3rd generation, UHV bakeable, bimorph deformable X-ray mirror with slope errors < 50 nanoradians for a range of...

A new class of 3rd generation, bimorph deformable, X-ray mirrors have been developed, which are UHV "bakeable" to 200°C and provide diffraction-limited performance for achromatic focusing and wavefront correction of high-intensity photon beams. Optical metrology was used to reduce slope errors to ∼ 42 nrad rms and height errors to ∼ 200 pm rms for concave, flat, and convex elliptical curvatures. Curved X-ray mirrors with slope errors < 50 nrad rms and height errors < 500 pm rms are required for nano-focusing and coherence applications at low-emittance synchrotron light and free electron laser facilities. In recent years, impressive technical progress has been made to fabricate fixed-curvature X-ray mirrors, approaching diffraction-limited performance. However, for many scientific applications, active optics with a deformable surface profile are required to intermittently change the focal distance or size of the X-ray beam, or to make fine adjustments to the X-ray wavefront. What we believe to be a new class of high-grade, actively deformable optics have been developed, which provide diffraction-limited performance for achromatic focusing and wavefront correction of X-ray beams. 3rd generation, bimorph deformable, X-ray mirrors have piezoelectric PZT actuators bonded to the silicon substrate using silver nano-particles. They can be safely thermally annealed to 200°C and are ultra-high vacuum compatible, making them suitable for a wide range of X-ray energies, including soft X-rays. We present a comprehensive optical metrology study of a 32-channel, 3rd generation bimorph mirror mounted in an opto-mechanical holder to assess suitability for routine beamline operation. Fizeau interferometry and slope profilometry were performed to characterize the range, drift, stability, repeatability, and resolution of bending. Voltages to individual electrodes were optimized to minimise surface errors, based on metrology feedback and a constrained, linear algebra solver. Slope errors of ∼ 42 nrad rms and height errors ∼ 200 pm rms were achieved for three different curvatures (concave, flat, and convex). Metrology testing also demonstrated the extreme resolution of bending (2 nm changes in the height profile by incrementally applying 0.1 V shifts to all piezo actuators) and long-term curvature stability of 0.1% rms over 16 hours. Hysteresis, creep, and short-term drift of the bimorph's profile were observed, which will be the subject of future research.