Showing papers on "Total external reflection published in 1992"

X‐ray scattering from interfacial roughness in multilayer structures

Abstract: A quantitative theory of the nonspecular scattering of x rays from multilayer structures having rough interfaces is presented. The results are valid for arbitrary polarization and angles of incidence (measured from the normal) less than the critical angle for total external reflection. A structural model is adopted wherein each interface is assumed to be described by a surface having statistically random roughness with a well‐behaved power spectrum. In addition, the model accounts for arbitrary correlation of the roughness between different interfaces. Calculations are presented for a variety of roughness configurations to investigate the dependence of the nonspecular scattering on the fundamental structural parameters. In particular, it is shown that the scattering from correlated roughness exhibits characteristic resonance behavior (quasi‐Bragg diffraction).

Fourier Analysis of Interference Structure in X-Ray Specular Reflection from Thin Films

Abstract: Interference oscillation observed in X-ray total external reflection from thin films was analyzed by the Fourier transform algorithm. The peak position in Fourier space was in good agreement with the layer thickness, and was determined independently from the surface/interface roughness. The principle of the present technique and its application to SiO2/Si thin films are shown. The advantages of the experiments using tunable synchrotron X-rays are also discussed.

X-Ray Diffraction from Laterally Structured Surfaces: Total External Reflection and Grating Truncation Rods

Abstract: A high-resolution study of x-ray scattering from a laterally structured surface is reported. Total external reflection displays a distinct dip in the reflectivity for incident angles < c (critical angle) and around a Bragg peak a system of truncation rods is found. Generalized Fresnel equations in conjunction with a kinematical theory provide a quantitative explanation of both findings. The method is well suited for characterising modulations on a mesoscopic length scale.

X-ray space modulation by diffraction on an ultrasonic superlattice

Abstract: X-ray diffraction on the surface of the YX cut of a LiNbO 3 crystal modulated by the surface acoustic wave was investigated under the total external reflection conditions. It is shown that X-ray diffraction is determined by the amplitude and by the period of the surface acoustic wave. The experimental and theoretical studies of X-ray diffraction have revealed the possibility of frabricating X-ray optical elements with space-time modulation of the diffracted radiation founded on the changes of the wavelength of the surface acoustic wave. These experiments have been made at CNRS (Grenoble, France) using a rotating copper anode.

Determination of residual stress in Cr‐implanted Al2O3 by glancing angle x‐ray diffraction

Abstract: We report x‐ray diffraction measurements of residual stress in sapphire crystals implanted with Cr+ ions. Stress is determined by measuring both in‐plane and out‐of‐plane lattice constants. Bragg peak positions are measured to determine average stress, while peak widths are measured to determine its variation. Using angles of incidence close to the critical angle for total external reflection of x rays, we compare measurements confined to within ∼2.5 nm of the surface and measurements over the ∼80 nm thickness of the implanted region. These x‐ray residual stress determinations are consistent with those based on indentation crack length, but were less by a factor of 10 than reports based on cantilever bending.

Effect of Index of Refraction on Radiation Characteristics in a Heated Absorbing, Emitting, and Scattering Layer

Abstract: The index of refraction can considerably influence the temperature distribution and radiative heat flow in semitransparent materials such as some ceramics. For external radiant heating, the refractive index influences the amount of energy transmitted into the interior of the material. Emission within a material depends on the square of its refractive index, and hence this emission can be many times that for a biackbody radiating into a vacuum. Since radiation exiting through an interface into a vacuum cannot exceed that of a blackbody, there is extensive reflection at the internal surface of an interface, mostly by total internal reflection. This redistributes energy within the layer and tends to make its temperature distribution more uniform. The purpose of the present analysis is to show that, for radiative equilibrium in a gray layer with diffuse interfaces, the temperature distribution and radiative heat flux for any index of refraction can be obtained very simply from the results for an index of refraction of unity. For the situation studied here, the layer is subjected to external radiative heating incident on each of its surfaces. The material emits, absorbs, and isotropically scatters radiation. For simplicity the index of refraction is unity in the medium surrounding the layer. The surfaces of the layer are assumed diffuse. This is probably a reasonable approximation for a ceramic layer that has not been polished. When transmitted radiation or radiation emitted from the interior reaches the inner surface of an interface, the radiation is diffused and some of it thereby placed into angular directions for which there is total internal reflection. This provides a trapping effect for retaining energy within the layer and tends to equalize its temperature distribution. An analysis of temperature distributions in absorbing-emitting layers, including index of refraction effects, was developed by Gardon (1958) to predict cooling and heat treating of glass plates. The interfaces were optically smooth; the resulting specular reflections were computed from the Fresnel reflection laws. This provides a somewhat different behavior than for diffuse interfaces. A similar application was for heating that occurs in a window of a re-entry vehicle (Fowle et al., 1969). A number of recent papers (Rokhsaz and Dougherty, 1989; Ping and Lallemand, 1989; Crosbie and Shieh, 1990) further examined the effects of Fresnel boundary reflections and nonunity refractive index. Other examples of analyses of both steady and transient heat transfer to single or multiple plane layers (Amlin and Korpela, 1979; Tarshis et al., 1969) have used diffuse assumptions at the interfaces as in the present study

Multiple Reflection From Isotropic Chiral Media and the Enhancement of Chiral Asymmetry

Abstract: Multiple reflection of light at the interface of an isotropic chiral medium (either transparent or absorbing) and a transparent achiral dielectric is studied as a function of angle of incidence and number of reflections to ascertain whether there is an enhancement (with respect to single-pass reflection) of (i) the difference in reflected left and right circularly polarized waves, and (ii) the ellipticity of reflected linearly polarized waves. It is shown that for ordinary reflection, where the mean refractive index n of the chiral medium is greate than the refractive index no of the incident medium, multiple reflection does not lead to a significant enhancement. However, in the vicinity of total reflection (no > n) the chiral asymmetries resulting from N reflections are significantly greater than those for one reflection; at critical angle the enhancement is approximately by a factor N.

Protective device for selectively reflecting high-intensity light over a broad spectral bandwidth

Abstract: A protective device designed to protect eyes and sensitive photodetectors from high-intensity light over a broad spectral bandwidth while permitting low-intensity and ambient light to be transmitted thereto. In a first embodiment, the device comprises a stack of nonlinear materials having the same linear index of refraction but alternating between a relatively high nonlinear index of refraction and a relatively low nonlinear index of refraction. The respective optical thicknesses of the nonlinear materials are unordered. In a second embodiment, the device comprises a stack of nonlinear materials having the same linear index of refraction and the same optical thickness of a one-half wavelength but unordered nonlinear indices of refraction. In a third embodiment, the device comprises a stack of nonlinear materials having linear indices of refraction alternating between a relatively high linear index of refraction and a relatively low index of refraction and nonlinear indices of refraction which are unordered. The respective optical thicknesses of the nonlinear materials are designed so that the materials are optimally transmissive in the linear domain. In a fourth embodiment, the device comprises a stack of nonlinear materials having the same linear index of refraction but unordered optical thicknesses and nonlinear indices of refraction.

The vector refraction law for shock waves

Abstract: The law is formulated in vector form and is shown to be a powerful principle for studying the refraction of shock waves. A variety of criteria for the onset of irregular refraction are discussed. The refraction index matrix is defined and it is shown that it arises naturally from the law. A projection matrix is also defined and it is found to be useful for operating on the vector wave impedance. It is expected that the methods described here will be useful for the numerical solution of problems in the refraction of shocks by materials with continuous changes in properties. The refraction law is violated in fast-slow refraction by the reflected wave over-running the incident shock to produce an irregular refraction which is either the anomalous type or the Mach-reflection-refraction type. For slow-fast refraction the law is violated by the transmitted wave becoming a precursor and also over-running the incident shock. The precursor may either be a shock or an evanescent compression wave band.

X-Ray Standing Wave Studies of the Liquid/Solid Interface and Ultrathin Organic Films

Abstract: X-ray standing waves generated above a mirror surface during total external reflection have been used to locate a heavy atom layer which was embedded hundreds of angstroms above the mirror surface in a Langmuir-Blodgett multilayer. This same method has also been used to map out the ion distribution in the diffuse double layer that forms at the electrolyte / charged surface interface.

An in situ x-ray diffraction method for the structure of amorphous thin films using shallow angles of incidence

Abstract: A technique for the structural characterization of thin amorphous films employing synchrotron radiation parallel beam x‐ray optics at grazing angles of incidence is detailed. At incident angles near to the critical angle for total external reflection, sampling of specimens may be achieved via the evanescent mode. The parallel beam geometry allows the use of a technique in which a 2θ detector, incorporating a parallel plate collimator, scans diffraction data for a given incident angle. For a specified wavelength, the incident angle chosen will determine the penetration of the radiation into the sample (∼10–1000 A). The data must be corrected for significant peak shifting resulting from x‐ray refraction, as well as for the effects associated with conventional θ:2θ scans. Preliminary data resulting from the first application of this technique to amorphous hydrogenated silicon:carbon thin films, deposited onto crystalline silicon substrates, will be presented and discussed. Conventional θ:2θ powder diffractio...

SM—Series Mössbauer spectrometer for total external reflection SM 1101ter

Abstract: Design, operation and capabilities of the SM 1101TER Mossbauer spectrometer for total external reflection investigations are described. The first experimental results are presented.

Differential reflection of circularly polarized light from a naturally optically active medium

Abstract: The difference in specular reflection of left and right circularly polarized light from a naturally optically active sample (an absorbing chiral liquid) was observed and measured as a function of wavenumber under conditions of double-pass total reflection. The imaginary part of the refractive index reached a maximum value on the order of 10-4 (at 21,000 cm-1); the real and imaginary parts of the complex gyrotropic parameter were on the order of 10-8 and 5 X 10-7, respectively. In accord with recent predictions, the observed differential circular reflection exceeded the intrinsic chiral parameters by some three orders of magnitude. This is the first observation of natural molecular optical activity by coherent light scattering.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

X-Ray Optics of Arrays of Reflective Surfaces

Abstract: We present a new way of focusing X-rays by capillary arrays, using total external reflection at grazing angles from the interior surfaces of hollow channels1–4 . Thin Bragg-difraction curved crystals can also be used in a transmissive mode to focus X-rays5 and the crystal netplanes can be considered as arrays of reflective surfaces.

MBE grown GaAs on GaAs (001): UHV X-ray diffraction measurements

Abstract: MBE grown gallium arsenide crystals with (001) orientation were investigated with a three-crystal X-ray diffractometer under UHV conditions. In the region of total external reflection (Q<0.3AA-1) no Kiessig fringes occur and hence no electron density difference between the substrate and the MBE layer exists. In the tails of the 004 Bragg reflection, modulations are observed. They are ascribed to a phase shift at the substrate-layer interface originating from a misfit parallel to the surface normal or a thin intermediate layer. The method can be applied to other thin film systems like oxidized or buried layers.

Calculation of surface x‐ray diffraction patterns in total external reflection conditions

Abstract: The high brilliance of the new generation of synchrotron radiation sources improves the grazing incidence x‐ray diffraction techniques and leads to great advances in the determination of crystal surface structures. The evaluation of the diffraction pattern intensity can be obtained in the distorted‐wave approximation introduced by Vineyard. In conditions of total reflection, typical penetration depths are about 50 A and only the atoms in the near‐surface layers interact with the electric field and produce the diffraction pattern. This model is the ground for a simulation code developed in order to calculate the diffracted peaks intensity of reconstructed and/or relaxed surfaces. An example of application, the Au (110) (1×2) reconstructed surface, is given. The intensity and the position of the diffraction peaks are determined for different values of the azimuthal angle and for different diffraction orders. The program takes into account the parameters of the beamline Advanced Line for Overlayer Interface and Surface Analysis (ALOISA) which will be available at the storage ring ELETTRA in Trieste.

Reconstruction of continuous and discontinuous refracted index profiles from power measurements of reflective fields

Abstract: The problem of retrieving a (possibly) discontinuous refractive index profile from the knowledge of only the modulus of the reflection coefficient is considered. The reconstruction of a parabolic profile obtained with the method is shown. The data have been simulated over 128 points with Delta k=4 m/sup -1/. After a few iterations, the reconstruction appears to be very good. >

Depth Selective Real Structure Analysis of Semiconductor Superlattices Using Grazing Incidence X-Ray Diffraction

Abstract: The periodicity of superlattices(SL) and its deviation perpendicular to the surface can be detected using X-ray grazing incidence diffraction (GID). Depth resolution is obtained by measuring rod scans at angles of incidence smaller or larger than the critical angle of total external reflection. First experimental curves measured from a lattice matched SL are interpreted in terms of a kinematical approach. The capability of the method for a depth resolved characterization is dicussed by simulation of the GID pattern for a partially relaxed strained layer SL.

Investigation of the Heavy-Atom Distribution in a Langmuir-Blodgett Film by an X-Ray Total External Reflection and Fluorescence Study

Abstract: Distributional arrangement of heavy atoms inside a Langmuir-Blodgett (L-B) film was found by means of X-ray fluorescence technique under total external reflection. The thin L-B film (8 monolayers) of stearic acid deposited on silicon substrate was inserted in the solution of lead stearate. The lead fluorescence was measured as a function of the glancing angle of a monochromated(13.5 keV) synchrotron X-ray beam. The analysis indicates that the lead atoms are distributed throughout the L-B film.