X-ray natural birefringence in reflection from graphite

The existence of the x-ray natural birefringence in reflection from graphite is demonstrated. This novel x-ray effect is measured as the polarization-plane rotation and ellipticity appearing upon reflection of linearly polarized synchrotron radiation from highly-oriented pyrolytic graphite. Polarization analysis is employed to measure the x-ray rotation and ellipticity spectra across the carbon 1s edge. Extraordinarily large birefringence rotation values of up to +/-90 degrees as well as ellipticity values up to 30 degrees are detected across the carbon 1s absorption edge. To analyze the origin of the measured spectra, the Stokes parameters as well as the x-ray natural linear dichroism in reflection and absorption are determined across the C 1s edge. The strong birefringence near the C 1s absorption edge is shown to result directly from the optical anisotropy of hexagonal graphite. Measurements carried out on isotropic amorphous carbon films show in contrast a negligible birefringence. With its large birefringence and its reduced heat load sensitivity graphite bears potential as a tunable x-ray phase shifting lambda/4 or lambda/2 plate for future applications on new, high-intensity light sources.