In principle, diffractive optics, particularly Phase Fresnel Lenses (PFLs), offer the ability to construct large, diffraction-limited, and highly efficient X-ray/$gamma$-ray telescopes, leading to dramatic improvement in angular resolution and photon flux sensitivity. As the diffraction limit improves with increasing photon energy, gamma-ray astronomy would offer the best angular resolution over the entire electromagnetic spectrum. A major improvement in source sensitivity would be achieved if meter-size PFLs can be constructed, as the entire area of these optics focuses photons. We have fabricated small, prototype PFLs using Micro-Electro-Mechanical Systems (MEMS) fabrication techniques at the University of Maryland and measured near diffraction-limited performance with high efficiency using 8 keV and higher energy X-rays at the GSFC 600-meter Interferometry Testbed. A first generation, 8 keV PFL has demonstrated imaging corresponding to an angular resolution of approximately 20 milli-arcseconds with an efficiency $sim$70$%$ of the theoretical expectation. The results demonstrate the superior imaging potential in the X-ray/$gamma$-ray energy band for PFL-based optics in a format that is scalable for astronomical instrumentation. Based upon this PFL development, we have also fabricated a `proof-of-principle refractive-diffractive achromat and initial measurements have demonstrated nearly uniform imaging performance over a large energy range. These results indicate that the chromaticity inherent in diffractive optics can be alleviated.
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