Thin Slab Of Material Research Articles

Coherent control of birefringence and optical activity

We show that polarization effects due to anisotropy and chirality affecting a wave propagating through a thin slab of material can be controlled by another electromagnetic wave. No nonlinearity of the metamaterial slab is required and the control can be exercised at arbitrarily low intensities. In proof-of-principle experiments with anisotropic and chiral microwave metamaterials, we show that manifestations of linear and circular birefringence and dichroism can be modulated by the control wave from their maximum value to zero.

Multiscale modeling and simulation of organic solar cells

In this article, we continue our mathematical study of organic solar cells (OSCs) and propose a two-scale (micro- and macro-scale) model of heterojunction OSCs with interface geometries characterized by an arbitrarily complex morphology. The microscale model consists of a system of partial and ordinary differential equations in an heterogeneous domain, that provides a full description of excitation/transport phenomena occurring in the bulk regions and dissociation/recombination processes occurring in a thin material slab across the interface. The macroscale model is obtained by a micro-to-macro scale transition that consists of averaging the mass balance equations in the normal direction across the interface thickness, giving rise to nonlinear transmission conditions that are parametrized by the interfacial width. These conditions account in a lumped manner for the volumetric dissociation/recombination phenomena occurring in the thin slab and depend locally on the electric field magnitude and orientation. Using the macroscale model in two spatial dimensions, device structures with complex interface morphologies, for which existing data are available, are numerically investigated showing that, if the electric field orientation relative to the interface is taken into due account, the device performance is determined not only by the total interface length but also by its shape.

Tuning of on-metal UHF RFID inlay tags loaded with a thin magneto-dielectric slab

The input impedance of UHF radio frequency identification (RFID) inlay tags is analyzed when a thin material slab is placed between the antenna and a metal surface.The two-layer slab is made of a thin magneto-dielectric EmersonC View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26903

Characterizing Metasurfaces/Metafilms: The Connection Between Surface Susceptibilities and Effective Material Properties

A metafilm is a type of metasurface, a two-dimensional equivalent of a metamaterial, and is essentially a surface distribution of electrically small scatterers arranged in a judiciously chosen pattern. In previous work, we have shown that the most appropriate way to characterize a metafilm is by its effective electric and magnetic surface susceptibilities, and we discussed methods for retrieving these surface susceptibilities. Nevertheless, some researchers have continued to characterize metasurfaces in terms of bulk effective material properties. In this letter, we first clarify sign conventions (and nomenclature) used for surface susceptibilities and correct sign errors in previous publications. We then discuss the connection between the surface susceptibilities of a metafilm and effective material properties of a thin material slab. The two subtle but important aspects discussed here are: 1) the interpretation of a thin material slab by its surface susceptibilities, and 2) the equating of a metafilm (which has its own unique surface susceptibilities) to a thin material slab with effective material properties.

Diffraction-free beams in thin films

The propagation and transmission of Bessel beams through nano-layered structures has been discussed recently. Within this framework we recognize the formation of unguided diffraction-free waves with the spot size approaching and occasionally surpassing the limit of a wavelength when a Bessel beam of any order n is launched onto a thin material slab with grazing incidence. On the basis of the plane-wave representation of cylindrical waves, a simple model is introduced providing an exact description of the transverse pattern of this type of diffraction-suppressed localized wave. Potential applications in surface science are put forward for consideration.

Virtual circular array using material-embedded linear source distributions

Linear arrays of sources embedded inside a thin material slab are designed such that they radiate the same fields as a circular array of radiators in free space in a two-dimensional configuration. The design is based on the coordinate transformation approach, where a circular domain containing a distribution of line sources is transformed into a rectangular slab. The source distribution in the transformed space forms two closely-spaced linear arrays and the resulting material slab is matched along the boundary with free space. Full-wave simulations are performed for verification.

Transverse effects in a thin slab of material with local-field induced intrinsic optical bistability

We consider a thin slab of dense material exhibiting local-field induced intrinsic optical bistability irradiated by a transversely uniform optical field (holding beam). We study the transverse effects that can arise when local excitations are created by means of a narrow optical beam (writing beam). We show that whereas diffraction effects are negligible, diffusion effects make the excitation-domain walls to move inward or outward in the transverse direction, with a speed that depends on the holding-beam intensity and the diffusion coefficient. Conditions can be found, however, for which the wall movement is counterbalanced by the field transverse gradient so that stable narrow excitation domains can be created that remain after the writing beam has been switched off. Such domains can be read on the transverse profile of the output field and can be erased by means of another narrow beam. Some issues concerning potential applications in optical parallel information processing are discussed.

Diffraction of the surface H-polarized wave by an angular break of a thin dielectric slab

This paper deals with a recent advance in applications of the Sommerfeld-Malyuzhinets technique to the problem of diffraction of a surface wave by an angular break of a thin material slab. The solution is represented by Sommerfeld intergrals, which are then substituted into boundary conditions. The unknown spectral functions satisfy coupled Malyuzhinets functional equations. The latter are reduced to Fredholm second kind integral equations, which are solved numerically. The scattering diagram of the cylindrical wave arising from the edge of the structure is computed. Bibliography: 3 titles.

A convenient method for complex permittivity measurement of thin materials at microwave frequencies

A practical problem in the reflection method for measuring permittivity of thin materials is the difficulty in ensuring the sample is placed exactly at the waveguide flange. A small position offset of the dielectric slab will give rise to significant errors in calculating the permittivity. To circumvent this problem, a measurement method using a waveguide partially filled with a thin material slab has been developed. The material sample can be easily prepared and inserted into the guide through a longitudinal slot on the broad wall of the waveguide. Multiple material slabs can be measured rapidly because one does not have to disconnect the waveguide system for sample placement. The method is verified with measurement of Teflon, glass and FR4 fibreglass. The measured permittivity show good agreement with published data. Subsequently, the permittivity of a vegetation leaf was measured. The method presented in this paper is particularly useful in measuring the permittivity of a thin and narrow slab of natural materials such as a paddy leaf.

Evolution of maximal coherence and nonlinear mixing in an ionizing three-level medium

We have carried out a numerical investigation of the influence of photoionization, causing neutral atom depletion and accompanying electron impact dephasing, on a stimulated Raman adiabatic passage (STIRAP) scheme in a dense medium of three-level atoms. The nonlinear mixing from the large (maximal) coherence generated during the population transfer is calculated within the framework of a time-dependent model for a single atom with simplified propagation appropriate to a thin slab of material. For definiteness and consistency of the atomic parameters employed we have treated a scheme in atomic calcium. For generation of intense vacuum ultraviolet fields, laser intensities in the range from 100 ${\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ to 1 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ are required, and these are found to cause significant ionization of the atom. We find that at atomic densities $g{10}^{16}/{\mathrm{cm}}^{3}$ the effects of electron impact dephasing on the coherence evolution is very significant and will dominate over other dephasing mechanisms. In contrast at lower densities $\ensuremath{\sim}{10}^{14}/{\mathrm{cm}}^{3}$ (compatible with a sealed vapor cell) the conversion efficiency is relatively unaffected by electron impact dephasing. We conclude that in general photoionization depletion and electron impact dephasing will be significant in maximal coherence wave mixing and STIRAP population transfer schemes operated at high density.

Stable propagation of an ordered array of cracks during directional drying.

We study the appearance and evolution of an array of parallel cracks in a thin slab of material that is directionally dried, and show that the cracks penetrate the material uniformly if the drying front is sufficiently sharp. We also show that cracks have a tendency to become evenly spaced during the penetration. The typical distance between cracks is mainly governed by the typical distance of the pattern at the surface, and it is not modified during the penetration. Our results agree with recent experimental work, and can be extended to three dimensions to describe the properties of columnar polygonal patterns observed in some geological formations.

The Pencil Beam Problem for Screened Rutherford Scattering

We consider the problem of describing steady-state transport of a perpendicularly incident pencil beam of particles through a thin slab of material. The scattering is assumed to be described by the continuous slowing down approximation in energy and by the screened Rutherford formula in angle. For very small screening parameters, it is well known that the scalar flux, as a function of depth and radius, is described reasonably well by the classic Fermi-Eyges formula. However, realistic screening parameters, such as encountered in medical physics applications, are not small enough for this formula, which is Gaussian in radius, to be accurate. A correction to the spatial component of the Fermi-Eyges formula for screened Rutherford scattering is developed. This correction exhibits an algebraic, rather than exponential, falloff of the scalar flux with radius. Comparisons with benchmark Monte Carlo calculations confirm the inaccuracy of the scalar flux spatial distribution of the Fermi-Eyges formula for realistic screening parameters and demonstrate the good results obtained with the present formalism. Contact is made with earlier work by Molière.

Long-wavelength end-effect undulator radiation (Transition Undulator Radiation)

As first pointed out by K.-J. Kim (KIM K.-J.,Phys. Rev. Lett.,76 (1996) 1244) undulator radiation contains a broad-band component in the long-wavelength region. This radiation is due to the change in longitudinal velocity of an electron upon entering and leaving an undulator. The radiation pattern is a hollow cone, peaked in the forward direction, with an opening angle of approximately 1γ, with a spectrum covering a wide range, including the infra-red and the visible. The radiation is radially polarized, analogous to transition radiation, and exhibits interference effects between the entrance and exit ends of the undulator, similar to the interference effects observed for transition radiation from a thin slab of material. A straightforward application of formulas from Jackson (JACKSON J. D.,Classical Electrodynamics, 2nd edition (J.Wiley & Sons, N.Y.) 1975) results in a closed-form exact expression for the low-frequency limit of this novel radiation effect, Transition Undulator Radiation or TUR.

A Non-Gaussian Treatment of Radiation Pencil Beams

The problem of describing steady-state transport of a perpendicularly incident particle beam through a thin slab of material is considered. For a scattering kernel sufficiently peaked in momentum transfer to allow a Fokker-Planck description of the scattering process in both energy and angle, an approximate closed form solution to this problem was obtained almost 50 yr ago and is referred to as the Fermi-Eyges formula. It is shown that a Fermi-Eyges-like formula can be derived for a broader class of scattering kernels. This class consists of scattering described by the continuous slowing-down approximation (the Fokker-Planck description in energy), but not sufficiently forward peaked in angle to allow an angular Fokker-Planck representation. This generalized formula reduces to the classic Fermi-Eyges result for scattering operators with a valid Fokker-Planck limit and also describes problems that, while involving a forward-peaked scattering kernel, do not possess a Fokker-Planck description. A classic example of such a kernel is the Henyey-Greenstein kernel, and the Fermi-Eyges-like solution in this case exhibits more beam spreading than that predicted by the classic Fermi-Eyges formula. In particular, the scalar flux is non-Gaussian in the radial coordinate, as contrasted with the Gaussian Fermi-Eyges result.

Long-range surface modes supported by thin films.

A detailed analysis of the surface modes of a thin slab of material of dielectric constant ${\mathrm{\ensuremath{\epsilon}}}_{2}$ (=${\mathrm{\ensuremath{\epsilon}}}_{\mathit{r}2}$-i${\mathrm{\ensuremath{\epsilon}}}_{\mathit{i}2}$) surrounded symmetrically by dielectric media is presented. Results show that in the thin-film limit, as well as the well-known long-range surface plasmon for a thin metal layer and the TM guided mode for a thin dielectric, a long-range surface mode exists for almost any value of ${\mathrm{\ensuremath{\epsilon}}}_{2}$. This is even true if the imaginary part of ${\mathrm{\ensuremath{\epsilon}}}_{2}$, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{i}2}$, is much larger than the real part ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{r}2}$. We also find that a long-range surface mode may arise from the coupling between two surfaces which individually cannot support a surface mode. These are a pair of special coupled-surface modes which may exist below a certain critical film thickness and which have two separate propagation vectors each with the same field symmetry. It is also found that the inverse situation may pertain, that is for certain relative values of dielectric constants even though ordinary surface modes may exist, below a critical thickness the resulting coupled long-range mode no longer exists. The analysis has also been extended to practical situations with weakly absorbing surrounding media and to circumstances where the dielectric constants of the surrounding media are slightly different. Both of these effects modify the dispersion relations obtained for the simple case and introduce further limit thicknesses into the problem. Analytic formulas in the thin-film limit are presented for all the above situations and field distributions and energy flow (Poynting vector) profiles presented to illustrate as necessary the nature of the modes supported by these systems. Finally experimental results are presented which illustrate the rather sweeping conclusion that a long-range surface mode may exist on a thin film for almost all values of ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{r}2}$ and ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{i}2}$. This result paves the way for a range of optics experiments on absorbing structures.

Effects of selective pulses on the measurement of T2 and apparent diffusion in multiecho MRI.

The effects of using spatially selective and nonuniform radiofrequency pulses on the apparent T2 of a train of multiple echoes have been calculated by computer simulation. It is demonstrated that the use of selective pulses produces errors in estimates of T2 in imaging experiments because regions of the slice are effectively off resonance and therefore receive spin rotations less than in an ideal case. Components of the transverse magnetization that are incompletely rotated by 180 degrees refocusing pulses produce longitudinal components that are later returned to the transverse plane and add to subsequent echoes. These so-called ghost components influence the precise value of T2 estimated, but even when completely removed the resultant estimate is too short. The apparent T2 is influenced by the magnitude of the nutation angle inhomogeneity across the slice and paradoxically is not always more accurate with larger numbers of echoes. The reduction of apparent T2 caused by increasing the slice-selection gradient across a thin slab of material replicates the effects previously ascribed to diffusion and affords an alternative explanation for these without recourse to motion of the spins.

Diffraction and Microscopy Studies of the Structure of Grain Boundaries

It is now well established that the grain boundary between two crystals consists of a thin region at the interface which is at least several atom distances thick. As is well known, the structure and properties of this relatively thin slab of material is of considerable scientific and technological interest. This paper is concerned with the study of the structure of grain boundaries. A good starting point for this work, therefore, is to ask what are the important questions that need to be answered about the structure of the grain boundary region. These questions are: 1) Is the boundary struc ture periodic? If so, what is the unit cell symmetry and dimensions? 2) What patterns of atomic relaxation exist in the boundary? Can these relaxation patterns be described in terms of primary or secondary grain boundary dislocations? 3) What is the detailed atomic structure of the grain boundary?

Thermal diffusivity at incandescent temperatures by a modulated electron beam technique

Apparatus is described for the determination of the thermal diffusivity by Cowan's method of a thin slab of material heated to incandescence by an electron beam. The electron beam is amplitude modulated to vary sinusoidally with time and the phase difference between the resulting temperature fluctuations of the two faces of the slab yields the thermal diffusivity of the sample. Results obtained for the thermal diffusivity and hence the thermal conductivity of platinum, tantalum, molybdenum and tungsten are given. The Lorenz functions of all four metals were found to be constant over the temperature ranges studied. Thermal conductivity values determined for three types of graphite, indium oxide, uranium dioxide, a U-49 at.%C alloy and boron nitride are also given. Results obtained with the apparatus to date cover a range of thermal conductivities of from 0.02 to 1.1 w cm-1 degK-1 within various temperature ranges between 1200 and 3000°K.

Measurement of Conducted Flow of Heat in a Sea Ice Cover

IN any micro-meteorological study or heat economy survey, the measurement of the conducted flux of heat at, or below, the Earth's surface is important. The two methods normally used to achieve this either involve measuring the temperature gradient if the thermal conductivity can be accurately established, or insertion of a heat flux meter, the basis of which is the measurement of temperature gradient across a thin slab of material of known conductivity. The accuracy of heat flux meters is limited unless the effective conductivity of the device used matches that of the medium in which the flow of conducted heat is being measured. For example, in using a flux meter with square horizontal surfaces with a width to thickness ratio of 12 : 1, a 20 per cent mismatch in conductivities results in an approximately 5 per cent error in indication.

Pulse Method of Measuring Thermal Diffusivity at High Temperatures

Equations are derived giving the temperature distribution within a thin slab of material which has received a short pulse of energy on one surface, for the case in which energy loss at the surfaces (by radiation or convection) is not negligible. An analysis of these equations indicates that measurement of thermal diffusivity by the pulse method should be feasible even when losses are so large that the maximum temperature of the far face is only 10 or 20% of the no-loss value; this includes nearly all materials, and temperatures to 2500°K or higher.