Non-normal Angles Research Articles

Photon Doppler velocimetry surface return study for common surface preparations

Photon Doppler velocimetry (PDV) is a ubiquitous diagnostic method in dynamic compression experiments. It typically requires the roughening of the test surface to ensure scattering of the probe light back into the PDV probe at non-normal angles of incidence. Although surface scattering plays an essential role in the quality of PDV data, surface scattering properties for common materials and preparations have not previously been studied with specific relevance to PDV. Here, we present detailed measurements of the bidirectional reflectance distribution function for four different surface preparations (Scotch-Brite hand and drill roughened, ball-rolled, and grit blasted) and four different materials (copper, aluminum, stainless steel, and tantalum). These measurements employ a conventional PDV probe and obtain a diffraction limited angular resolution with 10 pW accuracy and 2.6% repeatability. In addition to scattering data, we employ scattering theory and simulations to accurately emulate the measured data. We also present a straightforward method to derive the average scattering distribution from surface profilometry and observe several qualitative aspects of the scattering data that may help to optimize PDV signals.

Effect of Skull Porous Trabecular Structure on Transcranial Ultrasound Imaging in the Presence of Elastic Wave Mode Conversion at Varying Incidence Angle

With the advancement of aberration correction techniques, transcranial ultrasound imaging has exhibited great potential in applications such as imaging neurological function and guiding therapeutic ultrasound. However, the feasibility of transcranial imaging varies among individuals because of the differences in skull acoustic properties. To better understand the fundamental mechanisms underlying the variation in imaging performance, the effect of the structure of the porous trabecular bone on transcranial imaging performance (i.e., target localization errors and resolution) was investigated for the first time through the use of elastic wave simulations and experiments. Simulation studies using high-resolution computed tomography data from ex vivo skull samples revealed that imaging at large incidence angles reduced the target localization error for skulls having low porosity; however, as skull porosity increased, large angles of incidence resulted in degradation of resolution and increased target localization errors. Experimental results indicate that imaging at normal incidence introduced a localization error of 1.85 ± 0.10 mm, while imaging at a large incidence angle (40°) resulted in an increased localization error of 6.54 ± 1.33 mm and caused a single point target to no longer appear as a single, coherent target in the resulting image, which is consistent with simulation results. This first investigation of the effects of skull microstructure on transcranial ultrasound imaging indicates that imaging performance is highly dependent on the porosity of the skull, particularly at non-normal angles of incidence.

Bifunctional Metamaterials Using Spatial Phase Gradient Architectures: Generalized Reflection and Refraction Considerations.

We report the possibility of achieving normal-incidence transmission at non-normal incidence angles using thin interfaces made of metasurface structures with an appropriately-designed positive spatial phase distributions. The reported effect represents a consequence of generalized reflection and refraction, which, although having been studied for discovering exotic effects such as negative refraction, to the best of our knowledge fails to address normal incidence conditions in positive phase distribution and its underlying consequences. Normal-incidence conditions can be angle-tuned by modifying the vales of the phase distribution gradients. Furthermore, for configurations around the normal-incidence angles, the metasurface will exhibit a bifunctional behavior—either divergent or convergent. All these properties are essential for applications such as optical guiding in integrated optics, wave front sensing devices, polarization controllers, wave front-to-polarization converters, holographic sensors, and spatially-resolved polarization measurement.

Acoustic Transmission Factor through the Rat Skull as a Function of Body Mass, Frequency and Position

In many transcranial ultrasound studies on rats, the transmission factor is assumed to be independent of animal weight and losses resulting from non-normal incidence angles of the beam are not accounted for. In this study, we measured acoustic transmission factors through 13 excised skulls of male Sprague-Dawley rats weighing between 90 and 520g, at different positions on each skull and at 1, 1.25, 1.5, 1.75 and 2MHz. Our results revealed that insertion loss through rat skull increases linearly with both body mass and frequency and strongly depends on the position, decreasing from the front to the back and from the midline to the lateral sides. Skull thickness also scales linearly with body mass. Reflection explains the main part of the insertion loss compared with attenuation and aberration. These data are helpful in predicting the acoustic pressure at the focus in the brain.

Energy dependence of He-ion-induced tungsten nanofuzz formation at non-normal incidence angles

We report measurements of He-ion-beam induced tungsten nanofuzz formation for normal and non-normal incidence angles in the energy range 218eV–10keV. At 218eV, the fuzz tendrils are fine and grow randomly away from the interface in the direction of the surface normal. Above 480eV, the fuzz tendrils become increasingly coarser, and their growth direction is in the direction of the incident beam. This change is attributed to the ion-induced displacement damage which becomes effective once the displacement damage threshold energy is exceeded, and produces additional near-surface trapping sites in those portions of the surface that are in direct line of sight of the incident beam which can nucleate He clusters and initiate bubble growth. Once the surface morphology roughens sufficiently for shadowing to occur, the subsequent fuzz growth occurs preferentially toward the incident ion beam. Molecular dynamics (MD) simulations were carried out to determine the displacement damage threshold energies in the near-surface region along the three major crystallographic directions. It was found that the tungsten bulk values are established within the first 2–4 atomic layers below the tungsten surface. SRIM simulations based on the MD energy thresholds indicate that vacancy damage production in the near-surface region quickly dominates over sputtering in near-surface lattice modification effects as the energy above the damage threshold increases.

High-quality-factor planar optical cavities with laterally stopped, slowed, or reversed light.

In a planar optical cavity, the resonance frequencies increase as a function of in-plane wavevector according to a standard textbook formula. This has well-known consequences in many different areas of optics, from the shifts of etalon peaks at non-normal angles, to the properties of transverse modes in laser diodes, to the effective mass of microcavity photons, and so on. However, this standard formula is valid only when the reflection phase of each cavity mirror is approximately independent of angle. There is a certain type of mirror-a subwavelength dielectric grating near a guided mode resonance-with not only a strongly angle-dependent reflection phase, but also very high reflectance and low losses. Simulations show that by using such mirrors, high-quality-factor planar cavities can be designed that break all these textbook rules, leading to resonant modes that are slow, stopped or even backward-propagating in the in-plane direction. In particular, we demonstrate experimentally high-Q planar cavities whose resonance frequency is independent of in-plane wavevector-i.e., the resonant modes have zero in-plane group velocity, for one polarization but both in-plane directions. We discuss potential applications in various fields including lasers, quantum optics, and exciton-polariton condensation.

Large-scale fabrication of achiral plasmonic metamaterials with giant chiroptical response.

A variety of extrinsic chiral metamaterials were fabricated by a combination of self-ordering anodic oxidation of aluminum foil, nanoimprint lithography and glancing angle deposition. All of these techniques are scalable and pose a significant improvement to standard metamaterial fabrication techniques. Different interpore distances and glancing angle depositions enable the plasmonic resonance wavelength to be tunable in the range from UVA to IR. These extrinsic chiral metamaterials only exhibit significant chiroptical response at non-normal angles of incidence. This intrinsic property enables the probing of both enantoimeric structures on the same sample, by inverting the tilt of the sample relative to the normal angle. In biosensor applications this allows for more precise, cheap and commercialized devices. As a proof of concept two different molecules were used to probe the sensitivity of the metamaterials. These proved the applicability to sense proteins through non-specific adsorption on the metamaterial surface or through functionalized surfaces to increase the sensing sensitivity. Besides increasing the sensing sensitivity, these metamaterials may also be commercialized and find applications in surface-enhanced IR spectroscopy, terahertz generation and terahertz circular dichroism spectroscopy.

Fano resonances of dielectric gratings: symmetries and broadband filtering.

The guided mode resonances (GMRs) of diffraction gratings surrounded by low index materials can be designed to produce broadband regions of near perfect reflection and near perfect transmission. These have many applications, including in optical isolators, in hybrid lasers cavities and in photovoltaics. The excitation of rapid GMRs occurs in a background of slowly varying Fabry-Perot oscillation, which produces Fano resonances. We demonstrate the critical role of the polarity of adjacent Fano resonances in the formation of the broadband features. We design gratings for photovoltaic applications that operate at wavelengths where material absorption must be considered and where light is incident at non-normal angles.

Ultrafast Laser Patterning of Thin Films on 3-D Shaped Surfaces for Strain Sensor Applications

A femtosecond laser patterning process of thin film sensors on component surfaces is reported. This method is particularly useful for patterning of strain sensors which are sputter deposited directly on curved surfaces. The ablation behavior of NiCr film irradiated by femtosecond laser pulses at non-normal angles of incidence is modeled and experimentally verified with linear and circular beam polarization for incidence angles up to 80°. It is shown that the ablation threshold behavior can be described when including polarization and angle dependent Fresnel reflection into the laser ablation model. The laser process is finally demonstrated by patterning NiCr thin film sensors on a non-planar surface of a mechanical component for a machine tool.

빗각 증착으로 제조된 TiN 박막의 특성

Oblique angle deposition (OAD) is a physical vapor deposition where incident vapor flux arrives at non-normal angles. It has been known that tilting the substrate changes the properties of the film, which is thought to be a result of morphological change of the film. In this study, OAD has been applied to prepare single and multilayer TiN films by cathodic arc deposition. TiN films have been deposited on cold-rolled steel sheets and stainless steel sheet. The deposition angle as well as substrate temperature and substrate bias was changed to investigate their effects on the properties of TiN films. TiN films were analyzed by color difference meter, scanning electron microscopy, nanoindenter and x-ray diffraction. The color of TiN films was not much changed according to the deposition conditions. The slanted and zigzag structures were observed from the single and multilayer films. The relation between substrate tilting angle (<TEX>${\alpha}$</TEX>) and the growth column angle (<TEX>${\beta}$</TEX>) followed the equation of <TEX>$tan{\alpha}=2tan{\beta}$</TEX>. The indentation hardness of TiN films deposited by OAD was low compared with the ones prepared at normal angle. However, it has been found that <TEX>$H^3/E^2$</TEX> ratio of 3-layer TiN films prepared at OAD condition was a little higher than the ones prepared at normal angle, which can confirm the robustness of prepared films.

빗각 증착으로 제조한 Al 박막의 특성

Oblique angle deposition (OAD) is a physical vapor deposition method which utilizes non-normal angles between the substrate and the vaporizing source. It has been known that tilting the substrate changes the properties of the film deposited on it, which was thought to be a result of morphological change of the film. In this study, OAD has been applied to prepare single and multilayer Al films by magnetron sputtering. The magnetron sputtering source of 4 inch diameter was used to deposit the films. Al films have been deposited on Si wafers and cold-rolled steel sheets. The multilayer films were prepared by changing the tilting angle upside down at each layer interval, which means that when the first layer was deposited at an angle of <TEX>$+45^{\circ}$</TEX>, the second layer was deposited at an angle of <TEX>$-45^{\circ}$</TEX>, and vice versa. The microstructure, surface roughness and reflectance of the films were investigated using a scanning electron microscope, a surface profiler and a spectrophotometer, respectively. The corrosion resistance was measured and compared using the salt spray test. The single layer film prepared at an oblique angle of <TEX>$60^{\circ}$</TEX> prepared at other angles. However, for the multilayer films, the film prepared at an oblique angle of <TEX>$45^{\circ}$</TEX> showed the most compact and featureless structure. The multilayer films were found to exhibit higher corrosion resistance than the single layer films.

Acoustical Absorption of Porous Pavement

Acoustical absorption coefficients of more than 140 pavement cores were obtained by the impedance tube method with two microphones and cross-spectral analyses. The effectiveness of the impedance tube in predicting noise reduction for different mixes was evaluated by comparing the correlations between onboard sound intensity levels and absorption. Theoretical predictions of acoustical absorption due to friction between air and porous matrix and thermal relaxation were compared with measured results for an idealized porous structure. The model was used to infer porosity, tortuosity, and pore size from measured acoustical absorption spectra for manufactured porous asphalt and extrapolate test results to nonnormal angles of incidence, assuming isotropy of the porous structure. This model will help improve mix designs to increase the absorption of pavement surfaces and can be used to estimate porous pavement properties.

Measurements of Flows Over Isolated Valleys

Wind-tunnel measurements of the flow over an isolated valley both normal and at an angle (45°) to a simulated neutrally stable atmospheric boundary layer are presented. Attention is concentrated on the nature of the flow within the valley itself. The work formed part of a wider study that included detailed field measurements around an African desert valley and some limited comparisons with that work are included. A scale of about 1:1000 was used for the laboratory work, in which an appropriate combination of hot wire and particle image velocimetry was employed. For a valley normal to the upwind flow, it is shown that the upstream influence of the valley extends to a distance of at least one half of the axial valley width upstream of the leading edge, whereas differences in mean flow and turbulence could be identified well beyond two valley widths from the downwind edge. Non-normal wind angles lead to significant along-valley flows within the valley and, even at two valley heights above the valley ridge level, there remains a significant spanwise flow component. Downwind turbulence levels are somewhat lower in this case, but are still considerably higher than in the undisturbed boundary layer. At both flow angles, there are significant recirculation regions within the valleys, starting from mean separation just beyond the leading edge, but the strong spanwise flow in the 45° case reduces the axial extent of the separated zone. The flow is shown to be in some ways analogous to flow over an isolated hill. Our results usefully enhance the field data and could be used to improve modelling of saltation processes in the field.

Extreme-ultraviolet efficiency measurements of freestanding transmission gratings.

We report new, near-normal-incidence, transmission grating efficiency results at selected extreme-ultraviolet wavelengths between 4.5 and 30.5 nm for two transmission gratings, one with a period of 200 nm and the other with a period of 400 nm. These gratings consist of opaque gold bars separated by open spaces that have been produced by photolithography techniques commonly used to produce electronic components. The gold bars and the open spaces are nominally of the same width. Both gratings have a thickness of 470 nm. The transmission efficiency at the central, first, and, when possible, second order of diffraction was measured. In addition, guided-wave phenomena at nonnormal angles of incidence, as well as transmission differences depending on which side of the grating was illuminated, were investigated. The observed guided-wave effects allow one to selectively enhance the transmission of the grating at desired wavelengths, as is realized with a blazed reflection grating.

Porous broadband antireflection coating by glancing angle deposition.

We deposited graded-index SiO2 films using glancing angle deposition to produce high-transmission antireflection coatings on glass. Because of the accurate control over the thin-film microstructure provided by this technique, we were able to create graded densities with a Gaussian profile resulting in transmission values greater than 99.9% for a single-layer interface with bandwidths up to 460 nm. The graded-index layer also provides low reflectance at nonnormal angles of incidence with transmission values degrading little for incidence angles up to 30 degrees.

Solid particle erosion of diamond coatings under non-normal impact angles

This paper describes an erosion study, which examines the effect of impact angle on the erosion behaviour of diamond coatings deposited on tungsten substrates by chemical vapour deposition (CVD). The coatings were 37–60μm in thickness and were erosion tested using angular silica sand with a mean diameter of 194μm at a particle velocity of 268ms−1. The impact angles used were 30, 45, 60 and 90°. The results show that the damage features, termed “pin-holes” are generated at all angles, though the number of impacts required for pin-hole initiation is significantly increased at lower angles. This work provides useful information in attempting to explain the mechanism by which damage is generated during the high velocity sand erosion of CVD diamond.

Review of cathodic arc deposition technology at the start of the new millennium

The vacuum cathodic arc has been known as a means of producing coatings since the second half of the 19th century. This makes it one of the oldest known vacuum coating techniques. In the last century it has been recognized that the copious quantities of ions produced by the process provides certain coating property advantages. Specifically, ions can be steered and/or accelerated toward the parts to be coated. This, in turn, can provide enhanced adhesion, film density, and composition stoichiometry in the case of compound coatings. The ions generated by the cathodic arc have high ‘natural’ kinetic energy values in the range 20–200 eV, leading to enhanced surface mobility during the deposition process and even ion subplantation. In many cases, dense coatings are achieved even when the ions arrive at non-normal angles. The ion energy can be further manipulated by the plasma immersion biasing technique. Macroparticle contamination has been alleviated by a variety of novel plasma filters. The purpose of this review is to describe recent developments in macroparticle filtering and arc control. These developments promise to broaden the range of applications to the semiconductor, data storage, and optical coatings industry.

Modelling oblique hypervelocity impact phenomena using elementary shock physics

Summary During their missions in space, spacecraft are subjected to high velocity impacts by orbital debris particles. Such impacts are expected to occur at non-normal incidence angles and can cause severe damage to the spacecraft as well as to its external flight-critical components. In order to ensure crew safety as well as the proper function of internal and external spacecraft systems, the characteristics of debris clouds generated by such impacts must be known. In this paper, a first-principles-based analytical model is developed for the characterization of the penetration and ricochet debris clouds created by the hypervelocity impact of a spherical projectile on a thin aluminum plate. This model employs normal and oblique shock wave theory to characterize the penetration and ricochet processes. The predictions of the model are compared against numerical and experimental results.

Long-wavelength cutoff filters of a new type

A method for the design of cutoff filters is proposed that is based on the use of light at nonnormal angles of incidence and on the use of coating materials with a large dispersion of the optical constants. The optical constants are presented of several films made from indium tin oxide--a material that satisfies this requirement. The filtering is due to absorption, critical angle effects, or both. The theory of this type of filter is described, and the calculated performance of two long-wavelength cutoff filters based on the use of indium tin oxide films is given. The angular performance and the producibility of one of these filters are examined. Some applications for these filters are considered, and the feasibility of producing similar filters for other spectral regions is discussed.

A generalized scattering matrix approach for analysis of quasi-optical grids and de-embedding of device parameters

A generalized scattering matrix approach to analyzing quasi-optical grids used for grid amplifiers and grid oscillators is developed. The approach is verified by a novel method for de-embedding, in a waveguide simulator, the active device parameters of a differential pair high electron mobility transistor (HEMT) from the single unit cell of a grid amplifier. The method incorporates the additional ports presented to the active device into a method of moments solution of the embedding periodic array. The port(s) defined at the device or load location are within the plane of the array, and not terminated in a microstrip line with a known characteristic impedance. Therefore the generalized scattering matrix for the embedding array is normalized to the calculated input impedance(s) at these port(s). The approach described here uses a Floquet representation of the fields incident and reflected from the grid as the remaining ports in the generalized scattering matrix. The use of Floquet modes allows analysis of general geometries and nonnormal incident angles without the need for magnetic and electric wall assumptions. By developing a generalized scattering matrix for the embedding periodic array, this approach now allows conventional amplifier design techniques and analysis methods to be applied to quasi-optical grid amplifier and oscillator design. The major advantage of this unification for grid amplifier design being that the stability of the design can be predicted.