Angular Incidence Research Articles

Response of the ArcCHECK® device at 6MV and 15MV for VMAT and IMRT quality control.

To study the response of the ArcCHECK® device as VMAT and IMRT verification system. Various tests analyzing the linearity, the repeatability and the angular dependence of the device response, its dependence with the pulse repetition rate and the leakage losses were performed. The long-term response in dose measurements and the uniformity of the detectors conforming the system were controlled using a statistical process control program. The Elekta Infinity™ 6 and 15MV photon beams were used. The device showed excellent repeatability and linearity. The differences between the responses obtained for any pair of angular incidences were less than 2%. The absorbed dose increased by 3% when the pulse repetition rate varied from 50 to 600MU/min. Results are in overall agreement with those found in previous works for the ArcCHECK®, in which a reduced number of the device diodes were analyzed, and for the MapCheck®, an older 2D device that used the same diodes. Charge losses were found to be negligible except for some of the diodes of the device. The statistical process control program is a very useful tool to control the correct functioning of the device in the long term. The results of the analysis carried out indicate that the working and stability conditions of the ArcCHECK® device are adequate for its purpose. The dependence with the pulse repetition rate should be considered when VMAT or similar treatments are evaluated. A control program for the statistical monitoring of the device would be desirable and useful.

Plasmon-polariton gap and associated phenomenon of optical bistability in photonic hypercrystals

Photonic hypercrystals are a novel type of metamaterial combining the properties of photonic crystals and hyperbolic metamaterials. The transmission of transverse magnetic polarized light through photonic hypercrystal is studied. Along with the conventional gaps, the angular incidence of such radiation at the plasmon frequency of the perpendicular component of permittivity tensor of hyperbolic metamaterials gives rise to the appearance of the unconventional plasmon-polariton gap. Some properties of this gap and the associated phenomenon of optical bistability at the edge of this gap are explored.

Propagation and Attenuation Characteristics of an Ultrasonic Beam in Dissimilar-Metal Welds.

Ultrasonic inspection of welds joining dissimilar metals in nuclear power plants has proven to be a challenge, because the ultrasonic waves are subject to diffraction, distortion, scattering, and noise. These perturbations are due to their interactions with coarse-grained microstructures having anisotropic and heterogeneous metallurgical properties that can promote ultrasonic attenuation. In this paper, to improve the reliability of ultrasonic testing for dissimilar-metal welds (DMWs), ultrasonic beam characteristics for DMWs with a buttering layer were investigated in order to analyze the beam distortion phenomenon caused by inhomogeneous anisotropic properties and coarse grains. Ultrasonic testing was performed on DMW specimens using single ultrasonic transducers to investigate the behavior of the ultrasonic beam in the welds. According to the anisotropic and heterogeneous properties, when passing through the weld and the buttering layer of the DMW, ultrasonic waves were distorted and attenuation was high. In particular, in the case of using angular incidence that passed through the weld and the buttering layer in turn, the received ultrasonic data did not contain accurate internal information. From this, it was verified that internal defects may be detected by transmitting ultrasonic waves in different directions. Finally, the existing limitations on the application of non-destructive ultrasonic testing to dissimilar-metal welds were verified, and a solution to the measurement method was proposed.

Design of a Miniaturized 2.5-D Frequency Selective Surface With Angular Incidence and Polarization Stability

In this article, a miniaturized 2.5-D frequency selective surface with the band-stop response at 900 MHz is presented. The proposed FSS unit cell comprises rectangular spiral elements printed on both sides of the dielectric substrate. These spirals are connected with plated through-hole vias to form a “2.5-D closed loop”. The large perimeter of the closed loop increases the effective inductance and causes a reduction in resonant frequency and size of the unit cell. The structure is designed to operate at 900 MHz for shielding the interference from other GSM sources. The size of the proposed unit cell is 0.021λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.021λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> which is very compact with a figure-of-merit λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /p = 47.6, where λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the free-space wavelength of resonant frequency andp is the periodicity of unit cell. Further, the effect of substrate parameters on resonance performance has been studied. The proposed structure is polarization-independent and angularly stable for both TE and TM polarizations. A prototype of the FSS is fabricated and measured, where the measured responses show good agreement with the simulated responses.

Analytical and comprehensive optimization design for multilayer diffractive optical elements in infrared dual band

We present an analytical and comprehensive optimization design for multilayer diffractive optical elements (MLDOEs) in infrared dual band for angular incidence. The diffraction efficiency of MLDOEs is affected by both wavelength and incident angle. Accordingly, the proposed optimization design considers comprehensive factors for its further design and evaluation. It can realize high diffraction efficiency and optimal microstructures of MLDOEs in dual band for angular incidence. By using numerical simulations, we demonstrate the modulation transfer function (MTF) at the cutoff frequency in an infrared dual-band optical system. It was found that such a design provides more accurate results, which helps achieve both higher MTF and smaller microstructure heights.

On the Orbital Angular Momentum Incident Fields in Linearized Microwave Imaging.

Orbital angular momentum (OAM) is gaining great attention in the physics and electromagnetic community owing to an intriguing debate concerning its suitability for widening channel capacity in next-generation wireless communications. While such a debate is still a matter of controversy, we exploit OAM generation for microwave imaging within the classical first order linearized models, i.e., Born and Rytov approximation. Physical insights into different fields carrying ℓ-order OAM are conveniently exploited to propose possible alternative imaging approaches and paradigms in microwave imaging.

Heavily Doped Semiconductor Metamaterials for Mid‐Infrared Multispectral Perfect Absorption and Thermal Emission

AbstractA metamaterial perfect absorber based on layered, doped and undoped semiconductors is experimentally and theoretically investigated. Design rules are given to control the multispectral, narrow, strong absorption features (&gt;98% absorption) in the mid‐IR spectrum. The proposed sub‐wavelength grating structures support localized surface plasmons and photonic resonances associated to the quarter wavelength optical thickness of the undoped spacer layer. The resonances hybridize depending on the geometric setup of the metamaterial and the material properties of the heavily doped semiconductor. The angular response of the resonances is studied to determine the acceptance angle. While the absorption is best at near‐normal incidence, the spectral position of the absorption bands is weakly dependent on the angular incidence up to angles of 45°. Complementary to the polarization‐dependent grating design of the metamaterial, an alternative based on a crossed grating nanopattern is also investigated for polarization insensitivity. Furthermore, it is demonstrated that the metamaterial perfect absorber provides a tailored thermal emission spectrum. Based on the heavily doped InAsSb/GaSb metamaterial platform, integrable and miniaturized perfect absorbers and thermal sources can be built.

Design and fabrication of flexible FSS polarizer

A low profile flexible linear to circular polarizer made of paper substrate is proposed in this paper. The polarizer is constructed from frequency selective surface (FSS) comprising of crossed dipole unit cell elements. The polarizer converts the linearly polarized incident electromagnetic wave to circular polarized wave at 7.0 GHz. The polarizer provides the advantage of use in conformal system, which is substantiated using bending analysis. The proposed reflective polarizer provides a 3 dB axial ratio bandwidth of 13% at the center frequency of 7.0 GHz. In addition, the proposed flexible polarizer provides stable response for angular incidences up to 30°. Prototype of the flexible FSS polarizer is fabricated and its simulation results are validated with experimental measurements.

Optical performances of antireflective moth-eye structures. Comparison with standard vacuum antireflection coatings for application to outdoor lighting LEDs

Protection windows or lenses for outdoor white LED lighting generate high optical losses. Moth-eye structures elaborated by plasma etching offers a simple and low cost way for conferring broadband antireflective properties to protection optics. In this study, we optimized this technology and compared it to “standard” antireflective coatings elaborated by electron-beam evaporation: MgF2 single layer and broadband SiO2/ZrO2 multilayers.First glass substrates have been treated in order to compare nominal antireflective performances of these three vacuum technologies. Reactive ion etching process was then optimized for maximising transmission in normal and angular incidence in the wavelength range of white LEDs (400–750 nm). Best results with moth-eye structure were achieved on glass substrate etched on both sides: we measured 97,5% of white LED light transmission comparatively to 92% with untreated substrate. In a second time, the influence on antireflective properties of different ageing conditions generated during outdoor normative tests have been studied. Spectrophotometry and surface energy measurement highlighted the degradation of MgF2 coating under high humidity ageing conditions: we noticed a decrease of 1% of antireflective performance. Only moth-eye structures gave high environmental stability as they kept their white LED light transmission after ageing tests. In a third time, we introduced a novel comparison approach for such “technological study”: These three antireflective vacuum solutions were confronted from an economical and environmental impact point of view. Moth-eye structures elaborated by plasma etching offer a simple and low cost technology. Nevertheless, the use of greenhouse gas in this reactive ion etching process could be problematic for a large-scale industrialization.

Measurements and Modelization of the Rosario City Heat Island, Argentina - Preliminary Results

In relation to climate, cities introduce a perturbation with respect to the adjacent (non-urban) lands, since houses, buildings, industries, streets, among others, are made of materials that usually have different optical and thermal properties. In particular, they storage heat in a larger fraction than bare land. Consequently, it is of interest to analyse the main variable that can characterize this difference, the ambient temperature. We present results of the measurements done with a thermometer (with an estimated maximum uncertainty of 0.2°C) during the period June 2013 – June 2014, in different months of the year with clear sky and non-intense windy climatic conditions. The data were obtained at noon (around 13:00 local hour = Universal Time – 3 hours), traversing the city in the East-West and North-South directions. Particularly, in this work, we analyse the ambient temperature variation between the central dry Montenegro plaza and the Green Urquiza park. The first one is located among medium-height buildings (3 stories as a mean), while the second one is placed about 1770 meters away, in the border of the city, very near the coast of the large Paraná river (having it 60 km wide, including the delta islands). The mean temperature difference in the yearly investigated period was +0.7°C, being higher in the central dry plaza than in the green park. Results for different climatic periods are: for autumn-winter, +0.3°C and for spring-summer, +1.1°C. A possible explanation of this behaviour is the different angular incidence of solar radiation at the altitudinal level that air temperature is measured, being lower in autumn-winter (with the Sun at approximately 40° mean elevation angle) than in spring-summer (~70° mean elevation angle). We have also made model calculations, employing the Urban Weather Generator (UWG) model, developed by Bueno et al. at Massachusetts Institute of Technology, USA. As for the introduction of the climatological and building parameters, we have used climatological data from a rural meteorological station in the Energy Plus weather file format (.epw) and a parametric description of urban area in .xml format. We have also obtained a representation of the temperature behaviour in the central plaza, considering three scenarios. Scenario A represents the rural climate; scenario B portrays the current situation and scenario C verifies the Energy efficiency and solar protection in building N° 8757 Ordinance introduced for the first time in Argentina, by the Municipality of Rosario city. The highest temperature was found in scenario B, being 2.5 °C higher than in scenario A. Scenario C presents a temperature 1.5 °C higher than scenario A. In conclusion, the urban heat island data obtained is of interest in the design of acclimatisation systems (since normally the non-urban airport temperature data is employed). Also, they can be used as a simulation at present of what will happen in the future, due to the temperature increase produced by the emission of greenhouse gases, responsible of the global warming.

Angular dependency on heavy-ion-induced single-event multiple transients (SEMT) in 65 nm twin-well and triple-well CMOS technology

The extreme reduction in minimum transistor-to-transistor spacing has resulted in multiple transistors being often affected by a single ion strike. The charge sharing becomes a prevalent phenomenon, and it usually brings about single-event multiple transients (SEMT) in combinational logic circuits. In this paper, the angular dependency of heavy-ion-induced SEMT is characterized in heavy-ion experiments. We find that angular heavy-ion incidence can induce single-event five transients in 65 nm bulk CMOS technology. Moreover, the characteristics of SEMT are different in twin-well and triple-well technology.

Omnidirectional Reflector based on Transparent Oxide Materials for Light Extraction Enhancement of Organic and Inorganic Solid-state Light Emitting Devices

High performance reflector is significant structure for efficiency improvement of organic and inorganic solid-state light emitting devices because a large difference in the refractive index between the light source and air leads to multiple internal reflections at the reflector. Then we focused on wide angular light incidence and designed a total internal omnidirectional reflector (TI-ODR) based on transparent oxide layers. The simulation results showed that light extraction structure with TI-ODR gives 17% and 4% efficiency enhancement compared to typical Ag based mirror for organic LED (n=1.8) and inorganic LED (n=2.4), respectively. Then the TI-ODR on blue LED structure was fabricated and that showed enhancement effect of about 8%. The layers are composed of conductive layer for current distribution, low-index thick layer for total internal reflection (TIR) of wide angular light, and high-index layer for the reinforcement of adhesion of layers.

Ballistic transport in graphene Y-junctions in transverse electric field

We investigate the prospects for current modulation in single layer graphene Y-junctions in the ballistic regime, under an external electric field. Overcoming the inability of inducing field effect in graphene nanoribbons by a stacked gate, the proposed in-plane electric field setup enables a controlled current transfer between the branches of the Y-junction. This behavior is further confirmed by changing the angular incidence of the electric field. The ballistic transmission functions are calculated for the three terminal system using the non-equilibrium Green’s function formalism, in the framework of density functional theory, under finite bias conditions. The edge currents dominating the transport in zigzag nanoribbons are strongly influenced by the induced dipole charge, facilitating the current modulation even for the metallic-like character of the Y-junctions. Spin polarization effects indicate the possibility of achieving spin filtering even in the absence of the external field provided the antiferromagnetic couplings between the edges are asymptotically set. Overall, our results indicate a robust behavior regarding the tunability of the charge current in the two outlet ports, showing the possibility of inducing field effect control in a single layer graphene system.

Ground Based Bistatic Scatterometer Measurement for the Estimation of Growth Variables of Ladyfinger Crop at X-Band

The present study describes the ground based bistatic scatterometer measurements of ladyfinger crop at its various growth stages in the specular direction with the azimuthal angle ( $$ \phi = 0 $$ ) for the angular incidence angle ranging from 20° to 60° at the interval of 10° at HH and VV polarization. An outdoor ladyfinger crop bed of an area 4 × 4 m2 was specially prepared for the ground based bistatic scatterometer measurements. The crop growth variables like vegetation water content, leaf area index, fresh biomass, and plant height were also measured at the time of each bistatic scatterometer measurement. The specular bistatic scattering coefficients were found to be decreasing with the crop growth variables up to the maturity stage and then after it increased slightly. The linear regression analysis was carried out between specular bistatic scattering coefficient and crop growth variables at all the incidence angles for HH and VV polarization to select the optimum angle of incidence and polarization for the estimation of crop growth variables. The potential of subtractive clustering based adaptive neuro-fuzzy inference system was applied for the estimation of crop growth variables. The estimated values for different crop growth variables were found almost close to the observed values.

Can the narrow red bands of dragonflies be used to perceive wing interference patterns?

Despite numerous studies of selection on position and number of spectral vision bands, explanations to the function of narrow spectral bands are lacking. We investigate dragonflies (Odonata), which have the narrowest spectral bands reported, in order to investigate what features these narrow spectral bands may be used to perceive. We address whether it is likely that narrow red bands can be used to identify conspecifics by the optical signature from wing interference patterns (WIPs). We investigate the optical signatures of Odonata wings using hyperspectral imaging, laser profiling, ellipsometry, polarimetric modulation spectroscopy, and laser radar experiments. Based on results, we estimate the prospects for Odonata perception of WIPs to identify conspecifics in the spectral, spatial, intensity, polarization, angular, and temporal domains. We find six lines of evidence consistent with an ability to perceive WIPs. First, the wing membrane thickness of the studied Odonata is 2.3 μm, coinciding with the maximal thickness perceivable by the reported bandwidth. Second, flat wings imply that WIPs persist from whole wings, which can be seen at a distance. Third, WIPs constitute a major brightness in the visual environment only second after the solar disk. Fourth, WIPs exhibit high degree of polarization and polarization vision coincides with frontal narrow red bands in Odonata. Fifth, the angular light incidence on the Odonata composite eye provides all prerequisites for direct assessment of the refractive index which is associated with age. Sixth, WIPs from conspecifics in flight make a significant contribution even to the fundamental wingbeat frequency within the flicker fusion bandwidth of Odonata vision. We conclude that it is likely that WIPs can be perceived by the narrow red bands found in some Odonata species and propose future behavioral and electrophysiological tests of this hypothesis.

What is a troland?

Trolands are a widely used measure of retinal illuminance in vision science and visual optics, but disagreements exist for the definition and interpretation of this photometric unit. The purpose of this communication is to resolve the confusion by providing a sound conceptual basis for interpreting trolands as a measure of angular flux density incident upon the retina. Using a simplified optical analysis, we show that the troland value of an extended source is the intensity in micro-candelas of an equivalent point source located at the eye's posterior nodal point that produces the same illuminance in the retinal image as does the extended source. This optical interpretation of trolands reveals that total light flux in the image of an extended object is the product of the troland value of the source and the solid angle subtended by the source at the first nodal point, independent of eye size.

Calibration of a thermoluminescent dosimeter worn over lead aprons in fluoroscopy guided procedures

Fluoroscopy guided interventional procedures provide remarkable benefits to patients. However, medical staff working near the scattered radiation field may be exposed to high cumulative equivalent doses, thus requiring shielding devices such as lead aprons and thyroid collars. In this situation, it remains an acceptable practice to derive equivalent doses to the eye lenses or other unprotected soft tissues with a dosimeter placed above these protective devices. Nevertheless, the radiation backscattered by the lead shield differs from that generated during dosimeter calibration with a water phantom. In this study, a passive personal thermoluminescent dosimeter (TLD) was modelled by means of the Monte Carlo (MC) code Penelope. The results obtained were validated against measurements performed in reference conditions in a secondary standard dosimetry laboratory. Next, the MC model was used to evaluate the backscatter correction factor needed for the case where the dosimeter is worn over a lead shield to estimate the personal equivalent dose Hp(0.07) to unprotected soft tissues. For this purpose, the TLD was irradiated over a water slab phantom with a photon beam representative of the result of a fluoroscopy beam scattered by a patient. Incident beam angles of 0° and 60°, and lead thicknesses between the TLD and phantom of 0.25 and 0.5 mm Pb were considered. A backscatter correction factor of 1.23 (independent of lead thickness) was calculated comparing the results with those faced in reference conditions (i.e., without lead shield and with an angular incidence of 0°). The corrected dose algorithm was validated in laboratory conditions with dosimeters irradiated over a thyroid collar and angular incidences of 0°, 40° and 60°, as well as with dosimeters worn by interventional radiologists and cardiologists. The corrected dose algorithm provides a better approach to estimate the equivalent dose to unprotected soft tissues such as eye lenses. Dosimeters that are not shielded from backscatter radiation might underestimate personal equivalent doses when worn over a lead apron and, therefore, should be specifically characterized for this purpose.

Omnidirectional Reflector with Total Internal Reflective Interface for Light Extraction Enhancement of Solid‐State Light Source

Light extraction efficiency is key to the performance of solid‐state light sources such as the light‐emitting diode. The reflectance of the bottom mirror has a significant impact on the light extraction efficiency because a large difference in the refractive index between the light source and air leads to multiple internal reflections and optical loss. Therefore, efficiency can be drastically improved with even a slight increase in the bottom mirror reflectance. This paper focuses on wide angular light incidences. An omnidirectional reflector (ODR) is then designed that included a low‐index thick layer (MgF2), a pair of quarter‐wavelength‐thick layers (Ta2O5/MgF2), and a metal layer (Ag) on a solid‐state light source with n = 2.4. The first thick layer works as a perfect reflector for wide angular and broadband incident light since total internal reflection (TIR) occurs at the interface. In the simulations, the angular integrated reflectance for our designed reflector (TI‐ODR) reaches 99.7%. The reflectance is higher than the typical ODR that is composed of a quarter‐wavelength‐thick layer and Ag (98.6%) or an Ag reflector alone (97.9%). The TI‐ODR is then fabricated onto a gallium nitride (GaN)‐based light‐emitting diode and obtained a light extraction efficiency enhancement factor of approximately 1.1.

Angular Effects of Heavy-Ion Strikes on Single-Event Upset Response of Flip-Flop Designs in 16-nm Bulk FinFET Technology

Radiation particles are incident on an integrated circuit (IC) from all angles. For planar technologies, angular incidence increases the deposited charge in a given volume, resulting in higher collected charge at a node and more transistors collecting charge due to increased charge sharing. For FinFET technologies, the physical structure of a FinFET is very different from that of a planar transistor. As a result, deposited and collected charge at a node for angular incidences will be different from what has been published for planar technologies. 3D TCAD simulations and heavy-ion experiments were carried out to investigate the angular effects on flip-flop (FF) single-event upsets (SEU) at the 16-nm bulk FinFET technology. Results show different SEU cross-section trends for the FinFET technology compared to planar technologies. Results show increased upset probability and SEU cross-sections with increasing tilt angles, but those are reduced with increasing roll angles for low-LET heavy-ion incidence. The main reason for this behavior is posited to be variations in charge track length within active Si regions.

Enhanced Absorption and &lt;1% Spectrum-and-Angle-Averaged Reflection in Tapered Microwire Arrays

We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of <1% of the incident 400–1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0° to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays. Time-resolved microwave photoconductivity decay measurements yielded charge-carrier lifetimes of 0.75 μs (more than an order of magnitude higher than vapor–liquid–solid-grown Si microwires) in the tapered microwires, resulting in an implied Voc of 0.655 V. The high absorption and high aspect ratio in these ordered microwire arrays make them an attractive platform for high-efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.