This PDF file contains the editorial “Editorial: 1990 Rudolph Kingslake Medal and Prize” for OE Vol. 30 Issue 08

This PDF file contains the editorial “Guest Editorial: X-Ray/EUV Optics” for OE Vol. 30 Issue 08

Si/SiO₂ and Si/Si₃N₄ multilayers have been fabricated using a locally made reactive diode ri-sputtering system. The layer alternation is obtained by modulating a partial pressure of oxygen or nitrogen near the sample using a silicon target with argon as sputtering gas. O₂ and N₂ partial pressure conditions were optimized to deposit stoichiometric SiO₂ and Si₃N₄ films without significant reaction with the silicon target. In situ kinetic ellipsometry was used to monitor both thick film and multilayer deposition. The different interfaces appear very sharp with a little contamination of the silicon layers especially using oxygen. The multilayers were characterized by grazing x-ray reflection (Cu-K α line), and the reflectivity was measured in the soft x-ray range (120-350 Å) by synchrotron radiation. Both Si/SiO₂ and Si/Si₃N₄ multilayers exhibit well-defined Bragg peaks with very narrow bandpasses (two to three times lower than the conventional Mo/Si multilayer), and high absolute reflectivities (up to ≅22% at 130 Å). Finally, thermal stability of Si/Si₃N₄ multilayers was evaluated. We did not find any degradation after annealing up to 800°C, which is extremely high compared to conventional Mo/Si multilayers, which are generally destroyed above 500°C.

We have investigated the optimum deposition conditions for fabricating Mo/Si multilayers on Si wafers by means of electron-beam deposition, taking the substrate temperature and deposition rate as parameters. Mo/Si multilayers made at substrate temperatures Of room ternperature to 420°C and at deposition rates of 0.02 to 1.0 nm/s were evaluated by transmission electron micrographs of their cross sections, x-ray diffraction patterns, and their spectral reflectances for soft x rays of 10 to 17 nm. The optimum deposition parameters were found to be 100 to 150°C and 0.02 to 0.2 nm/s, respectively, for substrate temperature and deposition rate. The Mo/Si multilayer made under the optimum condition showed fairly smooth layered structure and a reflectance of ~30% for unpolarized soft x ray of 14.6 nm at an incident angle of 20 deg.

We have developed seven compact soft x-ray/EUV (XUV) multilayer- coated and two compact FUV interference-film-coated Cassegrain and Ritchey-Chrétien telescopes for a rocket-borne observatory, the Multi-Spectral Solar Telescope Array. We report here on extensive measurements of the efficiency and spectral band pass of the XUV telescopes carried out at the Stanford Synchrotron Radiation Laboratory.

Multilayer x-ray optics are currently finding use in several types of applications. Although the performance of silicon-based and carbon-based multilayer mirrors is adequate for some of these applications, new material pairs that would improve performance at several crucial wavelengths are necessary to make many new technologies feasible. We describe a procedure for selecting these new materials that considers both the optical and structural requirements of thin films for high-reflectivity mirrors. We identify material pairs that merit future study for use in both amorphous and superlattice multilayer mirrors. Preliminary results on the growth of two of these new material pairs, Ag/B and Pd_xB_1-x/B, are summarized. In work performed to date on Ag/B, the polycrystalline layers that formed were not structurally perfect enough to form good multilayer mirrors. However, further study of this material pair is warranted. The Pd_xB_1-x/B multilayers were produced by alternately depositing palladium and boron. X-ray and electron diffraction indicate that these multilayers are amorphous and are of high structural quality. Auger electron spectroscopy and x-ray photoelectron spectroscopy experiments on single interface samples indicate that these materials react to form Pd_xB_1-x/B with x ≈ 0.9.

The fabrication of linear, circular, and elliptical reflective zone plate lenses by a multistep process using microfabrication technologies is reported. The diffractive elements were generated by electron-beam lithography on a Mo/C multilayer mirror obtained by means of the triode sputtering technique. The patterns were transferred anisotropically into the multilayer mirror by reactive ion etching in a fluorinated plasma. An intermediate metallic mask made by the lift-off process was used for the transfer process. The groove depth could be monitored by following the reflectivity ofthe structure with a helium-neon laser during etching. Linear, circular (with 0.4-μm outer zones), and elliptical Fresnel zone plates (with 0.8-μ.m outer zones) were produced. Test patterns with a 50-nm top and 90-nm bottom resolution were produced. The groove profile and dimensional control were also investigated.

We describe the development of the water window imaging x-ray microscope based on normal-incidence multilayer x-ray mirrors. The narrow bandpass response inherent in multilayer x-ray optics is accurately tuned to wavelengths within the "water window." Similar doubly-reflecting multilayer optical systems have been fabricated for our astronomical rocket-borne x-ray/EUV telescopes. Previous theoretical studies performed during the MSFC X-Ray Microscope Development Program established that high-resolution multilayer x-ray imaging microscopes are possible by using either spherical (Schwarzschild configuration) optics or aspherical configu rations. These microscopes require ultrasmooth mirror substrates, which have been fabricated using advanced flow polishing methods. Hemlite-grade sapphire microscope optic substrates have been accurately figured and polished to a smoothness of 0.5-Å rms, as measured by the Zygo profilometer. We describe the current status of fabrication and testing of the optical and mechanical subsystems for the water window imaging x-ray microscope. This new instrument should yield images of carbon-based microstructures within living cells of unprecedented spatial resolution and contrast, without need for fixatives, dyes, and chemical additives.

Considerable effort has been devoted recently to the design, analysis, fabrication, and testing of spherical Schwarzschild microscopes for soft x-ray applications in microscopy and projection lithography. The spherical Schwarzschild microscope consists of two concentric spherical mirrors configured such that the third-order spherical aberration and coma are zero. Because multilayers are used on the mirror substrates for soft x-ray applications, it is desirable to have a small number of reflecting surfaces in the microscope. In order to reduce the microscope aberrations and increase the field of view, generalized mirror surface profiles have been considered in this study for a two-mirror microscope. Based on incoherent, sine wave modulation transfer function (MTF) calculations, the object plane resolution of a 20x microscope has been analyzed as a function of the object height and numerical aperture (NA) of the primary for several spherical Schwarzschild, conic, and aspherical reflecting two-mirror microscope configurations. The ultimate resolution of an aspherical, two-mirror microscope appears to be about 200 Å when using 100-Å radiation. Better resolution can be achieved when shorter wavelength radiation is used.

Free-electron laser (FEL) sources, driven by rf linear accelerators, have the potential to operate in the extreme ultraviolet (XUV) spectral range with more than sufficient average power for high-volume projection lithography. For XUV wavelengths from 100 to 4 nm, such sources will enable the resolution limit of optical projection lithography to be extended from 0.25 to 0.05 μm with an adequate total depth of focus (1 to 2 μpm). Recent developments of a photoinjector of very bright electron beams, high-precision magnetic undulators, and ring-resonator cavities raise our confidence that FEL operation below 100 nm is ready for prototype demonstration. We address the motivation for an XUV FEL source for commercial microcircuit production and its integration into a lithographic system, including reflecting reduction masks, reflecting XUV projection optics and alignment systems, and surface-imaging photoresists.

Photos obtained during 5 mm of observation time from the flight of our 25-cm-diam normal-incidence soft x-ray (λ = 63.5 Å) telescope on September 11, 1989, are analyzed and the data are compared to the results expected from tests of the mirror surfaces. These tests cover a range of spatial periods from 25 cm to 1 Å. The photos demonstrate a resolution close to the photon shot noise limit and a reduction in the scattering of the multilayer mirror compared to a single surface for scattering angles above 1 arcmin, corresponding to surface irregularities with spatial penods below 10 μm. Our results are used to predict the possible performance of future telescopes: We conclude that sounding rocket observations might be able to reach a resolution around 0.1 arcsec. Higher resolutions will require flights of longer durations and improvements in mirror testing for the largest spatial periods.

The Multi-Spectral Solar Telescope Array (MSSTA) is a sounding rocket-borne observatory designed to produce ultrahigh-resolution full-disk images of the sun. The MSSTA utilizes an array of Ritchey-Chrétien, Cassegrain, and Herschelian normal incidence telescopes operating in the soft x-ray (44- to 100-Å), the extreme ultraviolet (EUV: λλ ~ 100 to 1000 Å), and the far ultraviolet (FUV: λλ ~ 1000 to 2000 Å) portions of the electromagnetic spectrum. These normal incidence multilayer and thin film interference coated optical systems are designed to produce narrow band solar images at selected wavelengths within this very broad spectral regime. The MSSTA mission places extremely strenuous requirements upon the imaging detector. The desire for ultrahigh-resolution (~0.1 to 0.3 arcsec) images of the solar disk and corona out to 1.5 solar radii demands an information storage capacity that at the present time can be met only by the highest quality photographic emulsions. Because a tremendous range exists in the intensity levels and contrast characteristics of solar x-ray/EUV/FUV emission features, the MSSTA photographic films must have a wide latitude to allow extremely bright, high-contrast features associated with flares and active regions to be recorded without saturation, while maintaining the ability to capture the faint, low-contrast structures in coronal holes, polar coronal plumes, network structures, and coronal loops. The MSSTA flight films must have high to ultrahigh-resolution and be sensitive over this very broad and difficult portion of the electromagnetic spectrum. The gelatin overcoat of the film must be maintained at a minimum, while the 70-mm film must retain the ability to be transported through conventional Pentax 645 cameras with minimum degradation due to scratches in the delicate emulsion. Furthermore, because the payload must be evacuated to prevent the delicate EUV and soft x-ray filters from failing due to acoustic vibrational stresses during launch, the MSSTA flight films must also have low-outgassing rates. These films must also be provided with antistatic backing to prevent electrostatic sparks from degrading the images as the dry film is unspooled through the cameras. We describe the performance and characteristics required of the MSSTA photographic films for solar observations in the soft x-ray/EUV and FUV wavelength regimes. We discuss the properties of the important new emulsions selected for flight and present data on the response characteristics of a tabular grain Experimental XUV 100 film and an uncoated Experimental Spectroscopic 649 emulsion.

We have developed compact soft x-ray, extreme ultraviolet (EUV), and far-ultraviolet (FUV) multilayer coated telescopes for the study of the solar chromosphere, corona, and corona/solar wind interface. Because these systems operate at short wavelengths (~40 Å < λ < 1550 Å), the modest apertures of 40 to 127 mm allow observations at very high angular resolution (0.1 to 0.7 arcsec). In addition to permitting traditional normal incidence optical configurations (such as Cassegrain, Ritchey-Chrétien, and Herschelian configurations) to be used at soft x-ray/EUV wavelengths, multilayer coatings also allow a narrow wavelength band (λ/Δλ ~ 15 - 100) to be selected for imaging. The resulting telescopes provide a very powerful and flexible diagnostic instrument for the study of both the fine-scale structure of the chromosphere/corona interface and the large-scale structure of the corona and corona/solar wind interface. In previous papers, we have described a new solar rocket payload, the Multi-Spectral Solar Telescope Array (MSSTA), which is composed of 17 of these compact telescopes. In the present paper, we report on the ability of the MSSTA payload to obtain temperature diagnostic information about the optically thin solar plasma. We also discuss applications of this information to studies of coronal structure.

The multilayer mirrors used in the normal-incidence optical systems of the Multi-Spectral Solar Telescope Array (MSSTA) are efficient reflectors for soft x-ray/extreme ultraviolet (EUV) radiation at wavelengths that satisfy the Bragg condition, thus allowing a narrow band of the soft x-ray/EUV spectrum to be isolated. However, these same mirrors are also excellent reflectors in the visible, ultraviolet, and far-ultraviolet (FUV) part of the spectrum, where normal incidence reflectivities can exceed 50%. Furthermore, the sun emits far more radiation in the ultraviolet and visible part of the spectrum than it does in the soft x-ray/EUV. For this reason, thin foil filters are employed to eliminate the unwanted longer wavelength solar emission. With the proper choice of filter materials, the filters can also be used to eliminate EUV radiation at longer wavelengths, where the increasing specular reflectivity of multilayer mirrors and the high intensity of solar emissions can cause "contamination" of the image in the narrow band defined by the Bragg condition. In addition, filters can eliminate higher order multilayer reflections. Finally, filter absorption edges can sometimes be utilized to reduce the width of the primary bandpass. The MSSTA instrument uses various combinations of thin foil filters composed of aluminum, carbon, tellurium, potassium bromide, beryllium, molybdenum, rhodium, and phthalocyanine to achieve the desired radiation rejection characteristics. We discuss issues concerning the design, manufacture, and predicted performance of MSSTA filters.

We describe the design of a high-resolution stigmatic extremeultraviolet (EUV) spectroheliometer, configured for flight on a Black Brant sounding rocket, which consists of a 45-cm Gregory telescope coupled to a spectrometer employing a single toroidal diffraction grating in a normalincidence Rowland circle mounting and an imaging pulse-counting multianode microchannel array (MAMA) detector system. The toroidal diffraction grating is fabricated by a technique employing an elastically deformable submaster grating that is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The spectroheliometer will produce spatially resolved spectra of the chromosphere, transition region, and corona with an angular resolution of 0.4 arcsec or better, a spectral resolution λ/Δλ of about 10⁴ in first order, and a temporal resolution of the order of seconds. Because of the geometric fidelity of the MAMA detector system, the spectroheliometer will be able to determine Doppler shifts to a resolution of at least 2 mÅ at wavelengths near 600 Å (~1.0 km s^-1), depending on the level of the accumulated signal. The unique characteristics of the spectroheliometer will be used in combination with plasma-diagnostic techniques to study the emperature, density, and velocity structures of specific features in the solar outer atmosphere.

We discuss the fabrication and testing of electroformed replica Wolter I optics made from gold-coated lacquered mandrels. We also discuss testing of gold- and palladium-coated lacquered test flats. X-ray (5 keV for Wolter I mirror and 8 to 40 keV for test flats) and optical (Wyko NCP-1 000 profiler) measurements were used to evaluate the mirrors.

The use of the unique polarization properties of synchrotronradiation in the hard x-ray spectral region (Ε > 3 keV) is becoming increasingly important to many synchrotron radiation researchers. The radiation emitted from bending magnets and conventional (planar) insertion devices (IDs) is highly linearly polarized in the plane ofthe particle's orbit. Elliptically polarized x rays can also be obtained by going off axis on a bending magnet source, albeit with considerable loss of flux. The polarization properties of synchrotron radiation can be further tailored to the researcher's specific needs through the use of specialized insertion devices such as helical and crossed undulators and asymmetrical wigglers. Even with the possibility of producing a specific polarization, there is still the need to develop x-ray optical components that can manipulate the polarization for analysis and further modification of the polarization state. A survey of techniques for producing and analyzing both linear and circular polarized x rays will be presented with emphasis on those techniques that rely on single-crystal optical components.

Several mechanisms can induce a detectable amount of linear polarization (≥l%) in spectral lines emitted by the outer solar atmosphere at EUV/FUV wavelengths (i.e., 100 Å ≤ λ ≤ 1500 Å): (1) Polarization in FUV lines (up to 20%) can be originated by resonance scattering of radiation anisotropically illuminating the emitting atoms. Modifications of this polarization can then result from the presence of a magnetic field (Hanle effect). (2) Impact line polarization can arise from anisotropic collisional excitation ofthe EUV-emitting atoms by particles (electrons, protons) with non-Maxwellian velocity distributions. We suggest how new technological developments in the production of ultrasmooth, low-scatter flow-polished mirror substrates and high-quality multilayer and interference film coatings can make possible some new optical instruments for the observation of these polarization effects. We give the relevant observational parameters for all-reflective FUV/EUV imaging polarimeters. A coronagraph/polarimeter, operating at hydrogen Lyman a, could provide, via the Hanle effect, the first direct measurement of coronal vector magnetic fields. An EUV polarimeter, operating at EUV helium lines (e.g., 304 Å, 584 Å), could observe impact polarization phenomena occurring in solar flares. The reflecting polarization analyzers for these instruments will operate at the Brewster angle and will be coated with thin-film interference coatings or multilayer coatings. We describe several FUV/EUV polarimeter designs based on these polarization optics.

αWe describe new EUV/FUV (100 Å ≤ λ ≤ 1500 Å) polarimeter instrument concepts for solar research. These instruments are designed to observe linear polarization in EUV/FUV spectral lines originating in the outer solar atmosphere, specifically: (1) a new coronagraph/polarimeter operating at 1215.7 Å (neutral hydrogen Lyman ), which could observe this line in the near solar corona and lead to the first direct measurements of both strength and direction of coronal magnetic fields and (2) a new multilayer EUV imaging polarimeter, operating at wavelengths of strong helium emission lines (e.g., 304 Å, 584 Å), which could observe impact polarization phenomena and provide information concerning the relative importance ofthermal and nonthermal processes in solarflares. The emission mechanisms we will address with these instruments include resonance scattering and impact polarization. Resonance scattering of chromospheric radiation anisotropically illuminating the emitting atoms in the corona can produce up to 20% linear polarization in FUV coronal lines. Modifications, via the Hanle effect, of this polarization would result from the presence of a magnetic field. In the EUV, detectable polarization may be produced by impact polarization, which results from anisotropic collisional excitation of the emitting atoms by particles (electrons, protons) with non-Maxwellian velocity distributions produced during flares. These coronagraph/polarimeter instruments employ all-reflective optical systems utilizing ultrasmooth, low-scatter normal incidence mirrors and reflective polarization analyzers comprised of advanced flow-polished substrates with state-of-the-art thin film FUV interference and EUV multilayer coatings. The reflecting polarization analyzers operate at the Brewster angle. We discuss several instrument configurations and provide theoretical calculations and performance predictions for coronagraph/polarimeter instruments utilizing an optical design similar to the Ritchey-Chrétien EUV/FUV telescopes developed for flight on the Stanford/MSFC/LLNL Multi-Spectral Solar Telescope Array (MSSTA).

We present further data on the linear polarization sensitivity of Csl x-ray/UV photocathodes. A radical physical origin-"nonlocal" photoemission-is proposed for the x-ray vectorial effect. Predictions from a uniaxial photocathode model are compared with measurements of (1) relative pulse quantum yields, relative current quantum yields, and electron number distributions for both s- (electric vector perpendicular to the plane of incidence) and p- (E parallel to the plane of incidence) polarized soft x rays in the wavelength range 4.6-125 Å; and (2) relative current quantum yields from illumination by partially polarized, broadband UV radiation with a modal wavelength of 1550 Å. An origin seated in the polarimeter geometry is also proposed for the off-axis "phase shift" phenomenon reported in previous papers. The impact of these measurements on (1) an understanding of soft x-ray interactions with materials, and (2) the design and sensitivity of the Spectrum X-Gamma photoemission polarimeter is discussed; our future plans for the investigation of photocathode physics are outlined.

Methods for calculating Fraunhofer intensity fields resulting from diffraction through one- and two-dimensional apertures are presented. These methods are based on the geometric concept of finite elements and on Fourier and Abbe transforms. The geometry of the two-dimensional diffracting aperture(s) is based on biquadratic isoparametric elements, which are used to define aperture(s) ofcomplex geometry. These elements are also used to build complex amplitude and phase functions across the aperture(s) which may be of continuous or discontinuous form. The transform integrals are accurately and efficiently integrated numerically using Gaussian quadrature. The power of these methods is most evident in two dimensions, where several examples are presented which include secondary obstructions, straight and curved secondary spider supports, multiple-mirror arrays, synthetic aperture arrays, segmented mirrors, apertures covered by screens, apodization, and phase plates. Typically, the finite element Abbe transform method results in significant gains in computational efficiency over the finite element Fourier transform method, but is also subject to some loss in generality.

The role of convection in controlling laser melted pool geometry such as pool shape, undercut, and ripples; defects such as variable penetration, porosity, and lack offusion; and properties such as homogeneity has been studied by a number of researchers. This paper reviews the state-of-the art research in this area.

Digital pulsed laser velocimetry (DPLV)is a novel full-field, twodimensional, noninvasive, quantitative flow visualization technique. A description of the technique, which uses direct digitization of single- or twophase flow images using a high-resolution imaging system and a highpower pulsed laser, is presented. Images (ten consecutive frames of data separated by a time increment of 150 ms) of single- and two-phase flow over a step are acquired with a high-resolution camera. Each of these ten frames contains the images of tracer articles or bubbles at that one instant of time. The image data was stored for further analysis by new image processing and analysis software developed specifically for flow experiments. An image finding, smoothing, and defining program was developed. This program groups appropriate pixels into particles, smooths the image, and calculates important parameters for the image. Another program was developed to match the particles from each of the frames into tracks through time. The program uses a statistical technique to determine the best possible path of the particles. The most important capability of pulsed laser velocimetry is the ability to use these image processing techniques to capture simultaneous and quantitative rather than qualitative information.

We experimentally and theoretically investigate optical beam propagation in nonlinear refractive materials having a thickness greater than the depth of focus of the input beam (i.e., internal self-action). A simple model based on the "constant shape approximation" is adequate for analyzing the propagation of laser beams within such media under most conditions. In a tight focus geometry, we find that the position of the sample with respect to the focal plane, z, is an important parameter in the fluence limiting characteristics of the output. The behavior with z allows us to perform a "thick sample Z-scan" from which we can determine the sign and magnitude of the nonlinear refraction index. In CS₂, we have used this method to independently measure the negative thermally induced index change and the positive Kerr nonlinearity with nanosecond and picosecond CO₂ laser pulses, respectively. We have experimentally examined the limiting characteristics of thick CS₂ samples that qualitatively agree with our analysis for both positive and negative nonlinear refraction. This analysis is useful in optimizing the limiting behavior of devices based on self-action.

This PDF file contains the editorial “Book Rvw: Optical Computing in Japan," edited by S. Ishihara for OE Vol. 30 Issue 08

This PDF file contains the editorial “Book Rvw: Dye Laser Principles with Applications," edited by F. J. Duarte and Lloyd W. Hillman for OE Vol. 30 Issue 08