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Abstract: The precise exploitation and efficient control of residual stresses are crucial for the development of high-performance thin-film optics. Multi-beam optical sensors (MOS) have been widely used for in situ residual stress measurements; however, this only provides information on the average residual stress at a single position. To overcome this limitation, speckle-based curvature optical metrology (SCOM) has been implemented at the multilayer deposition system (MDS) at Diamond Light Source. SCOM delivers two-dimensional curvature mapping, which has enabled direct visualization of the variation of spatial residual stress and substrate deformation for the first time. Benchmarking of SCOM measurements against MOS reveals excellent agreement in the extracted curvature for a deformable mirror. Importantly, SCOM offers a significantly larger dynamic range, allowing accurate measurements under the extreme curvature conditions that are frequently encountered in thick, high-residual stress films. This approach was also used to evaluate the residual stress evolution of a range of important materials, including Mo, Si and WSi2, which have been systematically investigated across varying deposition thicknesses and working pressures.

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Abstract: Reciprocity—the principle that a response is identical along the forward and backward paths—is a fundamental concept across physics. Non-reciprocity occurs when this symmetry is broken, resulting in direction-dependent behaviour. Achieving optical non-reciprocity typically requires complex metamaterials, exotic media or strong fields. Researchers have overlooked the possibility that conventional materials could support optical non-reciprocity. Here, through the Stokes–Mueller formalism, we predict a pathway to non-reciprocal absorption and emission of orthogonal linear polarizations. We test this idea using solution-processed films of CdS, CdSe and CdTe magic-size clusters with comparable circular and linear dichroism, and demonstrate non-reciprocal absorption and emission of linearly polarized light. Based on these findings, several design rules and practical applications are presented. Our work reveals that non-reciprocal linear dichroism and emission can be achieved in readily processable materials by harnessing chiral–linear optical interference, providing opportunities within polarization-based quantum optics and photonics such as direction-dependent optical routing or polarization-multiplexed encryption.

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Abstract: 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.

Open Access

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Abstract: A novel aberration-free X-ray compound refractive kinoform lens design based on the Cartesian oval curve is presented, designated as the OVAL-OK (OVAL Overlap Kinoform) lens. Material infilling of the kinoform step structure maintains focal spot dimensions while reducing focal intensity and reproducibility of structures. A SU-8 OVAL-OK lens fabricated through X-ray lithography achieved vertical focal sizes of 70.8 nm (knife-edge scanning) and 56 nm (wavefront propagation analysis) under 15 keV X-ray illumination, using a 120 μm × 200 μm (horizontal × vertical) aperture and 40.8 mm working distance. The lens exhibits a horizontal structural depth of 170 μm and a minimum feature size of 5 μm. The observed discrepancy between direct knife-edge measurements and wavefront-derived values is attributable to the combined effects of geometric, diffraction, coherence, instrumental instability, etc. These results demonstrate the potential for achieving sub-50 nm 2D focusing in future iterations through enhanced structural depth and expanded aperture dimensions.

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Abstract: The internal layer structure and optical performance of coating for X-ray free-electron laser (XFEL) applications were studied. Surface morphology analysis shows that the layer growth at 5 mTorr replicates the substrate morphology at low and mid frequencies but adds slight high-frequency roughness, with the RMS value increasing from 0.13 to 0.33 nm. To investigate the internal physical and chemical structure of , angle-resolved X-ray photoelectron spectroscopy (ARXPS) and depth profiling were performed. Together with soft X-ray (SXR) and hard X-ray (HXR) reflectivity measurements and the fitted results, a three-sublayer model with different composition and density was built for the 50 nm coating. The developed coating was further deposited on a 600 mm length mirror with a thickness variation of 0.7 nm (peak-to-valley). The radius of curvature changed slightly from 181 km before coating to 128 km after coating, and the slope errors were maintained at around 0.07 µrad, indicating a high-performance and large-size coating for the XFEL.