefficient of saturation was considered to be equal or different to unity (ζ=1 or ζ≠1). The photons intensities Q_j were determined as function of the pumping rate σ₀ of the active medium and analyzed the linear stability of the stationary solutions obtained in a circular laser containing a saturable absorber “LSA” for inhomogeneous broadening.     

Thermal stabilization of anomalies in inhomogeneous conducting structures

We study the thermal mechanism of suppression of the anomalies in the nonlinear characteristics of inhomogeneous media. A generalized expression for the effective nonlinear conductivity is derived that allows for heat transfer from hot regions. We study the nature of the divergences in two-and three-dimensional inhomogeneous structures as depending on the local parameters and the microgeometry of the system. Finally, we show that in the critical region the effective nonlinear conductivity may be much higher than the conductivity of the components. Zh. éksp. Teor. Fiz. 115, 1818–1832 (May 1999)     

An Analog of the Kubo formula for conductivity of spatially inhomogeneous media in spatially inhomogeneous electric fields

The average densities of currents and charges induced by a weak electromagnetic field in spatially inhomogeneous systems at a finite temperature are calculated. The Kubo formula for the electrical conductivity tensor is generalized to spatially inhomogeneous systems and spatially inhomogeneous fields. The contributions containing electric fields and derivatives of the fields with respect to coordinates are separated. It is shown that semiconductor quantum wells, wires, and dots can be treated as spatially inhomogeneous systems.     

Measuring the electrical conductivity and the hall coefficient for semiconductor structures inhomogeneous with depth

The well-known four-point probe method of combined measurements of electrical conductivity and the Hall coefficient of semiconductor layers is developed further in the present work. Formulas are derived that allow the electrical conductivity and the Hall coefficient to be averaged over layers inhomogeneous with depth. Analytical expressions for calculating correction factors for this measurement method are obtained, and practical recommendations for their calculation on a computer are also given. Lipetsk State Pedagogical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 46–51, January, 2000.     

Electromagnetic probing of the underlying surface and of the upper ground layer for the purpose of their monitoring and diagnostics

A short review is given of theoretical work carried out under the author on the problem of the electromagnetic probing of the underlying surface and the upper ground layer, including regions containing local and extended natural and artificial inhomogeneities. This work mainly involves electromagnetic boundary problems for homogeneous and inhomogeneous media when they are excited by electromagnetic sources of various types. Considerable attention is devoted to approximate methods for solving these problems. In the case of low-frequency probe fields the solutions are obtained taking account of the weak magnetic properties of soil. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 62–75, August, 1998.     

New method of creation of a rearrangeable local Coulomb potential profile and its application for investigations of local conductivity of InAs nanowires

We present a new technique to create a reconfigurable Coulomb potential profile. The potential profile on the sample surface covered with residual polymethyl methacrylate (PMMA) layer as charge accumulation substance is performed with a low DC voltage applied to conductive probe tip of scanning microscope. To characterize the resulted Coulomb potential profile Kelvin probe technique is used. The effectiveness of this method is demonstrated by performing measurements of the local conductivity of InAs quantum wires. These investigations revealed an inhomogeneous conductivity of the wires and the formation of a potential barrier in the wire at the contact pad interface when the electronic system of the wire is depleted.     

Simulation of Electromagnetic Wave Logging Response in Deviated Wells Based on Vector Finite Element Method

The vector finite element method of tetrahedral elements is used to model 3D electromagnetic wave logging response. The tangential component of the vector field at the mesh edges is used as a degree of freedom to overcome the shortcomings of node-based finite element methods. The algorithm can simulate inhomogeneous media with arbitrary distribution of conductivity and magnetic permeability. The electromagnetic response of well logging tools are studied in dipping bed layers with the borehole and invasion included. In order to simulate realistic logging tools, we take the transmitter antennas consisting of circular wire loops instead of magnetic dipoles. We also investigate the apparent resistivity of inhomogeneous formation for different dip angles.     

Two Hard Spheres in a Spherical Pore: Exact Analytic Results in Two and Three Dimensions

The partition function and the one- and two-body distribution functions are evaluated for two hard spheres with different sizes constrained into a spherical pore. The equivalent problem for hard disks is addressed too. We establish a relation valid for any dimension between these partition functions, second virial coefficient for inhomogeneous systems in a spherical pore, and third virial coefficients for polydisperse hard spheres mixtures. Using the established relation we were able to evaluate the cluster integral b ₂(V) related with the second virial coefficient for the Hard Disc system into a circular pore. Finally, we analyse the behaviour of the obtained expressions near the maximum density.     

Critical fields and currents in a weakly nonlinear medium near the percolation threshold

For fields above a critical value the expansion of the conductivity in powers of the field ceases to be valid and the weak-nonlinearity approximation no longer works. The density behavior of the critical fields in strongly inhomogeneous media near the percolation threshold is found on the basis of two criteria—an average criterion and a local criterion. The parameter values of the medium for which crossover—a change of the critical behavior—occurs are determined. Similar calculations are performed for the critical currents. Zh. Tekh. Fiz. 68, 5–8 (June 1998)     

Spatially resolved twin domain distribution and lattice parameter variations in the near-surface region of SrTiO<Subscript>3</Subscript> single crystals

We investigated the twin domain distribution and lattice parameter variations associated with the displacive phase transition in SrTiO₃ by means of X-ray diffraction with high spatial resolution. By using 4.5-keV photons, the probed region is the first micrometer near the surface. We find a very inhomogeneous domain distribution, showing both regions with large monodomains and highly twinned regions, as well as large needle domains. Also, the lattice parameters in these different regions vary substantially.     

An Extension of Spherically Symmetric Self-Similar Inhomogeneous Cosmological Models

An inhomogeneous cosmological solution with partial similarity is derived by extending the self-similar solutions in spherically symmetric spacetime containing pressureless matter. To show their physical difference from the previous self-similar one, we consider inhomogeneous cosmological models with inner and outer homogeneous regions and an intermediate inhomogeneous region, and derive the relations between observational quantities in them. It is found as a result that the models have different z-dependence of the local density parameter.     

Effective dichroism in forward scattering by inhomogeneous birefringent medium

The Mueller matrix model for inhomogeneous medium characterized by simultaneous linear and circular birefringence for single scattering in the forward direction is derived. We find that the presence of inhomogeneity results in the occurrence of new polarization effects. In particular, forward scattering by inhomogeneous birefringent medium exhibits linear and circular dichroism. We show that in the case of weak depolarization (when Cloude's entropy is less than 0.5), the initial parameters for linear and circular birefringence can be obtained from deterministic Mueller matrix associated with the largest eigenvalue of Cloude's coherency matrix. Sample calculations are given for quartz.     

3D imaging with a single-sided sensor: an open tomograph

An open tomograph to image volume regions near the surface of large objects is described. The central achievement in getting such a tomograph to work is the design of a fast two-dimensional pure phase encoding imaging method to produce a cross-sectional image in the presence of highly inhomogeneous fields. The method takes advantage of the multi-echo acquisition in a Carr-Purcell-Meiboom-Gill (CPMG)-like sequence to significantly reduce the experimental time to obtain a 2D image or to spatially resolve relaxation times across the sensitive volume in a single imaging experiment. Depending on T(2) the imaging time can be reduced by a factor of up to two orders of magnitude compared to the one needed by the single-echo imaging technique. The complete echo train decay has been also used to produce T(2) contrast in the images and to spatially resolve the T(2) distribution of an inhomogeneous object, showing that variations of structural properties like the cross-link density of rubber samples can be distinguished by this method. The sequence has been implemented on a single-sided sensor equipped with an optimized magnet geometry and a suitable gradient coil system that provides two perpendicular pulsed gradient fields. The static magnetic field defines flat planes of constant frequency parallel to the surface of the scanner that can be selected by retuning the probe frequency to achieve slice selection into the object. Combining the slice selection obtained under the presence of the static gradient of the open magnet with the two perpendicular pulsed gradient fields, 3D spatial resolution is obtained.     

Measurements of terahertz electrical conductivity of intense laser-heated dense aluminum plasmas

We report the electrical conductivity of laser-produced warm dense aluminum plasmas measured using single-shot ultrafast terahertz (THz) frequency spectroscopy. In contrast with experiments performed at optical frequencies, measurements based upon THz probe reflectivity directly determine a quasi-dc electrical conductivity, and therefore the analysis does not require a free-electron Drude model based extrapolation to recover the near zero frequency conductivity. In fact, our experimental results indicate that the Drude model breaks down for warm (>0.6 eV), moderate-dense (<1.6 g/cm(3)) aluminum at THz frequencies. A calculation of THz reflectivity over a non-Fresnel boundary in dense plasmas is also presented.     

Absorption in the fractional quantum Hall regime: trion dichroism and spin polarization

We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0相似文献   

Transport at surface nanostructures measured by four-tip STM

For in situ measurements of local electrical conductivity of well-defined crystal surfaces in ultrahigh vacuum, we have developed two kinds of microscopic four-point probe methods. One is a ‘four-tip STM prober’, in which independently driven four tips of scanning tunneling microscope (STM) are used for four-point probe conductivity measurements. The probe spacing can be changed from 500 nm to 1 mm. The other one is monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several μm. These probes were installed in scanning-electron-microscopy/electron-diffraction chambers, in which the structures of sample surfaces and probe positions were in situ observed. The probes can be positioned precisely on aimed areas on the sample with aid of piezo-actuators. With these machines, the surface sensitivity in conductivity measurements has been greatly enhanced compared with macroscopic four-point probe method. Then the conduction through the topmost atomic layers (surface-state conductivity) and influence of atomic steps upon conductivity could be directly measured. The STM prober is mainly described here.     

Contribution to the theory of “incompressible” regions in an inhomogeneous magnetized 2D electronic system

A modification of the theory of “incompressible” regions in an ideal spinless inhomogeneous magnetized 2D electronic system near points on the electron density profile n(x) with an integer filling factor is proposed. Such regions leads to the appearance of a finite capacitance between the parts of the 2D system that are separated by an incompressible channel, so that capacitive methods can be used to investigate such a system. The Corbino configuration is especially convenient for these purposes. The parameters of the “incompressible” channel in a Corbino disk with a spatially inhomogeneous 2D electronic system in the presence of an individual point, near the channel, on the electron density profile with an integer magnetic filling factor are determined. The magnetocapacitance between the edges of the Corbino disk separated by an incompressible interlayer is found for cases of practical interest. It is shown that this magnetocapacitance contains direct information about the width of the integer strip. Fiz. Tverd. Tela (St. Petersburg) 41, 1103–1109 (June 1999)     

金属圆柱腔体中使用非均一背景增强微波断层成像

针对基于圆柱金属腔体的微波断层成像系统,提出了一种利用非均一背景增强系统获取目标信息能力的方法.该方法通过在腔体内放置已知物体构成非均一背景,这样不但能利用背景的先验信息,而且可以增加等效辐射源对目标进行探测.首先,利用矩量法计算圆柱金属腔体内非均一背景的格林函数和离散积分算子,并对离散积分算子进行奇异值谱和条件数分析,在理论上证明该方法的可行性;然后,利用基于有限元的对比源逆成像法对均一背景、有耗非均一背景和无耗非均一背景三种情况进行仿真研究;最后对仿真结果进行了误差分析和比较.仿真结果表明,该方法可以提高反演收敛速度和结果准确度,有耗非均一背景略优于无耗非均一背景.该方法可以在不改变硬件系统和算法的情况下得到更准确的反演结果,可应用于医学成像与工业无损探测.     

Signal dynamics of the photothermal response in three-level media

In this paper, we consider the process of optical recording and decay of transient spatial structures in a system with a nonlinear response caused by incoherent saturation of three-level centers. The main attention is devoted to the thermal mechanism of phase recording in media with a temperature-independent heat conductivity. The relaxation of an inhomogeneous temperature field and the diffraction of a probe beam from the transient structure are studied. The designed theoretical model satisfactorily describes the obtained experimental results and allows the evolution of a temperature field to be viewed.