Polarization effect in resonant soft X-ray scattering study of helical liquid crystal phases.

Liquid crystals form many interesting nano-scale structures, many of which can be probed with X-ray scattering techniques, such as layering in smectics, hexagonal packing of cylinders in discotics. Typically hard X-rays are used due to its high penetrating power. However, in the hard X-ray regime, the scattering contrast of some LC nanostructures can be extremely low due to their weak electron density modulation. Here we show that by utilizing the coupling between x-ray polarization and molecular bond orientation, it is possible to use polarized soft x-rays at carbon resonant k-edge to probe helical strictures, i.e. the helical nanofilament B4 phase [1], the newly discovered twist bend nematic phase [2], and the blue phase [3], which are structures with no or minimal electron density modulation. Furthermore, we will discuss the relationship between the scattering anisotropy and the polarization of the incoming x-rays. These work show the great potential of resonant soft x-ray scattering in investigating structures of high orientational order.