Polarisation Control and Analysis

I16 is equipped with a complex in-vacuum quarter-wave phase retarder to vary the incident polarization of the light from the linear horizontal provided from the insertion device to circular left or right from 2.8 to 10 keV and, linear with arbitrary orientation from zero (// to the synchrotron orbit) to 90 (perpendicular to the synchrotron orbit) from 3.5 to 9 keV.  The latter requires the simultaneous alignment and operation of two plates together and the rotation of the plates scattering planes about the incident beam direction using the angle ppchi.

The best plate to use depends on the  energy of the incident beam, that dictates the transmission through the diamond and the angular separation of the working points of the plates given their effective thickness.  Generally speaking a large offset for the quarter wave condition (>5 mdeg), with a good transmitted beam >5-10% gives a high polarization rate.

For energies > ~ 6 keV, the performance of the 111 and 220 reflections are very similar and the reflection can be chosen to minimise multiple scattering conditions.

An even better polarization rate can be obtained with two plates acting sequentially on the oncoming beam.    

The simplest possible setup is with only one plate in the beam at a time to obtain circularly polarized beam.

The diamond crystals share the same orientation and the same shape with 5 mm edges parallel to the <1,1,0> direction and the flat face normal to the  <0,0,1>. In this geometry it is possible to choose between the (1,1,1) reflections and the (2,2,0)-symmetric ones.

The thin Si membrane circle surface normal is along the <1,1,1> direction.

For energies > ~ 6 keV, the performance of the 111 and 220 reflections is very similar and the reflection can be chosen to minimise multiple scattering conditions.

As a rough guide, the following choices are appropriate:

I16 Diffractometer in horizontal scattering showing pi-pi and pi-sigma polarisation analysis

The polarisation of the scattered beam can be analsysed using the fully motorised Stokes rotation stage. An analyser crystal will be aligned close to 45 degrees from the scattered beam and rotated about the beam using the Stokes rotation. Either the in-vacuum Merlin area detector or the APD point-detector can be used.

A selection of anaylser crystals is available to achieve the best polarisation of scattered beam and also match the mosaic of the sample diffraction.

Some of the available analyser crystals are shown below:

By making use of the vertical and horizontal geometries of the 6-circle diffractometer, it is possible to access the 4 linear polarisation states for magnetic scattering: sigma-sigma, sigma-pi, pi-sigma and pi-pi.

The azimuthal angle is the rotation of the crystal about the wavevector tranfer (Q=k_f-k_i) of the reflection under study. Standard Bragg diffraction from isotropic charge density (atoms) does not vary with azimuthal angle, but magnetic moments and anisotropic electron density (e.g. electronic ordering) can couple to the polarisation direction of the incident beam, creating a variation in the scattered intensity. This variation, or "azimuthal dependence" can be used to determine the magnetic moment direction, or be used to separate different types of resonant scattering processes. 

On I16 it is possible to perform azimthal angle scans in many geometries, making full use of the 6-axis diffractomter. Combining these scans with polarisation analysis can provide unparalled detail on exotic electronic phenomena.

Table of available analyser crystals for all resonant edges in the I16 energy range:

If you have any comments, suggestions or corrections, please contact a member of the beamline staff.