Off-specular Angle Research Articles

Magnetic properties of aligned multiferroic Janus nanofiber agglomerates measured with the scattered magneto-optical Kerr effect

We report magnetic properties of electrospun multiferroic nanofibers assembled into linear aggregates with an external magnetic field, and measured with the magneto-optical Kerr effect (MOKE) in a non-specular or scattering geometry (ScMOKE). Compared with bulk magnetometry, ScMOKE’s sensitivity to subtle differences between aggregates offers a route to determine local multiferroic coupling in disordered nanomaterials. CoFe2O4-BaTiO3 nanofiber agglomerates are assembled prior to measurement by suspending and aligning the fibers in a transparent air-cured polyvinyl alcohol solution. We detect the polarization change in light scattered from the fibers, collected at an off-specular angle in order to eliminate the background caused by substrate reflection. Averaged hysteresis loops from different agglomerates show a variety of unique structures. For our optical spot size of ∼15 m, multiple fibers, within a single agglomerate, are detected simultaneously, suggesting ScMOKE can distinguish local magnetization reversal fields that vary from fiber to fiber, as well as magnetic interactions between fibers.

Use of a novel infrared wavelength-tunable laser Mueller-matrix polarimetric scatterometer to measure nanostructured optical materials.

Nanostructured optical materials, for example, metamaterials, have unique spectral, directional, and polarimetric properties. Samples designed and fabricated for infrared (IR) wavelengths have been characterized using broadband instruments to measure specular polarimetric transmittance or reflectance as in ellipsometry or integrated hemisphere transmittance or reflectance. We have developed a wavelength-tunable IR Mueller-matrix (Mm) polarimetric scatterometer which uses tunable external-cavity quantum-cascade lasers (EC-QCLs) to tune onto and off of the narrowband spectral resonances of nanostructured optical materials and performed full polarimeteric and directional evaluation to more fully characterize their behavior. Using a series of EC-QCLs, the instrument is tunable over 4.37-6.54 μm wavelengths in the mid-wave IR and 7.41-9.71 μm in the long-wave IR and makes measurements both at specular angles, acting as a Mm polarimeter, and at off-specular angles, acting as a Mm scatterometer. Example measurements of an IR thermal metamaterial are shown.

Polarizing properties and structure of the cuticle of scarab beetles from the Chrysina genus.

The optical properties of several scarab beetles have been previously studied but few attempts have been made to compare beetles in the same genus. To determine whether there is any relation between specimens of the same genus, we have studied and classified seven species from the Chrysina genus. The polarization properties were analyzed with Mueller-matrix spectroscopic ellipsometry and the structural characteristics with optical microscopy and scanning electron microscopy. Most of the Chrysina beetles are green colored or have a metallic look (gold or silver). The results show that the green-colored beetles polarize reflected light mainly at off-specular angles. The gold-colored beetles polarize light left-handed near circular at specular reflection. The structure of the exoskeleton is a stack of layers that form a cusplike structure in the green beetles whereas the layers are parallel to the surface in the case of the gold-colored beetles. The beetle C. gloriosa is green with gold-colored stripes along the elytras and exhibits both types of effects. The results indicate that Chrysina beetles can be classified according to these two major polarization properties.

Tailoring broadband light trapping of GaAs and Si substrates by self-organised nanopatterning

We report on the formation of high aspect ratio anisotropic nanopatterns on crystalline GaAs (100) and Si (100) substrates exploiting defocused Ion Beam Sputtering assisted by a sacrificial self-organised Au stencil mask. The tailored optical properties of the substrates are characterised in terms of total reflectivity and haze by means of integrating sphere measurements as a function of the morphological modification at increasing ion fluence. Refractive index grading from sub-wavelength surface features induces polarisation dependent anti-reflection behaviour in the visible-near infrared (VIS-NIR) range, while light scattering at off-specular angles from larger structures leads to very high values of the haze functions in reflection. The results, obtained for an important class of technologically relevant materials, are appealing in view of photovoltaic and photonic applications aiming at photon harvesting in ultrathin crystalline solar cells.

Polarizing properties and structural characteristics of the cuticle of the scarab Beetle Chrysina gloriosa

The scarab beetle Chrysina gloriosa is green with gold-colored stripes along its elytras. The properties of light reflected on these areas are investigated using Mueller-matrix spectroscopic ellipsometry. Both areas reflect light with high degree of left-handed polarization but this effect occurs for specular reflection for the gold-colored areas and for off-specular angles for the green areas. The colors and polarization phenomena originate from reflection of light in the cuticle and a structural analysis is presented to facilitate understanding of the different behaviors of these two areas. Scanning electron microscopy (SEM) images of the cross section of beetle cuticles show a multilayered structure. On the gold-colored areas the layers are parallel to the surface whereas on the green-colored areas they form cusp-like structures. Optical microscopy images show a rather flat surface in the gold-colored areas compared to the green-colored areas which display a net of polygonal cells with star-shaped cavities in the center. Each of the polygons corresponds to one of the cusps observed in the SEM images. Atomic force microscopy images of the star-shaped cavities are also provided. The roughness of the surface and the cusp-like structure of the green-colored areas are considered to cause scattering on this area.

Low-energy bulk plasmon of nickel

High-resolution electron energy loss spectroscopy has been used to study low-energy bulk excitation modes of planar-bound electrons (bulk plasmons) in nickel. We observed for the first time a bulk plasmon at about 1.2 eV, in agreement with dielectric theory. The behavior of its amplitude with the off-specular angle ensures the dipolar nature of such mode. On the other hand, the intensity of the plasmon peak is vanishing upon ion bombardment due to the sputtering-induced modification of dielectric function.

Comparative surface dynamics of amorphous and semicrystalline polymer films

The surface dynamics of amorphous and semicrystalline polymer films have been measured using helium atom scattering. Time-of-flight data were collected to resolve the elastic and inelastic scattering components in the diffuse scattering of neutral helium atoms from the surface of a thin poly(ethylene terephthalate) film. Debye-Waller attenuation was observed for both the amorphous and semicrystalline phases of the polymer by recording the decay of elastically scattered helium atoms with increasing surface temperature. Thermal attenuation measurements in the specular scattering geometry yielded perpendicular mean-square displacements of 2.7•10(-4) Å(2) K(-1) and 3.1•10(-4) Å(2) K(-1) for the amorphous and semicrystalline surfaces, respectively. The semicrystalline surface was consistently ∼15% softer than the amorphous across a variety of perpendicular momentum transfers. The Debye-Waller factors were also measured at off-specular angles to characterize the parallel mean-square displacements, which were found to increase by an order of magnitude over the perpendicular mean-square displacements for both surfaces. In contrast to the perpendicular motion, the semicrystalline state was ∼25% stiffer than the amorphous phase in the surface plane. These results were uniquely accessed through low-energy neutral helium atom scattering due to the highly surface-sensitive and nonperturbative nature of these interactions. The goal of tailoring the chemical and physical properties of complex advanced materials requires an improved understanding of interfacial dynamics, information that is obtainable through atomic beam scattering methods.

Dynamics of fluctuations in smectic membranes

We present a comprehensive account of the dynamics of layer-displacement fluctuations in smectic liquid-crystal membranes as studied by x-ray photon correlation spectroscopy (XPCS) and neutron-spin echo (NSE). Combining these two techniques at fast relaxation times, three distinct relaxation regimes can be distinguished. For thin membranes, at the specular Bragg position oscillatory relaxation occurs, which transforms for thicker samples into exponential decay. Above a critical off-specular angle, in XPCS exponential relaxation is observed that does not depend on the scattering angle. This indicates relaxation times that are independent of the wavelength of the fluctuations. In this regime the relaxation of the fluctuations is dominated by the surface tension. Using NSE larger off-specular angles can be reached than by XPCS, for which the relaxation time decreases with the scattering angle. This regime is dominated by the bulk elasticity of the smectic membrane. The results are compared with theoretical models for the fluctuation behavior of smectic membranes, in which effects of the mosaic distribution and of the center of mass movement of the smectic membranes must be incorporated.

Principal Mueller matrix of reflection and scattering measured for a one-dimensional rough surface

Reflectance R, ellipsometric parameters (c, D), depolariza- tions D, and cross-polarized depolarization Dv of specular reflection, and scattering by a rough stainless steel surface are measured using a null ellipsometer. The three polarization elements (Px ,Py ,Pz) of a principal Mueller matrix are obtained from D, c, and D. The measured c and D of specular reflection for incident angle u.70 deg are fit to the Fresnel equations to obtain n and k. The measured specular R(u) is fit to Beck- mann's scattering theory to obtain the root mean square (rms) rough- ness s. The fits (5 392 nm) is of the same order as the stylus measured s (5 484 nm). The D of specular reflection is small ( , 0.07). Scattering measurements are made with variable sample orientation at a detection direction 40 deg backward from the incident direction. The Dv of scatter- ing is very small (, 0.002). The values of c, D, Px , Pz , and Dv of scattering are about constant for an off-specular angle (OSA) within 660 deg and can be explained by the facet model with single scattering. The measured bidirectional reflectance distribution function (BRDF) in this region can be converted to slope angle distribution with OSA52 slope angle. The values of c, D, Px , Pz , and Dv of scattering deviate signifi- cantly and symmetrically from a constant level for uOSAu.60 deg, for which the simple facet model with single scattering does not apply.

Adsorption sites and microstructures ofCO2on Fe(111) derived from specular and off-specular HREELS

The dependence on surface temperature and adsorbate pressure of static and time-dependent HREEL (high-resolution electron-energy-loss) spectra provides direct experimental evidence for molecular adsorption of ${\mathrm{CO}}_{2}$ on Fe(111) at 100 K in one linear state and two bent states. The data demonstrate that these adsorption states can be distinguished unambiguously by their binding energies and their thermal stabilities. Comparison of HREELS data taken under specular geometry with those taken at off-specular angles allows a complete determination of the individual symmetries and microstructures of ${\mathrm{CO}}_{2}$ in these states.

Space-Based Visible Space Object Photometry: Initial Results

One of the objectives of the Midcourse Space Experiment surveillance experiments is to collect photometric and radiometric data on a selected set of resident space objects from the long-wavelength infrared through the short wavelength ultraviolet region of the spectrum. This was done using the Spatial Infrared Imaging Telescope III, Space-BasedVisible (SBV),ultravioletandvisibleimager,andspectrographicimagerinstrumentpackagesonboard thespacecraft. Analysis of these data will primarily emphasize 1 )construction and validation of phenomenological models for a broad range of resident space object classes, for example, spin- and three-axis stabilized satellites, rocket bodies, and orbital debris, over a wide range of solar phase angles and 2 ) discrimination between classes of resident space objects and identie cation of discriminating characteristics within a resident space object class. We concentrate on the initial results from the SBV instrument. We provide an overview of the surveillance data collected to date, discuss its limitations, and present examples of the data collected on two classes of resident space objects. Nomenclature Ap = projected area of resident space object, m 2 B i V = stellar color index Mv = Johnson V-band visual magnitude MSBV = Space-Based Visual (SBV) magnitude R = range, m d sun = solar declination angle W = off-specular angle w = Midcourse Space Experiment parallax angle

Observation of temporary-negative-ion states in condensed methanol via vibrational excitation induced by slow electron scattering.

We report the measurements of electron-impact vibrational excitation of amorphous methanol condensed on a polycrystalline Pt substrate over incident energies of 1--20 eV at a large off-specular angle. The spectra obtained show that vibrations within solid methanol in its ground state are rather sensitive to the variation of impact energy and there is tangible evidence of the formation of at least three compound temporary-negative-ion (TNI) states. The low-energy feature lying between 3 and 6 eV is assigned tentatively to a $^{2}$A\ensuremath{'} valence-shape resonance with a $^{2}\mathit{N}$,(8a\ensuremath{'}\ensuremath{\sigma}*${)}^{1}$] configuration, whose parent state is the ground state $^{1}$A\ensuremath{'} of the molecule. The origin of the second broad structure centered around 7.0 eV in the excitation functions may be threefold: a Feshbach-type resonance $^{2}$A\ensuremath{''} of the electronic configuration $^{2}[$(2a\ensuremath{''}${)}^{1}$,(3sa\ensuremath{'}${)}^{2}$] (about 7 eV); a nearby shape resonance $^{2}$A\ensuremath{'} of $^{2}[$(N,(9a\ensuremath{'}\ensuremath{\sigma}*${)}^{1}$] (about 8 eV); and probably also a minor contribution from another $^{2}[$(7a\ensuremath{'}${)}^{1}$,(3sa\ensuremath{'}${)}^{2}$] Feshbach resonance $^{2}$A\ensuremath{'} (about 9 eV). The third broad bandlike resonant feature, extending from 11 up to 17 eV, may consist of overlapping TNI states, dominated by the $^{2}[$(6a\ensuremath{'}${)}^{1}$,(3sa\ensuremath{'}${)}^{2}$] Feshbach resonance $^{2}$A\ensuremath{'}. Plausible decay channels from their configurations are discussed. \textcopyright{} 1996 The American Physical Society.

Helium-atom-scattering study of multiphonon processes on LiF(001) with temperature variation for specular and off-specular angles.

High-resolution inelastic He-atom-scattering experiments, employing a time-of-flight energy resolution technique, have been carried out on LiF(001)〈100〉 for a wide range of temperatures and for several different incident beam angles. A quantum-mechanical multiphonon theory is used to explain the observed inelastic background. Numerical calculations based on the theory produce good agreement with experiment.

Scattering from disordered surfaces in the sudden approximation

The angular intensity distribution of atomic beams scattered from partially disordered solid surfaces is investigated. Quantitative relations are established between correlation functions that describe the structural disorder, and several features of the scattered intensity distribution. To simplify the collision dynamics, the study is carried out in the framework of the sudden approximation, which assumes that momentum changes perpendicular to the surface are large compared with momentum transfer due to surface corrugation. Simple relations are obtained between the structural correlation functions and the scattering at near-specular angles, at extreme off-specular angles, and for directions close to that at which the surface scattering rainbow appears for the corresponding ordered lattice system. Detailed expressions for the case of step disorder are presented. The results suggest that molecular beam scattering should be a powerful probe of structural properties of disordered surfaces.

Roughness measurement with a laser scanning analyser

Abstract In the laser scanning analyser a laser beam is reflected from a rotating polygonal mirror so as to scan the workpiece surface at high speed, where it is again reflected into a photodetector which measures the reflected intensity. If the surface is rough, in addition to the specular reflectance the diffuse reflectance due to diffraction is measured as a function of the off-specular angle. In effect the system when used in this mode is a glossmeter which measures the optical properties of a section through the surface and is thus highly suitable for comparison with a stylus instrument. Experiments are described in which reflectance measurements on surfaces finished by various machining processes are compared with stylus roughness measurements. It is found that reflectance measurements are sensitive both to roughness and to the orientation of the lay. Reflectance measurements are found to vary as the average roughness and as the fourth power of the profile slope. There is no correlation with curvature and no evidence of the effect of longer surface wavelengths. There is some evidence that two different scattering mechanisms are involved. Possible applications of such a real-time non-contacting roughness measuring system are described.

Microtopographic Influence of Nominally Flat Metallic Surfaces upon Their Thermally Reflective Behaviour

The total reflectivity as well as the spatial distribution of the reflected energy is influenced by the topography of the reflecting surface. It has been commonly, but mistakenly, assumed that the angular variation of thermal radiation reflected from real surfaces has a directional distribution that falls between the diffuse and specular limits. For roughened surfaces, maxima in the reflected, total, radiant-intensity vector distribution occur at viewing directions other than at the angle of specular reflection, even when the mean optical roughness (i.e. r.m.s. roughness divided by wavelength of the incident radiation) is less than unity. Off-specular phenomena occur if the product of the mean normalized absolute surface slope and the predominant radiation wavelength is smaller than about 12. The magnitude of the off-specular angle is then a function of the mean surface slope, the r.m.s. roughness, the predominant wavelength of the incident radiation and its angle of incidence.