Magnetooptical images of inhomogeneous magnetic fields in metallic films with in-plane anisotropy

The magnetooptical (MO) images of the inhomogeneous field created by permanent magnets in magnetic metallic films with in-plane anisotropy are experimentally studied. The MO images recorded using the longitudinal Kerr effect are the superposition of two pictures, namely, a polar-sensitivity MO image and a longitudinal-sensitivity MO image. An analysis of these images after separation shows that the polar-sensitivity MO image reflects the distribution of the inhomogeneous field component that is normal to the surface of an indicator film in an analog manner. The longitudinal-sensitivity MO image reflects the angular distribution of the in-plane component of a stray field in an analog manner. The coincidence of the experimental and corresponding simulated MO images makes it possible to interpret the experimental images. In particular, it is shown that the specific features detected in the topological characteristics of the inhomogeneous field correspond to experimental singular points. Hidden magnetic images (magnetic bar codes) are shown to be visualized with metallic CoFe films. As an example, the stray field of a magnetic system made of cylindrical magnets is mapped.     

Imaging of percolative conduction paths and their breakdown in phase-separated (La1-yPry)0.7Ca0.3MnO3 with y=0.7

Local magnetization and current distribution in (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) crystals are studied by a magneto-optical (MO) imaging technique. MO images below 120 K visualize inhomogeneous magnetization and conduction paths that manifest the percolative conduction originated from the mesoscopic phase separation into ferromagnetic metals and antiferromagnetic insulators. Application of large amounts of current switches the current distribution from inhomogeneous to homogeneous concomitantly with a steep increase in resistivity. These phenomena are discussed in view of current induced collapse of the phase separation through a local heating.     

Orientational imaging of single molecules by annular illumination

The absorption dipole orientation of single fluorescent molecules is determined by mapping the spatial distribution of the squared electric field components in a high-numerical-aperture laser focus. Annular illumination geometry and the vicinity of a plane dielectric/air interface strongly enhance the longitudinal field component and the transverse fields perpendicular to the polarization direction. As a result, all three excitation field components in the focus are of comparable magnitude. The scheme holds promise to monitor rotational diffusion of single molecules in complex environments.     

Dipole and quadrupole surface plasmon resonance contributions in formation of near-field images of a gold nanosphere

Multipolar plasmon optical excitations at spherical gold nanoparticles and their manifestations in the particle images formatted in the particle surface proximity are studied. The multipolar plasmon size characteristic: plasmon resonance frequencies and plasmon damping rates were obtained within rigorous size dependent modelling. The realistic, frequency dependent dielectric function of a metal was used. The distribution of light intensity and of electric field radial component at the flat square scanning plane scattered by a gold sphere of radius 95 nm was acquired. The images resulted from the spatial distribution of the full mean Poynting vector including near-field radial components of the scattered electromagnetic field. Monochromatic images at frequencies close to and equal to the plasmon dipole and quadrupole resonance frequencies are discussed. The changes in images and radial components of the scattered electromagnetic field distribution at the scanning plane moved away from the particle surface from near-field to far-field region are discussed.     

Localized spin-wave resonance modes of ferromagnetic microstrips in the field of a magnetic probe

Some results of the micromagnetic modeling of forced magnetization oscillations in planar microstrips of NiFe with easy plane anisotropy and Co/Pt with perpendicular easy axis anisotropy in the field of a magnetic spherical probe are considered. It has been shown that the probe field provokes the appearance of a hedgehog–antivortex coupling state in the NiFe strips, due to its lateral components and a skyrmion magnetization state in the Co/Pt layer. These effects destroy spatial magnetization oscillations in the microstrips and lead to the appearance of additional resonances in the spectrum of oscillations corresponding to the modes localized in the probe field.     

Near-field characteristics of highly non-paraxial subwavelength optical fields with hybrid states of polarization

The vectorial structure of an optical field with hybrid states of polarization(So P) in the near-field is studied by using the angular spectrum method of an electromagnetic beam. Physical images of the longitudinal components of evanescent waves are illustrated and compared with those of the transverse components from the vectorial structure. Our results indicate that the relative weight integrated over the transverse plane of the evanescent wave depends strongly on the number of the polarization topological charges. The shapes of the intensity profiles of the longitudinal components are different from those of the transverse components, and it can be manipulated by changing the initial So P of the field cross-section. The longitudinal component of evanescent wave dominates the near-field region. In addition, it also leads to three-dimensional shape variations of the optical field and the optical spin angular momentum flux density distributions.     

Transverse magnetization transfer under planar mixing conditions in spin systems consisting of three coupled spins 1/2

Polarization transfer under planar mixing conditions is a widely used tool in modern NMR-experiments. In the case of two coupled spins 1/2 or a chain of three or more spins 1/2 with only nearest neighbor couplings, it is only possible to transfer a single magnetization component (longitudinal magnetization in the principle axis system of the planar coupling tensors). However, if all couplings in a three-spin system are non-zero, it turns out that all magnetization components can be efficiently transferred even under strictly planar mixing conditions. In this article a detailed theoretical analysis is presented based on analytical transverse coherence transfer functions and on the underlying commutator algebra. In addition, transverse magnetization transfer is demonstrated experimentally. The results show that in highly coupled spin systems, as for example in the case of partially aligned samples with many residual dipolar couplings, special care has to be taken to avoid phase distortions if planar mixing steps are used.     

Adaptive ultrafast nano-optics in a tight focus

We theoretically demonstrate the control of electromagnetic field properties on a sub-diffraction length scale, by polarization shaping of tightly focused femtosecond laser pulses. The field distribution in a tight focus is represented as a superposition of plane waves. The near-field of a model nanostructure is then obtained as a sum of the near-field distributions induced by the planar waves components. A self-consistent solution of Maxwell’s equations in the frequency domain yields the near-field distributions for planar wave illumination. Adaptive optimization of the incident polarization pulse shape using an evolutionary algorithm allows controlling of a number of observables, such as local nonlinear flux, simultaneous spatial and temporal control of the intensity evolution, and control of the local spectrum. The tight focusing reduces the controllability of the flux distribution in comparison to plane wave illumination. However, it is still possible to control the spatial and temporal field evolution for particular locations in the vicinity of the nanostructure. PACS 42.65.Re; 78.47.+p; 78.67.-n; 82.53.-k     

Measurement of micro weld joint position based on magneto-optical imaging

In a laser butt joint welding process,it is required that the laser beam focus should be controlled to follow the weld joint path accurately.Small focus wandering off the weld joint may result in insufficient penetration or unacceptable welds.Recognition of joint position offset,which describes the deviation between the laser beam focus and the weld joint,is important for adjusting the laser beam focus and obtaining high quality welds.A new method based on the magneto-optical(MO)imaging is applied to measure the micro weld joint whose gap is less than 0.2 mm.The weldments are excited by an external magnetic field,and an MO sensor based on principle of Faraday magneto effect is used to capture the weld joint images.A sequence of MO images which are tested under different magnetic field intensities and different weld joint widths are acquired.By analyzing the MO image characteristics and extracting the weld joint features,the influence of magnetic field intensity and weld joint width on the MO images and detection of weld joint position is observed and summarized.     

Features of Radiation of Electromagnetic Waves from a Thin Electric Antenna in a Planar Waveguide with Inhomogeneous Filling

We consider the problem of radiation of electromagnetic waves from a thin electric antenna located in a planar waveguide with perfectly conducting walls filled with an inhomogeneous dielectric. Cases where the antenna is fed by a time-harmonic e.m.f. and a bell-shaped impulse signal are analyzed. The current distribution along the antenna, its input admittance, the longitudinal component of the electric field, and the total complex power of the antenna are calculated numerically. We compare the antenna characteristics for waveguides with homogeneous and inhomogeneous filling.     

Synthetic T1-weighted brain image generation with incorporated coil intensity correction using DESPOT1

The increased use of phased-array and surface coils in magnetic resonance imaging, the push toward increased field strength and the need for standardized imaging across multiple sites during clinical trials have resulted in the need for methods that can ensure consistency of intensity both within the image and across multiple subjects/sites. Here, we describe a means of addressing these concerns through an extension of the rapid T(1) mapping technique - driven equilibrium single-pulse observation of T(1). The effectiveness of the proposed approach was evaluated using human brain T(1) maps acquired at 1.5 T with a multichannel phased-array coil. Corrected "synthetic" T(1)-weighted images were reconstructed by substituting the T(1) values back into the governing signal intensity equation while assuming a constant value for the equilibrium magnetization. To demonstrate signal normalization across a longitudinal study, we calculated synthetic T(1)-weighted images from data acquired from the same healthy subject at four different time points. Signal intensity profiles between the acquired and synthetic images were compared to determine the improvements with our proposed approach. Following correction, the images demonstrate obvious qualitative improvement with increased signal uniformity across the image. Near-perfect signal normalization was also observed across the longitudinal study, allowing direct comparison between the images. In addition, we observe an increase in contrast-to-noise ratio (compared with regular T(1)-weighted images) for synthetic images created, assuming uniform proton density throughout the volume. The proposed approach permits rapid correction for signal intensity inhomogeneity without significantly lengthening exam time or reducing image signal-to-noise ratio. This technique also provides a robust method for signal normalization, which is useful in multicenter longitudinal MR studies of disease progression, and allows the user to reconstruct T(1)-weighted images with arbitrary T(1) weighting.     

理想浅海波导中声场奇异点与声源深度的关系

为了有效利用声场奇异点蕴含的声源参数信息,研究了在理想浅海波导中,远场不同邻阶模态组的声场奇异点与声源深度之间的关系。推导计算了典型浅海声源声场的邻阶模态组奇异点位置,并通过仿真对奇异点的分布进行分析,结果显示邻阶模态组的阶数和阶差越大,奇异点分布越复杂。进一步研究发现,邻阶模态组第一组奇异点的深度和声源深度之间存在联系,并且基于奇异点与声压场的对应关系,在获得准确模态分布的前提下,可以通过两个邻阶模态组的第一组奇异点深度逆运算获得对应声源深度信息,也可以通过第一组奇异点深度反演获得声源深度信息。该文为获取浅海声源深度提供了思路。     

Inhomogeneous Double Optical Gating of High-Intensity Isolated Attosecond Pulse Generation in Crossed Metal Nanostructures

We propose and investigate an effective method for obtaining high-energy and high-intensity isolated attosecond pulses (IAPs) using the inhomogeneous double optical gating (DOG) technology in specifically designed metal nanostructures. First, using the homogeneous mid-infrared DOG technology modulated by a linearly near-infrared field, we obtain a harmonic yield of 2.5 orders of magnitudes higher than that from the single polarization gating (PG) technology. Further, introducing the crossed metal nanostructures along the driven and gating components, we can extend not only the harmonic cutoff but also enhance the harmonic yield attributed to the plasmonic field enhancement near the vicinity of metal nanostructures. As a result, we find a single harmonic plateau with smaller modulations. The supercontinuum is not very sensitive to the pulse duration of the near-infrared field, and the harmonic yields can be further enhanced with increase in the pulse intensity of the near-infrared field, showing a 108 eV supercontinuum with an intensity enhancement of 4 orders of magnitudes. Finally, by superposing the selected harmonics from the inhomogeneous DOG scheme, we obtain a 33 as SAP with an intensity increase of 4 orders of magnitudes.     

Distribution of the longitudinal component of the magnetic field of prominences from radioastronomical measurements

We present the results of prominence observations at RT-22 CrAO in April, May, and September 1993 at 13.5 mm (intensity and circular polarization) and at 8.2 mm (intensity) wavelengths carried out by circular scanning using the combined scalar horn. On the assumption of thermal cyclotron radiation we have measured the values of the longitudinal component of the magnetic field for 18 prominences. The histogram of the distribution of the longitudinal component indicates the presence of maxima in the vicinity of 8, 15, and 25– 30 G. Our results are compared with the distributions obtained from optical measurements.     

熔石英亚表面缺陷附近光强分布的数值模拟

 熔石英亚表面缺陷对光场的调制是导致激光辐照场破坏的主要因素。采用有限元方法对熔石英亚表面缺陷（平面和锥形划痕）周围的光强分布进行了数值模拟。结果表明：划痕形状、几何尺寸、方位角、光的入射角等是影响划痕周围光强分布的主要因素;前表面划痕对光强的增强效果比后表面弱;在理想形状的划痕截面和表面同时发生内全反射时,平面划痕周围的光强增强效果明显。锥形划痕周围的光强分布为正确解释交叉划痕的夹角平分线附近的损伤提供了理论依据。     

Modulated magnetic structure of weak rhombohedral ferromagnets α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga and FeBO<Subscript>3</Subscript>:Mg

The effect of diamagnetic impurities on the stability of the homogeneous magnetic state of rhombohedral antiferromagnets with weak ferromagnetism (α-Fe₂O₃:Ga and FeBO₃:Mg) is studied experimentally. It is shown that the application of an external magnetic field in the basal plane in the crystals under study in a certain temperature range induces a magnetic superstructure along the hard magnetization axis, which can be presented in the form of a ripplon phase with the azimuth of the local ferromagnetism vector oscillating about the direction of the field. The preferred orientation of the discovered modulated structures relative to crystallographic directions in the basal plane of α-Fe₂O₃:Ga and FeBO₃:Mg is studied, and the dependence of the spatial period of the superstructure on the applied magnetic field and temperature is analyzed. The magnetic-field-induced transition of the studied crystals from a homogeneous to an inhomogeneous magnetic state is described phenomenologically on the basis of the thermodynamic potential with gradient terms. In the discussion of physical reasons for magnetic order parameter modulation in weak ferromagnetic doped with diamagnetic ions, preference is given to the mechanism associated with the emergence of uniaxial magnetic centers with a random distribution of azimuths of easy axes in the basal plane of the crystal in the vicinity of impurities. A model describing the formation of a modulated magnetic state in α-Fe₂O₃:Ga and FeBO₃:Mg is proposed, according to which the competition between magnetoanisotropic and Zeeman interactions in the inhomogeneous magnetic phase of these crystals leads to periodic deviations in the direction of the local ferromagnetism vector from the direction of magnetization.     

纵向运动双散射散斑场的动态特性研究

研究了慢纵向运动双散射散斑场的统计性质,得出纵向运动双散射散斑振幅自相关函数、强度涨落自相关函数,并由此得出,夫朗和费衍射区内的接收平在上任意点上的自相关函数是时间的非平稳随机过程,而在光轴附近小区域内,强度涨落自相关函数是时间的平稳随机过程,相关时间与散射体纵向运动速度或反比,理论和实验结果相一致。     

Magnetooptical diffraction from a strip domain structure in the case of oblique incidence of light

The distribution of the magnetization component longitudinal with respect to the beam propagation is shown to be trapezoidal in shape in the case of oblique incidence of light on a strip domain structure with vertical domain walls. Features of intensity and polarization characteristics of the diffracted light field are studied with reflections of incident and diffracted beams from the entrance and the exit surfaces of the film taken into account.     

Magnetization reversal of ferromagnetic nanoparticles under inhomogeneous magnetic field

We investigated remagnetization processes in ferromagnetic nanoparticles under inhomogeneous magnetic field induced by the tip of magnetic force microscope (MFM) in both theoretical and empirical ways. Systematic MFM observations were carried out on arrays of submicron-sized elliptical ferromagnetic particles of Co and FeCr with different sizes and periods. It clearly reveals the distribution of remanent magnetization and processes of local remagnetization of individual ferromagnetic particles. Modeling of remagnetization processes in ferromagnetic nanoparticles under magnetic field induced by MFM probe was performed on the base of Landau–Lifshitz–Gilbert equation for magnetization. MFM-induced inhomogeneous magnetic field is very effective to control the magnetic state of individual ferromagnetic nanoparticles as well as to create different distribution of magnetic field in array of ferromagnetic nanoparticles.     

Remote ultrasonic defectoscopy of sound radiating objects through the air

A method of obtaining ultrasonic images of objects radiating ultrasonic acoustic waves in the air is proposed. Object images are retrieved by spatially matched filtering of the amplitude-phase distribution of the ultrasonic field measured in a planar rectangular region at a distance from the radiating object under investigation. The field is measured at different frequencies in a wide band, which allows one to reduce the level of noise and artifacts in the retrieved images by processing in a wide frequency band.