Near-normal Incidence Research Articles

Characterization of Pyromark 2500 Paint for High-Temperature Solar Receivers

Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The radiative properties, aging, and selective absorber efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective absorber coatings currently being developed. The solar absorptance ranged from ∼0.97 at near-normal incidence angles to ∼0.8 at glancing (80°) incidence angles, and the thermal emittance ranged from ∼0.8 at 100 °C to ∼0.9 at 1000 °C. After thermal aging at temperatures of ∼750 °C or higher, the solar absorptance decreased by several percentage points within a few days. It was postulated that the substrate may have contributed to a change in the crystal structure of the original coating at elevated temperatures.

Morphology evolution of fused silica surface during ion beam figuring of high-slope optical components

Ultra-precision and ultra-smooth surfaces are vitally important for some high performance optical systems. Ion beam figuring (IBF) is a well-established, highly deterministic method for the final precision figuring of extremely high quality optical surfaces, whereas ion sputtering induced smoothing, or roughening for nanoscale surface morphology, strongly depends on the processing conditions. Usually, an improper machining method would arouse the production of nanoscale patterns leading to the coarsening of the optical surface. In this paper, the morphology evolution mechanism on a fused silica surface during IBF of high-slope optical components has been investigated by means of atomic force microscopy. Figuring experiments are implemented on two convex spherical surfaces by using different IBF methods. Both of their surface errors are rapidly reduced to 1.2 nm root mean square (RMS) after removing similar deep material, but their surfaces are characterized with obviously different nanoscale morphologies. The experimental results indicate that the ion incidence angle dominates the microscopic morphology during the IBF process. At near-normal incidence, fused silica achieves an ultra-smooth surface with an RMS roughness value R(q) down to 0.1 nm, whereas nanoscale ripple patterns are observed at a large incidence angle with an R(q) value increasing to more than 0.9 nm. Additionally, the difference of incidence angles on various machined areas would influence the uniformity of surface quality, resulting from the interplay between the smoothing and roughening effects induced by ion sputtering.

Comparisons of angularly and spectrally resolved Bremsstrahlung measurements to two-dimensional multi-stage simulations of short-pulse laser-plasma interactions

A 2-D multi-stage simulation model incorporating realistic laser conditions and a fully resolved electron distribution handoff has been developed and compared to angularly and spectrally resolved Bremsstrahlung measurements from high-Z planar targets. For near-normal incidence and 0.5-1 × 1020 W/cm2 intensity, particle-in-cell (PIC) simulations predict the existence of a high energy electron component consistently directed away from the laser axis, in contrast with previous expectations for oblique irradiation. Measurements of the angular distribution are consistent with a high energy component when directed along the PIC predicted direction, as opposed to between the target normal and laser axis as previously measured.

Approximated method for modelling hemispherical reflectance and evaluating near-specular reflectance of CSP mirrors

Commercial first-surface solar-mirrors, which are alternatives to conventional glass mirrors, reflect solar radiation in a broader manner because of light scattering. However, in concentrated solar power all the radiation reflected in the solid angle of receiver-viewing is useful. This makes a mirror-evaluation methodology which takes into account not only specular, but also near-specular reflectance, mandatory. Two laminated mirrors and one traditional were analysed with a new method based on near-normal incidence reflectance measurements of the hemispherical spectrum, and near-specular at 405.5, 543.5 and 632.8nm, for several acceptance angles. For a given acceptance angle, the wavelength-behaviour of the ratio near-specular/hemispherical is found well modelled by the well-known Total Integrated Scattering relationship. The angular-behaviour of the hemispherical reflectance is predicted by a simplified optical model of the mirror. As final result, the behaviour of solar reflectance versus the incidence-angle over the investigated acceptance-angle range was obtained.

Polarized infrared reflectance spectra of brushite (CaHPO4⋅2H2O) crystal investigation of the phosphate stretching modes

Polarized infrared (IR) reflectance measurements at near-normal incidence were recorded from the ac-plane of a monoclinic brushite (CaHPO4⋅2H2O) crystal in the 800–1200cm−1 spectral range (P–O stretching modes). The adjustment of these data, on the basis of a dispersion analysis (DA) model for monoclinic case, allowed the determination of oscillators parameters for the four P–O stretching observed modes of the phosphate group.

Al/Zr multilayer mirror and its thermal stability for EUV application

Two kinds of Al/Zr (Al(1%wtSi)/Zr and Al(Pure)/Zr) multilayers in the region of 17-19nm were deposited on fluorine doped tin oxide coated glass by using direct-current magnetron sputtering technology. Based on the fitting data of grazing incident X-ray reflection and near-normal incident (EUV) reflectance, the interfacial roughness in the Al(1%wtSi)/Zr is lower than that in Al(Pure)/Zr because of the presence of silicon in Al. For the further characterization of Al(1%wtSi)/Zr multilayers, six samples deposited on Si substrates were annealed from 100 °C to 500 °C temperature in a vacuum furnace for 1 h. Based on the results of EUV and X-ray diffraction, the Al(1%wtSi)/Zr multilayer has a stable structure up to 200 °C, and keeps almost the similar EUV reflectivity as the non-annealed sample. After 300 °C, the amorphous of Al-Zr alloy is transformed to polycrystalline in the interface, which could be the reason for the decrease of EUV reflectivity. The polycrystalline Al-Zr compound does not destroy the multilayer completely even up to 500 °C.

High performance La/B4C multilayer mirrors with barrier layers for the next generation lithography

Results on optimization and manufacturing of La/B4C multilayers are reported. Such mirrors are promising optical elements for the development of the next generation of nano-lithography at an operation wavelength of 6.7 nm. A near-normal incidence reflectivity of up to 58.6% has been measured at the BESSY-II soft x-ray reflectometer beamline. This is the highest reflectivity measured so far at this wavelength. The potential to further increase the reflectivity of the multilayers is discussed.

Triple-wavelength, narrowband Mg/SiC multilayers with corrosion barriers and high peak reflectance in the 25-80 nm wavelength region

We have developed new, Mg/SiC multilayer coatings with corrosion barriers which can be used to efficiently and simultaneously reflect extreme ultraviolet (EUV) radiation in single or multiple narrow bands centered at wavelengths in the spectral region from 25 to 80 nm. Corrosion mitigation was attempted through the use of Al-Mg or Al thin layers. Three different multilayer design concepts were developed and deposited by magnetron sputtering and the reflectance was measured at near-normal incidence in a broad spectral range. Standard Mg/SiC multilayers were also deposited and measured for comparison. They were shown to efficiently reflect radiation at a wavelength of 76.9 nm with a peak reflectance of 40.6% at near-normal incidence, the highest experimental reflectance reported at this wavelength for a narrowband coating. The demonstration of multilayer coatings with corrosion resistance and multiple-wavelength EUV performance is of great interest in the development of mirrors for space-borne solar physics telescopes and other applications requiring long-lasting coatings with narrowband response in multiple emission lines across the EUV range.

Scattering of High-Incident-Energy Kr and Xe from Ice: Evidence that a Major Channel Involves Penetration into the Bulk

The scattering of high kinetic energy (1–6.5 eV) xenon and krypton atoms from ice was experimentally measured and theoretically modeled. The ice efficiently accommodates translational energy, but the extent to which the energy is quenched suggests the mechanism for the highest energies and near-normal incidence angles involves more than interaction with just the molecules at the ice surface. Simulations show that for these conditions the xenon penetrates into the selvedge. This penetration into the solid manifests experimentally in that the postcollision translational energy is essentially independent of the incident translational energy. This observation is in stark contrast with what is usually the situation for scattering from a surface. Finally, postexposure desorption measurements showed that some of the incident gas atoms were trapped in the bulk of the ice at temperatures well above where the absorption is thermodynamically stable. The above evidence leads to the conclusion that under conditions of...

Near-Normal Incidence Dark-Field Microscopy: Applications to Nanoplasmonic Spectroscopy

The spectroscopic characterization of individual nanostructures is of fundamental importance to understanding a broad range of physical and chemical processes. One general and powerful technique that addresses this aim is dark-field microscopy, with which the scattered light from an individual structure can be analyzed with minimal background noise. We present the spectroscopic analysis of individual plasmonic nanostructures using dark-field illumination with incidence nearly normal to the substrate. We show that, compared to large incidence angle approaches, the near-normal incidence approach provides significantly higher signal-to-background ratios and reduced retardation field effects. To demonstrate the utility of this technique, we characterize an individual chemically synthesized gold nanoshell and a lithographically defined heptamer exhibiting a pronounced Fano-like resonance. We show that the line shape of the latter strongly depends on the incidence angle. Near-normal incidence dark-field microscopy can be used to characterize a broad range of molecules and nanostructures and can be adapted to most microscopy setups.

Molecular dynamics simulation of low-energy bombardment of Pt (1 1 1) surface by Cu atoms with various incident angles

The low-energy bombardment of Pt (111) surface by Cu atoms with various incident angles (θ) is studied with MD simulations. In the case of near-normal incidence (θ≤20°), the result of energy deposition is similar to that of θ=0°. In contrast, in the case wherein the incident angles are higher than 60°, the incident atom cannot penetrate through the first layer and is scattered directly on the surface. The low-energy deposition has no obvious effect on the substrate. For 20°≤θ≤60°, the oblique incidence contributes to uniformity of nucleation and layer-by-layer growth of film as well as the layer-by-layer removal of atoms in the surface layers. Based on our MD simulations, the mechanism behind the deposition and thin film formation is related to the horizontal component and the vertical component of the impact momentum.

Differential Detection for Nanoplasmonic Resonance Sensors

A differential measurement design employing two nearly collinear optical beams can lead to surface plasmon polariton (SPP) sensors of increased dynamic range and signal-to-noise ratio. We demonstrate a differential measurement device that is based on wavelength interrogation, employs a single incident polarization state, and is combined with a 2-D nanohole array for operation at near-normal incidence, where this approach offers a decrease in the measurement time.

Erosion/accretion patterns and multiple beach cusp systems on a meso-tidal, steeply-sloping beach

This study examines daily variations in intertidal topography and beach cusps on a meso-tidal, steeply sloping beach during a five-month period. The observations are based on coastal imagery and frequent topographic surveys and are extracted from patterns of contours positioned 1 and 3m above MSL (lower/higher beach-face, respectively). The intertidal topography was very sensitive to variations in the wave regime, triggering rapid transitions from erosive to accretive states and vice versa, as well as transformations of the cusp systems, often involving severe horn erosion and beach-face scarping. Distinct beach cusp systems were observed at different levels of the beach, with their evolution being controlled by: (i) wave forcing; (ii) tidal modulation of the wave power at different levels of the beach; and (iii) interactions between the existing beach cusp systems. The lower beach-face rhythmical patterns changed constantly and merged with the upper beach-face cuspate features, typically when the significant wave height exceeded Ho>1.5m. The upper beach-face pattern was persistent during the monitoring period, re-emerging with similar spacing and alignment, despite being eroded during energetic events; this persistence is also maintained by the interactions between swash and irregularities (e.g. embayments) in the dune/backshore morphology. The spacing of the observed beach cusps ranged from λc,min=8m to λc,max=67m; these values were underestimated by the edge wave and self-organization theories and were better predicted by the Sunamura (2004) formula. Beach cusp growth was linked to breaker angles of near-normal incidence and narrow-banded frequency wave conditions. Bay infilling was found to be the most dominant cusp reduction mechanism for larger features (λc>30m), while smaller features vanished during erosive conditions. The most intense cusp growth episodes were linked to accretive conditions, while larger features were shown to emerge during more energetic, erosion events. These findings highlight the need for more longer-term observations of diverse beach morphologies to formulate a global theory regarding beach cusp formation and evolution.

Molecular dynamics simulation study of the growth of a rough amorphous carbon film by the grazing incidence of energetic carbon atoms

The morphological evolution of an amorphous carbon film deposited by energetic carbon atoms of 75 eV with various angles of incidence was investigated by molecular dynamics simulation. Normal or near-normal incidence of carbon atoms resulted in a smooth surface of the deposited film. In contrast, a bump-like surface structure emerged and led to rough surfaces at grazing incidences, in agreement with the experiments. The bifurcated growth mode was explained by the impact-induced transport of atoms on the growing surface. The downhill transport of atoms on a sloping surface dominates at normal incidence, which suppresses the evolution of surface irregularities to form a rough surface. However, the dominance of uphill transport at a grazing incidence made the surface irregularities grow to a seed structure, which provided the shadowing effect during carbon deposition. This mechanism mediates initial seed formation and subsequent roughening together with shadowing effects under grazing incidence.

Grazing incidence optics for X-ray astronomy

Cosmic X-ray sources are usually very weak and their detection, therefore, needs large area telescopes to gather light and sensitive detectors to enhance quantum efficiency. Conventional telescopes for visible light use refracting or reflective optics which is impractical for X-ray wavelengths because photon energies are greater than the binding energies of the typical atomic electrons leading to a refractive index for X-rays being less than unity. Thus single surface reflectivity for near-normal incidence is negligible for X-rays. However, by Snell’s Laws, total external reflection occurs and X-rays can be reflected from a surface up to a critical angle (usually about a degree for energies below 10 keV) given by cosine θ = n. This is known as the grazing angle. X-ray telescopes are made to exploit the grazing incidence from a set of co-axial and con-focal shells of paraboloidal and hyperboloidal mirrors. X-ray reflectors having high atomic number surfaces with low scattering are used to realize imaging capability for a telescope. I describe here various configurations required, and the various technologies used and their limitations, to make practical X-ray telescopes. A soft X-ray imaging telescope (SXT) using grazing incidence has been built at TIFR for ASTROSAT—an Indian Multiwavelength Satellite designed to cover a very broad band of X-rays, UV and optical. Astrosat is planned to be launched by a Polar Satellite Launch Vehicle in 2012 into a near-Earth Equatorial orbit. I will also describe the ongoing R&D for realizing telescopes for hard X-rays above 10 keV useful for both Astronomy and medical diagnostics.

Near-100% Bragg reflectivity of X-rays

Materials that exhibit strong reflectivity of hard X-rays at normal incidence are sought after for components such as hard-X-ray cavities, beamsplitters and delay lines. Here, researchers experimentally demonstrate hard-X-ray reflectivities of more than 99% from diamond crystals at near-normal incidence.

Characterization of a spherically bent quartz crystal for Kα x-ray imaging of laser plasmas using a focusing monochromator geometry

We have measured the key spectrometric properties (peak reflectivity, reflection curve width, and Bragg angle offset) of a spherically bent quartz 200 crystal using the x-ray emission from a laser-produced Ar plasma. This crystal can image Ar Kα x-rays at near-normal incidence (θB ≈ 81 degrees); our technique operates the same crystal as a high-throughput focusing monochromator on the Rowland circle at angles far from normal incidence (θB ≈ 68 degrees) to make a reflection curve with He-like x-rays from the same laser plasma. This approach, which is applicable to many commonly imaged x-ray emission lines and corresponding spherically bent crystals, permits the experimentalist to obtain an in-situ crystal characterization in the same reflection order as that used for operation.

Self-organized patterning on Si(001) by ion sputtering with simultaneous metal incorporation

This report focuses on the effect of simultaneous Fe incorporation in the self-organized pattern formation on Si(001) by low-energy ion-beam sputtering. Experimental observations giving evidence for the correlation between different ion-beam parameters, Fe concentration on the substrate, and the resulting topography are shown. It was observed that the incorporation of Fe affects the evolution of the topography and it is a requisite for the formation of ripples at near-normal incidence. It is shown also that Fe is not homogeneously distributed on the surface, but there is a higher concentration at the crests of the ripples than at the valleys. For the experimental setup used for this study, the Fe flux that reaches the surface is determined mainly by the acceleration voltage (U acc), while the ion energy (E ion) and ion-beam incidence angle (α) control the concentration of Fe in the steady state. The adjustment of these operational parameters of the ion source enables the fine-tuning of surface patterns.

Mapping the ionization state of laser-irradiated Ar gas jets with multiwavelength monochromatic x-ray imaging

Two-dimensional monochromatic images of fast-electron stimulated Ar Kα and He-α x-ray self-emission have recorded a time-integrated map of the extent of Ar(≈6+) and Ar(16+) ions, respectively, within a high density (10(20) cm(-3) atomic density) Ar plasma. This plasma was produced by irradiating a 2 mm wide clustering Ar gas jet with an ultrahigh intensity (10(19) W/cm(2), 50 TW) Ti:sapphire laser operating at 800 nm. Spherically bent quartz crystals in the 200 (for Kα) and 201 (for He-α) planes were used as near-normal incidence reflective x-ray optics. We see that a large (830 μm long) region of plasma emits Kα primarily along the laser axis, while the He-α emission is confined to smaller hot spot (230 μm long) region that likely corresponds to the focal volume of the f/8 laser beam. X-ray spectra from a Bragg spectrometer operating in the von Hamos geometry indicate that the centroids of the Kα and He-α emission regions are separated by approximately 330 μm along the laser axis.

Low energy Ar+ bombardment of GaN surfaces: A statistical study of ion reflection and sputtering

Statistical molecular dynamics simulations are performed to analyze the sputtering of w-GaN (wurtzite) and z-GaN (zinc blende) surfaces under 100 eV Ar+ ion bombardment. Ion reflection and physical sputtering mechanisms are investigated as a function of the ion impact angle and the crystalline nature of samples. The probability of ion reflection is lower for the w-GaN phase and increases with the angle of incidence θi. As θi becomes more glancing, the reflected ions become more energetic and their angular distribution tends to narrow. The sputtering yields of w-GaN and z-GaN surfaces are maximum for θi=45°. For near-normal incidence, the probability of sputtering is smaller for the w-GaN phase, suggesting that the atomic arrangement in the pristine state modifies the characteristics of the momentum transfer occurring between the ion and the surface atoms during the collision cascade. Atomic nitrogen sputters preferentially and represents 87% to 100% of sputtered species due to its lower mass. These statistical results differ from the predictions of continuous ion bombardment simulations since the surfaces are not allowed to evolve self-consistently during the gathering of impact statistics.