Design of a high-resolution extreme-ultraviolet imaging spectrometer with aberration-corrected concave gratings

A high-resolution extreme-ultraviolet imaging spectrometer is designed for the Japanese solar mission Solar-B. A spherical varied-line-space (SVLS) grating and a toroidal uniform-line-space (TULS) grating are chosen as candidates for use in the spectrometer to yield high spectral and spatial resolution within the spectral range 25-29 nm. The spectral image-focusing properties and the mechanical tolerances for fabrication and alignment are compared for the two types of grating. The SVLS design is found to be superior to the TULS design for off-plane spectral images and in ease of fabrication and optical alignment.     

Design for an aberration-corrected concave grating for a mid-infrared long-slit spectrometer

A new design for an aberration-corrected concave grating for the spectral region near 10 mum is presented. It was designed for use in the ground-based astronomical medium-resolution (lambda/Dlambda ~ 100) Mid-Infrared Camera and Spectrometer (MICS). It provides a flat focal plane for a wide spectral range (7.5-13.5 mum) with small aberrations, permitting efficient long-slit observations in the mid-infrared region. It permits a simple design of the spectrometer without collimator and camera mirrors, which is quite advantageous for cryogenic instruments. The grating has variable spacing grooves to reduce aberrations. In addition, the grating surface figure is designed to be toroidal and in the direction perpendicular to the grooves, aspherical, to suppress the aberrations further over a wide spectral range. The angle of the grooves is also varied to yield better efficiency near the blaze angle. The grating was fabricated by high-quality ultraprecision machining, which made these features possible. Test observations confirmed that the designed spectral resolution was achieved.     

Effects of modulation strength in guided-mode resonant subwavelength gratings at normal incidence

A comparative study of the reflection spectral resonances in weakly and strongly modulated subwavelength gratings is presented. The effects of strong modulation in resonant subwavelength gratings have been largely ignored in the literature. We show that the spectral stability of resonances as a function of angle of incidence around normal can be greatly enhanced with strongly modulated gratings while the desirable narrow line-width associated with weakly modulated gratings is still maintained.     

Technique for narrow-band imaging in the far ultraviolet based on aberration-corrected holographic gratings

We have developed a new family of imaging spectrometer designs that combine the imaging power of two-element telescopes with the aberration control of first-generation holographic gratings. The resulting optical designs provide high spatial resolution over modest fields of view at selectable wavelengths. These all-reflective designs are particularly suited for narrow-band imaging below 1050 A, the wavelength below which there are no transmitting materials in the UV. We have developed designs to efficiently map the spatial distribution of UV-emitting material. This mapping capability is absent in current and future astronomical instruments but is crucial to the understanding of the nature of a variety of astrophysical phenomena. Although our examples focus on UV wavelengths, the design concept is applicable to any wavelength.     

Antireflection quarter-wave-thick, high-spatial-frequency, surface-relief gratings at oblique incidence

Zero-reflectance conditions for quarter-wave-thick, high-spatial-frequency, rectangular-groove dielectric gratings are analyzed further at oblique incidence by a combination of the effective medium theory and the anisotropic thin-film theory. Numerical examples are given for gratings on glass and silicon substrates with refractive indices of 1.5 and 3.5, respectively.     

High-efficiency MoRu-Be multilayer-coated gratings operating near normal incidence in the 11.1-12.0-nm wavelength range

MoRu-Be multilayer coatings were applied to two diffraction gratings for the purpose of enhancing their normal-incidence efficiency in the 11.1-12.0-nm wavelength range. The grating substrates were replicas of a holographic master grating that had a blazed groove profile with 2400 grooves/mm and a 2-m radius of curvature. The relatively low average microroughness (0.8 nm) of the grating surfaces contributed to the relatively high groove efficiency of the grating substrates and the reflectance of the MoRu-Be multilayer coatings. The peak efficiency, measured with synchrotron radiation, was 10.4% in the second diffraction order at a wavelength of 11.37 nm.     

Erosion of MgO by solid particle impingement at normal incidence

A study has been made of the erosion of almost fully dense, fine-grained MgO by millimeter-scale WC-6 wt% Co spheres impinging at normal incidence with velocities between 10 and 90 m sec^–1. Scanning electron microscopy showed that the damage consisted of a central crater surrounded by an array of intergranular radial and/or median and/or lateral cracks characteristic of an elastic-plastic impact. The crater had a thin lining of plastically deformed material, but appeared to have been formed primarily by localized transgranular and integranular fracture processes, suggesting that any mode of irreversible deformation in the contact region will suffice to produce the changeover from Hertzian cracking to radial, median and lateral cracking. The accompanying gravimetric studies showed that mass loss, which was caused primarily by intersection of lateral cracks with the free surface and with radial and/or median cracks, increased threefold during the short incubation period in which the as-received surface evolved into its steady-state eroded condition. During this period the exponent relating erosion to impact velocity decreased asymptotically towards a value smaller than that predicted by any current theory of erosion in the elastic or the elastic-plastic impact regime. Nor are these theories any more successful at explaining the observed particle-size dependence of the erosion of polycrystalline MgO.     

Suppressed light transmission through corrugated metal films at normal incidence

In the study of light transmission through corrugated metallic film, it has been shown that along with the enhancement of light transmission, suppression may take place at specific values of the period and magnitude of corrugation for normal light incidence. Suppression was found to be due to the interplay between symmetric and assymetric surface plasmon polariton modes at their simultaneous excitation.     

Characteristics of ray traces at the back of biaxial crystals at normal incidence

Ray vector traces that can be obtained at the back of a biaxial crystal through crystal rotation are calculated numerically by variation of one of the three Euler angles. Negative and positive lossless biaxial crystals are considered at normal incidence. Interesting features of these traces are evidenced by reference to the circular ring that forms at internal conical refraction.     

Scattering by a dense layer of infinite cylinders at normal incidence

The solution for scattering by a layer of densely distributed infinite cylinders is presented. The layer is irradiated by an arbitrarily polarized plane wave that propagates in the plane perpendicular to the axes of the cylinders. The theoretical formulation utilized the effective field and quasi-crystalline approximation to treat the multiple scattering interactions in the dense finite medium. Governing equations for the propagation constants and amplitudes of the effective fields are derived for TM and TE mode incident waves, from which the scattered intensity distribution and scattering cross section for arbitrary polarization are obtained. The dense medium gives rise to coherent and incoherent scattered radiation that propagates in the plane normal to the axes of the cylinders. The coherent scattered radiation includes the forward component in the direction of the incident wave and the backward component in the direction of specular reflection. The incoherent scattered intensity distribution shows a pronounced forward peak that coincides with the angle of refraction of the effective waves inside the medium. Numerical results are presented to illustrate the scattering characteristics of a dense layer of cylinders as a function of layer thickness for a given solid volume fraction.     

Illustration of the use of multimode deformable plane mirrors to record high-resolution concave gratings: results for the Cosmic Origins Spectrograph gratings of the Hubble Space Telescope

To illustrate the efficiency of using a deformable plane mirror to record holographic gratings, we have computed the three gratings for the Cosmic Origins Spectrograph. Their working conditions are severe, since they have to correct the residual spherical aberration of the Hubble Space Telescope. Nevertheless, all images obtained are largely diffraction limited with regard to the resolution.     

Infrared quarter-wave reflection retarders designed with high-spatial-frequency dielectric surface-relief gratings on a gold substrate at oblique incidence

One- and two-dimensional high-spatial-frequency dielectric surface-relief gratings on a Au substrate are used to design a high-reflectance quarter-wave retarder at 70° angle of incidence and 10.6-μm light wavelength. The equivalent homogeneous anisotropic layer model is used. It is shown that equal and high reflectances (>98.5%) for the p and the spolarizations and quarter-wave retardation can be achieved with two-dimensional ZnS surface-relief gratings. Sensitivities to changes of incidence angle, light wavelength, grating filling factor, and grating layer thickness are considered.     

Numerical approximation of light scattering from tenuous cylindrical membranes at normal incidence

A numerical method has been developed to calculate the scattered field from a complex matrix of cylindrical membranes that are thin relative to wavelength with a refractive index close to that of the medium. The membranes are broken into a collection of adjacent parallel thin fibers, each fiber is assigned an internal field, and vector summation of the fields scattered by each fiber is used to calculate the total scattered field. For an incident wave in the TM mode the internal field of each membrane fiber is chosen to be equal to the incident field (the Born approximation). For the TE mode the internal field is determined by the local membrane orientation (φ) and relative refractive index (m) according to A sin 2φ for the component in the direction of incidence and 1 - A(1 - cos 2φ)for the transverse component, where A = (m(2) - 1)/2m(2). The method has application to light scattering by cell membranes in bundles of nerve axons.     

Necking and radial cracking around perforations in thin sheets at normal incidence

The formation of multiple necks and cracks around perforations in ductile materials is investigated. Expressions are obtained for the number of plane-strain radial necks formed by conical penetrators (with or without starter holes in the flat target), and also by round-ended projectiles into flat targets having starter holes or, in plain targets with no starter holes, after fracture of the initial circumferential neck has detached a circular cap of material (“discing”). Likewise, expressions are derived for the number of radial cracks which form subsequently in both cases and lead to “petalling”. The number of cracks is smaller than the number of necks. Experiments seem to agree with the analyses, although accurate counting of necks in thin sheets is illusory.     

Simple expressions for transmission and reflection matrix elements of a biaxial thin layer at normal incidence

The extended Jones matrix method is applied for determination of the transmission and reflection matrices for a normally incident plane wave upon an homogeneous and lossless biaxial thin layer. The elements of these matrices are expressed by simple analytical relations. By using these relations one can express analytically the polarization-dependent optical parameters to be determined by generalized ellipsometry.