Synchrotron Radiation Center

During the last few years, synchrotron radiation centers around the world have experienced a dramatic demand to provide radiation of variable polarization. The recent interest in the polarized nature of synchrotron radiation is spurred by applications such as magnetic circular dichroism (MCD) and circular intensity differential scattering (CIDS), which probe the magnetic electronic properties and chiral structure of solids and biological samples.

Considering the achievable flux and limited degree of circular polarization, Pcirc , intrinsic to crossed-field undulators, combined with the potential problems the operation of this device might impose on other users of the electron storage ring, the Synchrotron Radiation Center opted for a rather unconventional way of generating circular-polarized light. The use of a multiple-reflection polarizer is proposed as a highly effective tool by utilizing the phase difference between the s- and p-components of a multiple reflected light beam.

The Quadruple Reflection l/4 Phase Shifter is being developed by physicists Hartmut Höchst, Peter Bulicke and Rajesh Patel of the Synchrotron Radiation Center, and mechanical engineer Fred Middleton of the Physical Sciences Laboratory. The two-year-long project is expected to be completed by the end of this year.

The quadruple reflector uses a linearly polarized beam of light from a bending magnet or undulator beam line of the Aladdin storage ring. It converts the linearly polarized light into elliptical or a circular polarization by a series of oblique angle reflections from mirrors, which mix together and polarization components with a variable phase relationship.

Either the polarization alone, or a combination of polarization and intensity, can be optimized by selecting particular reflection angles, which is entirely under computer control. The concept of the quadruple reflector can be used over a wide energy range of 8 - 120 eV.

Two pairs of reflectors are used to keep the beam directed in the horizontal plane. After entering the apparatus, light reflects from the first reflector to a parallel second. Since the exiting light has the same angle of incidence, it leaves horizontally and parallel to the incoming light and to the instrument. The light then impinges on a second pair of parallel reflectors which are a mirror image of the first pair. Light reflects from the third reflector to the fourth, which is on the axis of rotation of the complete reflector housing assembly. Light then exits the apparatus.

 

Middleton, who designed the mechanical realization device, said that one requirement was to maintain a fixed angle in the rotators while the housing is rotated. This is an important factor since every reflector angle corresponds to a different polarization of light. To keep the light on track, the two pairs of reflectors are kept parallel to within less than a thousandth of a degree, and they counter-rotate equally to within less than a thousandth of a degree.

Optical encoding and computer control are used to maintain the reflector positions. As the housing drive is rotated, the reflector drive is simultaneously and equally counter-rotated to prevent relative motion between the internal reflector drive and the reflector housing. Relative motion would result in an angular change in the position of the reflectors.

The entire assembly is rotated to determine whether the light will be left or right polarized For each energy point in a scan, the polarization can be flipped automatically to measure a difference (dichroism) spectrum.