Reflective Optics

An important activity of the Center for X-ray Optics is research on x-ray mirrors and their use in optical devices to focus and deflect x-ray beams. The two kinds of mirror most widely used are glancing incidence reflectors and multilayer coatings.

Glancing Incidence x-ray mirrors

The complex refractive index ñ = 1 - d - i b of all materials is very close to 1, from the extreme ultraviolet (EUV) spectral region (wavelengths of a few tens of nanometer, or energies of a few tens of eV) through the x-ray region (wavelengths of less than 1/10 nm, energies of tens of keV). For an introduction to optical constants click here. For this reason ordinary mirrors, which operate at normal or near-normal angles of incidence, do not work throughout this region. However, x-rays and EUV can be reflected by mirrors operating at large angles of incidence, where the rays make a small angle (a few degrees or less) with the mirror surface. This called the glancing (or grazing) angle and is the complement (90 degrees - i) of the optical angle of incidence i. Reflection by such mirrors can be calculated using the software in the X-ray Interactions With Matter web page. Click here to try some examples; you will need to enter the material of the mirror, the surface roughness (if any), the polarization and the range of angles/energy over which you want to make the calculations. For example try the following:

We see that mirrors for deflecting or focusing x-rays must be used at small angles of (glancing) incidence. This means that the optical designs look very different from conventional optics. The figure shows a typical two-mirror set-up for re-focusing a point source to a point image.

Multilayer coatings

Research on multilayer coatings for x-ray mirrors is one of the principal activities of the Center for X-ray Optics. If you are not familiar with the principles of these optics, or would like to refresh your memory, you may want to have a look at this mini-tutorial.

CXRO has been making, testing and utilizing multilayer x-ray mirrors since 1984, and has made major contributions to understanding the physics and chemistry of these devices, and to their utilization in optical systems for a variety of research fields, including x-ray astronomy, plasma spectroscopy, x-ray lasers and synchrotron radiation research. CXRO has constructed several beamlines at the Advanced Light Source which either utilize these optics or are designed to test them and evaluate their performance. The X-ray Microprobe Beamline is in the first category, while the Calibration and Standards Beamline and the EUV Interferometry Beamline are in the second. CXRO also maintains other instrumentation for the test and evaluation of multilayer mirrors, including a 2-circle x-ray diffractometer operating at 8.0 keV (Cu Ka x-rays), a soft x-ray reflectometer using a laser-produced plasma as a source, and instruments for the measurement of mechanical properties such as film stress.

Some of the advances made by CXRO in the past few years include:

• Completion of the Calibration and Standards Beamline at the Advanced Light Source for the measurement of the properties of multilayers and other x-ray optical components.
• Development of multilayer mirrors to focus x-rays to a 1 micron spot size in an x-ray microprobe.
• Mo/Be multilayers with high reflectance at 110 eV, in a joint development (with Lawrence Livermore National Lab.).
• Development of a dispersion element for the analysis of light elements in an x-ray electron microprobe.
• Coating of Schwarzschild optics for Super-Maximum photo-electron microscope at the Elettra synchrotron radiation facility in Trieste, Italy.
• Reserch on new combinations of materilas to achieve multilayers with special proerties or for particular spectral regions.