Diamond optics for high heat load applications at XFELsThe radiation loads caused by the extremely high photon flux of hard x-ray free electron lasers (XFELs) quickly damage conventional optics such as Fresnel zone plates (FZPs) or diffraction gratings. This problem can be avoided by correct choice of materials used in the nanofabrication of these elements; for example it has been shown that FZPs made out of diamond sustain the full XFEL beam at Linac Coherent Light Source (LCLS) and can be used for nanofocusing. The low x-ray absorption, damage threshold and good heat conductivity of diamond make it an ideal material for XFEL optics.
The downsides of diamond are that it produces only a very weak phase shift in the hard x-ray range, and it is not a material commonly used in nanofabrication. This means that it is challenging to make diamond nanostructures with sufficiently small feature sizes and high aspect ratios for use in hard X-ray applications. We have developed a fabrication technique based on e-beam lithography and inductive coupled plasma reactive ion etching (ICP RIE) for the fabrication of diamond nano-optics. Several applications with differing requirements for the diamond structures have been developed and successfully tested at Linac Coherent Light Source (LCLS), a hard X-ray free electron laser in Stanford, CA:
• Diamond Zone Plates for XFELs
• Beam Splitter Gratings for Spectral Monitoring
• Split-and-Delay Scheme for Ultra-Fast Pump-Probe Experiments