Showing papers on "Total external reflection published in 1988"

Why some things are darker when wet

Abstract: Angstrom has proposed that rough absorbing materials are darker when wet because their diffuse reflection makes possible total internal reflection in the water film covering them, increasing the likelihood of the absorption of light by the surface His model is extended here in two ways: the probability of internal reflection is calculated more accurately, and the effect on absorption of the decrease of the relative refractive index (liquid to material instead of air to material) is estimated Both extensions decrease the albedo of the wetted surface, bringing the model into good agreement with experiment

Nonlinear selective reflection from an atomic vapor at arbitrary incidence angle

Abstract: We calculate the reflection coefficient for a light beam incident on the interface between a dielectric and a resonant atomic vapor to first order in the dipole polarization in the vapor. The angle of incidence and the polarization direction are chosen arbitrarily, and saturation is fully accounted for. The atoms are supposed to get deexcited at collisions with the surface. The resulting transient behavior of atoms leaving the surface is responsible for a nonlocal response. This spatial dispersion near the surface is known to produce a natural-linewidth-limited resonance in the vapor reflection coefficient at normal incidence. Our analysis shows that this sub-Doppler structure is broadened by the residual Doppler effect for non-normal incidence. This structure disappears at the critical angle for total internal reflection, where one predicts a conventional Voigt-type dispersion response, based on the complex-refractive-index approach. We also calculate the saturation broadening of the atomic response for large intensities. Beyond the critical angle, the spectral response suddenly shifts from dispersion to absorption line shapes. In the case of total internal reflection, the spatial dispersion leads to an additional Lorentzian broadening, which results from an effective imaginary Doppler shift at passage through the evanescent wave (transit-time broadening).

Simple method for focusing x rays using tapered capillaries.

Abstract: A new method of focusing x rays is described using appropriately tapered capillaries. The x rays are incident on the inner surface of the capillary below the critical glancing angle and reflect due to total external reflection. By appropriately narrowing the capillary, the x rays can thus be focused in a broad band of energies. The theory of the effect and optimum taper is described. A measurement verifying the focusing capability of the method is presented. The method appears practical for focusing bending magnet synchrotron radiation around 8 keV down to a diameter of 10 mum from an initial dimension of 1-mm(2) incident cross section with an attenuation of the total energy of ~2, i.e., an increase in the intensity per unit area of 6.5 x 10(3). Greater focusing is possible with softer x rays and from undulator sources. The wide-ranging applicability of the technique is discussed.

Reflection and transmission of self-focused channels at nonlinear dielectric interfaces.

Abstract: We show with an extensive numerical study that the global reflection and transmission properties of a finite-width optical self-focused channel incident at an oblique angle to an interface separating two self-focusing nonlinear media can be categorized into three distinct regimes of behavior as the incidence angle is varied through the angle for total internal reflection. The largest regime in parameter space is the nonlinear regime, where a channel undergoes either total internal reflection or transmission, in marked contrast to the well-known linear Snell’s law behavior. The beam asymptotics in this latter region are quantitatively explained by a recent equivalent-particle theory.

High-resolution X-ray diffraction studies of multilayers

Abstract: High-resolution X-ray diffraction studies of the perfection of state-of-the-art multilayers are presented. Data were obtained using a triple-axis perfect-crystal X-ray diffractometer. Measurements reveal large-scale figure errors in the substrate. A high-resolution triple-axis set up is required, therefore, to obtain structural information about the layers. Measurements of reflectivity from parallel layers are presented as well as measurements of the angular distribution of the layer normal over the illuminated spot. These measurements are supplemented with measurements of scattering obtained in the total external reflection regime.

Dynamical X-Ray Diffraction Profiles for Asymmetric Reflection form Crystals under Grazing Incidence Conditions

Abstract: Dynamical X-ray diffraction during total external reflection has been studied by measuring rocking curves from Bragg planes nearly perpendicular to the flat surface of a germanium single crystal. The rocking-curve profiles for both the diffracted and specular beams observed from slightly asymmetric Bragg planes were found to be quite different from those from the symmetric plane. Experiments using a parallel X-ray beam and a surface passivated sample confirmed the rocking-curve features arising from the excitation and de-excitation of the wave fields in shallow surface layers of the bulk crystal.

Near Surface Structure of Ion Implanted Si Studied by Grazing Incidence X-Ray Scattering

Abstract: X-rays incident on a surface under grazing angle may undergo total external reflection and excite an interior wave field damped exponentially into the bulk. These evanescent waves are a sensitive probe for the study the real structure in the near surface region. We report results on the influence of implantation defects on Bragg diffracted and on diffuse intensities. By detailed comparison of Bragg intensities with predictions of dynamical scattering theory we detect the presence of amorphous layers and determine their thickness. For the first time defect induced diffuse scattering underconditions of grazing incidence andexit is observed and compared to recent theoretical results. Strength and symmetry of implantation induced defects can be determined as well as their depth distribution which is compared to results of a TRIM simulation: the defect distribution is found to agree with that of the deposited collisional energy.

Effects of Refraction of X-Rays in Double-Crystal Topography

Abstract: We have investigated the effects of refraction of X-rays in double-crystal topography in the (+, -) Bragg-Bragg asymmetric arrangement, in which the angle of incidence of X-rays is very small. It is shown that in a double-crystal topograph of a sample having an uneven surface, the refraction effect gives rise to diffraction contrasts corresponding to the shape of the sample surface. This effect can be applied to an examination of surface roughness of a crystal and to a determination of a refractive index in the X-ray region. An attempt to derive the refractive index for silicon crystals has been made. The derived value was found to be in good agreement with the nominal one.

Microscopic description of the reflection of light under normal incidence: validity of the Lorentz approach to interpret a refractive index profile

Abstract: Optical techniques are widely used to study thin layers and more generally interfacial structures. The authors consider the problem raised by a microscopic interpretation of the refractive index and study the reflection of a plane wave under normal incidence, in the dipolar approximation. Calculations of the reflection on a thin homogeneous layer reveal significant differences between macroscopic and microscopic approaches, when the layer thickness is of a few lattice parameters, and it appears that this is mainly due to edge effects. The authors describe microscopically the reflection on a layered interface. It is shown that a macroscopic description may be recovered to very good approximation when an 'apparent' refractive index napp(z) is introduced. This index is related to both the local polarisability and its gradient. Comparison is established with the Lorentz refractive index profile.

Method for the measurement of the refractive index of a thin layer

Abstract: A method for the measurement of the refractive index of a thin layer, which is formed on another layer having a known refractive index, has the following steps: first, the thin layer is irradiated individually at a predetermined angle of incidence with S-polarised and P-polarised monochromatic light; then energy reflection ratios Rs and Rp are determined for the S-polarised and P-polarised monochromatic light; hereupon the light intensity of the reflected light from the thin layer is then determined and, finally, the refractive index of the thin layer is measured by means of a prescribed calculation on the basis of phase value changes of the light during its propagation from an upper surface of the thin layer to its lower layer, the phase changes corresponding to the refractive index of the medium and to the energy reflection ratios Rs and Rp being determined.

Refraction shift of bragg reflection of neutrons

Abstract: A refraction shift of the asymmetric Bragg reflection of thermal neutrons is observed. Germanium crystals are used to demonstrate this in an invariant geometry while the index of refraction is changed. Two values of the index-of-refraction jump at one crystal surface are created by simultaneous reflection of neutrons with fractional wavelengths, by immersing the crystal partially a liquid (heavy water), and by creating a jump of magnetic induction at the crystal surface. The observed Bragg reflection shifts reach values of 40 to 50 s of arc. The presented method can be used for measuring the indices of refraction. [Russian Text Ignored]

Total internal reflection effects on two-dimensional scattering measurements

Abstract: A simple geometric optics explanation is given for various total internal reflection anomalies which were obtained during the acquisition of experimental two-dimensional scattering data. This explanation appears accurately to indicate where the effects of the total internal reflection appear in the data but not the magnitude of the effect.