Single Crystal Diamond Compound Refractive Lenses

Next generation light sources, diffraction-limited storage rings, and high repetition rate free electron lasers will have significantly increased the average brightness of the generated X-ray beams. These machines will require X-ray refractive optics with precise dimensional control and smooth surfaces that are capable of handling large heat loads and preservation of the x-ray beam quality. How the problem is being addressed: In this project we will machine refractive lenses out of single crystal diamonds by femtosecond laser pulses. The key advantage of this approach is in the short duration of the laser pulse. Femtosecond pulses only ablate the material and do not lead to thermal fatigue, subsequent crystalline defect formation, and reduction in the quality of X-ray optical properties. With the choice of single crystal diamond for the lens, the scattering on grain boundaries, defects and voids associated with polycrystalline materials currently used will be eliminated and the coherence of the X-rays will be preserved. What was done in Phase I: We fabricated a set of diamond X-ray lenses by fs-laser cutting of various grades of single crystal diamonds. We performed white beam topography to determine which grades of diamond are acceptable for x-ray optics and what damage the fs laser cutting induced on the bulk material. We demonstrated a successful test of lens operation in an x-ray source reimaging configuration at the Advanced Photon Source (APS). We also performed an experiment with a stack of three lenses (compound refractive lens) focusing an undulator source at APS into a ~ 50 x 20 micron spot that approach commercial beryllium lens performance. Finally, we performed diamond slurry polishing of the lens surface. What was done in Phase II: In Phase II we built a custom fs-laser cutter system for complex shapes ablation on diamond surfaces. We developed a post-ablation polishing procedure which produces uniform polishing of the full lens surface. A large number of lenses had been tested at Advanced Photon Source (APS) of Argonne National Laboratory. These tests determined that there is a correlated shape error on the order of 1-3 microns. What will be done in Phase IIA: In Phase II we will adjust manufacturing process – both ablation and polishing to decrease the shape error to below 1 micron per APS specs. These lenses will be packaged in stainless steel or copper housing for the beamline – ready compound refractive lens. Applications and benefits: The technology developed here is required to utilize X-ray beams at fourth generation light sources to maximum potential. Diamond is virtually the only material that can withstand the heat load of the next generation light sources. If an inexpensive manufacturing method is established, diamond refractive optics would supersede the current alternative, which is based on beryllium and has safety concerns as well as a lower performance.