Focusing Optical Elements Research Articles

Quasicrystal hole array as X-ray focusing optical element

Quasicrystal hole arrays are expected to be adopted as new focusing optical elements, owing to the characteristic self-imaging effect and the possibility of overcoming the diffraction limit. Using one such element, we achieved soft X-ray focusing. We designed a fivefold symmetric quasicrystal based on a Penrose tiling pattern consisting of 301 1.5 μm diameter holes. The light source was 10 nm synchrotron radiation. The observed diffraction pattern agreed well with the results of Fraunhofer-approximation simulations. Furthermore, the focal spot size was estimated by direct CCD imaging and found to be sufficiently small, in line with simulation results.

Theory of angular-dispersive, imaging hard-x-ray spectrographs

A spectrograph is an optical instrument that disperses photons of different energies into distinct directions and space locations, and images photon spectra on a position-sensitive detector. Spectrographs consist of collimating, angular dispersive, and focusing optical elements. Bragg reflecting crystals arranged in an asymmetric scattering geometry are used as the dispersing elements. A ray-transfer matrix technique is applied to propagate x-rays through the optical elements. Several optical designs of hard x-ray spectrographs are proposed and their performance is analyzed. Spectrographs with an energy resolution of 0.1 meV and a spectral window of imaging up to a few tens of meVs are shown to be feasible for inelastic x-ray scattering (IXS) spectroscopy applications. In another example, a spectrograph with a 1-meV spectral resolution and 85-meV spectral window of imaging is considered for Cu K-edge resonant IXS (RIXS).

432-μm laser’s beam-waist measurement for the polarimeter/interferometer on the EAST tokamak

A far-infrared (FIR) polarimeter/interferometer (PI) system is under development for measurements of the current-density and the electron-density profiles in the EAST tokamak. The system will utilize three identical 432-μm CHCOOH lasers pumped by a CO2 laser. Measurements of the laser beam’s waist size and position are basic works. This paper will introduce three methods with a beam profiler and several focusing optical elements. The beam profiler can be used to show the spatial energy distribution of the laser beam. The active area of the profiler is 12.4 × 12.4 mm2. Some focusing optical elements are needed to focus the beam in order for the beam profiler to receive the entire laser beam. Two principles and three methods are used in the measurement. The first and the third methods are based on the same principle, and the second method adopts an other principle. Due to the fast and convenient measurement, although the first method is a special form of the third and it can only give the size of beam waist, it is essential to the development of the experiment and it can provide guidance for the choices of the sizes of the optical elements in the next step. A concave mirror, a high-density polyethylene (HDPE) lens and a polymethylpentene (TPX) lens are each used in the measurement process. The results of these methods are close enough for the design of PI system’s optical path.

Sub-nm accuracy metrology for ultra-precise reflective X-ray optics

The transport and monochromatization of synchrotron light from a high brilliant laser-like source to the experimental station without significant loss of brilliance and coherence is a challenging task in X-ray optics and requires optical elements of utmost accuracy. These are wave-front preserving plane mirrors with lengths of up to 1 m characterized by residual slope errors in the range of 0.05 μrad (rms) and values of 0.1 nm (rms) for micro-roughness. In the case of focusing optical elements like elliptical cylinders the required residual slope error is in the range of 0.25 μrad rms and better. In addition the alignment of optical elements is a critical and beamline performance limiting topic. Thus the characterization of ultra-precise reflective optical elements for FEL-beamline application in the free and mounted states is of significant importance. We will discuss recent results in the field of metrology achieved at the BESSY-II Optics Laboratory (BOL) of the Helmholtz Zentrum Berlin (HZB) by use of the Nanometer Optical Component Measuring Machine (NOM). Different types of mirror have been inspected by line-scan and slope mapping in the free and mounted states. Based on these results the mirror clamping of a combined mirror/grating set-up for the BL-beamlines at FLASH was improved.

Towards high diffraction efficiency zone plates for X-ray microscopy

Modern soft and hard X-ray microscopes use Fresnel zone plates as imaging or focusing optical elements. For short exposure times and to minimize the radiation load on the specimens, the diffraction efficiency of the zone plate objectives has to be maximized. Since the diffraction efficiency strongly depends on the height of the diffracting zone structures, the achievable aspect ratio of the nanostructures which can be fabricated by existing nanostructuring processes limits the zone plate efficiency. To increase the achievable aspect ratios especially for hard and multi-keV X-ray optics we develop a process to superimpose zone plates on top of each other. With this so-called stack process for zone plate fabrication the final aspect ratio is only limited by the number of single zone plate layers that can be stacked with sufficient accuracy. Several nanostructuring process steps have to be developed and/or improved to establish the stack process. For the first time, we present a polished zone plate on a subsequently etched support membrane. In addition, we present first results of stacked zone plates with two layers of nickel zones superimposed on top of each other.

Compact and Efficient Green VECSEL Based on Novel Optical End-Pumping Scheme

We propose a novel lens-less optical end-pumping scheme and report on the development of a highly efficient and compact green vertical-external-cavity surface-emitting laser (VECSEL). A constant-wave pump-power-limited maximum green output power of 1.1 W and an optical-to-optical conversion efficiency of as high as ~15.7% (=1.1 W/7 W) were achieved using a single chip pump laser diode placed directly behind the VECSEL structure without any beam focusing and shaping optical elements.

A new method of depth sensitive micro-X-ray fluorescence analysis

A new method to measure depth distributions of elements by micro-X-ray fluorescence analysis was developed. This method is based on the use of a focusing optical element and the knife-edge principle and gives depth resolutions of about 23 μm. Furthermore, this method also allows measuring surface profiles with a depth resolution of 7 μm. In this paper experimental results of depth distribution measurements of a layered sample and of the surface roughness profile of another sample are presented.

Synchrotron radiation induced μ-X-ray fluorescence spectroscopy on municipal solid waste fly ashes

The high brightness of synchrotron radiation sources of the 3rd generation and the development of focusing optical elements for X-rays make it possible to create beams of micrometer size with high intensity. These beams can be used to reveal spatially resolved information about structural and chemical properties of particles. Here, the possibility of using synchrotron radiation micro-X-ray fluorescence (μ-SRXRF) for the determination of the major and minor components and their distribution within municipal solid waste (MSW) fly ashes has been investigated. By using an excitation energy of 27.0 keV, trace elements of environmental concern, like Cd, Pb and Zn can be detected to their low concentrations (ppm) routinely. The aim of the work was to gain a better understanding of the factors that determine the environmental mobility of each trace element and, in particular, the potential for their dissolution and leaching. Such detailed investigation of the content and distribution of toxic metals on/in individual particles is a valuable complement to the usual elemental analysis of bulk samples.

SYNCHROTRON RADIATION SCANNING PHOTOEMISSION MICROSCOPY: INSTRUMENTATION AND APPLICATION IN SURFACE SCIENCE

X-ray photoelectron spectroscopy has become a true microscopic technique at third generation soft X-ray synchrotron sources, finding applications in many domains of academic and applied research. This paper describes the present status of scanning photoemission microscopy, where by using photon optics the photon beam can be focused to micron or submicron dimensions and imaging or spectroscopy can be performed. It discusses different photon focusing optical elements and describes the major components of the constructed scanning microscopes. The applications of imaging and spectroscopy with high lateral resolution in surface science are illustrated, and some recent results obtained in different laboratories are briefly reviewed.

Fiber-optic-based UV–visible absorbance detector for capillary electrophoresis, utilizing focusing optical elements

A compact and flexible fiber-optic cell for absorbance detection in capillary electrophoresis (CE) has been developed. Optical fibers with a large core diameter were combined with focusing optics and a circular aperture, the size of the aperture being matched to the I.D. and O.D. of the separation column, in order to maximize light throughput and reduce stray light. The detection limit for mesityl oxide was 1.5 μ M for a 100-μm I.D. column, and the linear dynamic range was larger than 1000 for columns from 100-μm I.D. down to 30-μm I.D. The design is suitable for high-speed CE, utilizing short columns.

Alternative fiber-optic backreflectance sensor for measurement of the effective focal lengths of optical elements and distances

An alternative and simple fiber-optic backreflectance sensor method for accurate measurement of either effective focal lengths of optical elements and short distances using a single-mode optical fiber and cw laser is presented. The method is based on the intensity sensing of focused backreflectance laser emission and the spatial location of three specific points: the focal point of the focusing optical element and two object points. The single-mode fiber is a key optical element and serves simultaneously as a point laser source for testing, an object for projecting and a highly sensitive point receiver of the focused backreflectance emission. The experimental and analytical results demonstrate the potential of the method for locating the spatial points and determining the effective focal length with accuracies exceeding 1 μm and 0.5%, respectively, as well as for short distance measurement with submicron accuracy.

An alternative fiber-optic backreflectance method for measurement of distances using a continuous wave laser

We present a new and simple fiber-optic backreflectance method for precise short distance measurement using a single-mode fiber and cw laser. The method is based on the intensity sensing of focused backreflectance laser emission, and the spatial location of three specific points: the focal point of the focusing optical element and two object points. The optical fiber is used in a multifunctional regime where it serves simultaneously as a point light source, an object for projecting, and a point receiver that is highly sensitive to spatial displacements of the focused backreflectance emission. The experimental and analytical investigations conducted demonstrate the usefulness of the method for distance measurement with submicron (about 370 nm) and millimeter accuracies in the near and far distance region, respectively, for a mirror target 1 m away.

Computer generated beam shaping and focusing optical elements for laser material processing

The paper presents a theoretical investigation of computer generated optical elements having a microstructure on a planar substrate. The field of application is the beam shaping in laser material processing, especially the generation of various intensity distributions and figures in the focus domain. These optical elements match the wavefront phase not only at the Fresnel zone border, but also within the zones. The inverse task of focusing has been solved based on the rules of geometrical optics. As an example, the surface profile has been evaluated for the focusing of a Gaussian laser beam into a cross-shaped intensity profile. The Fresnel-Kirchhoff integral has been applied to these elements and the results of diffraction calculations have been discussed for CO 2 laser and Nd: Yag laser applications. Furthermore, we have carried out a first experiment with a lower power CO 2 laser and compared the results with the theory. The cross-shaped focus obtained during the experiment was fully comparable with the theoretical result.

Simple fiber-optic autocollimation method for determining the focal lengths of optical elements

A novel simple fiber-optic autocollimation method for determining the focal lengths of lenses and objectives is presented. This method is based on the intensity sensing of reflected light, photoelectric measurement, and the use of single-mode optical fiber. This fiber serves simultaneously as a point light source for the formation of a collimated input laser beam and as a receiver that is highly sensitive to spatial displacements for the autocollimation backreflectance. The method permits us to locate the focal point spatially and to determine the back focal length of a focusing optical element with accuracies exceeding 1 and 2 microm, respectively, which are confirmed by both experimental and analytical investigations.

Second-generation scanning photoemission microscope at the National Synchrotron Light Source

We are commissioning a new generation scanning photoemission microscope (X1-SPEM II) at beamline X1A of the National Synchrotron Light Source (NSLS). Our first generation scanning photoemission microscope (X1-SPEM I) was the first to achieve submicron resolution. One of the major improvements is the replacement of the home-made single pass cylindrical mirror analyzer with a high energy resolution, multi-channel Hemispherical Sector Analyzer (HSA). The alignment scheme for the optical elements has also been redesigned. The advantages of these two major improvements will be discussed.A photoemission microscope requires a high brightness source and a good focusing scheme. In most cases, a monochromator is placed between the photon source and the focusing optical elements. Our X1-SPEM uses the soft x-ray undulator at the NSLS as a high brightness source. A Fresnel zone plate is coherently illuminated by the monochromatic beam selected by the spherical grating monochromator (250-800 eV range) to form a microprobe, less then 0.2 μm in size.

Study of SR beam line windows within the range of 17–80 nm

The use of a microchannel plate (MCP) as a support for an SR extraction window allows high temperature (up to 250°C) heating of the assembly with the installed filter to be performed. The MCP may be used as a collimator, with an appropriate MCP thickness to hole diameter ratio, or as a focusing optical element. Films on MCPs can also be manufactured from other substances. We are studying the possibility of manufacturing films from A1, In and Sn.

Recording the spatial distribution of coherent-radiation intensity

This paper discusses the feasibility of recording the spatial distribution of laser-radiation intensity, which is proven theoretically and confirmed by experiment. The authors consider the influence of the inhomogeneity and the wave front of radiation within the confines of a focusing optical element on the character of the diffracted image of the element.

A systematic method of estimating the performance of X-ray optical systems for synchrotron radiation. I. Description of various optical elements in position–angle space for ideally monochromatic X-rays

The performances of various optical elements are presented for ideally monochromatic X-rays in terms of position–angle space, which is used in the phase-space method. The optical elements discussed are a synchrotron radiation source, flat perfect crystals (reflection and transmission geometries), mosaic crystals, curved crystals of reflection (Johansson, Johann and logarithmic spiral types) and transmission geometries, and an elliptic total reflection mirror. The angular widths of acceptance and emergence for diffraction are properly taken into account in both the flat and curved crystals. It is shown that focusing optical elements such as a curved crystal and an elliptical mirror should not be treated simply as lenses.