Daniel J. Merthe

Methodology for Optimal In Situ Alignment and Setting of Bendable Optics for Diffraction-Limited Focusing of Soft X-Rays

Abstract. We demonstrate a comprehensive and broadly applicable methodology for the optimal in situ configuration of bendable soft x-ray Kirkpatrick-Baez mirrors. The mirrors used for this application are preset at the Advanced Light Source Optical Metrology Laboratory prior to beamline installation. The in situ methodology consists of a new technique for simultaneously setting the height and pitch angle of each mirror. The benders of both mirrors were then optimally tuned in order to minimize ray aberrations to a level below the diffraction-limited beam waist size of 200  nm (horizontal)×100  nm (vertical). After applying this methodology, we measured a beam waist size of 290  nm (horizontal)×130  nm (vertical) with 1 nm light using the Foucault knife-edge test. We also discuss the utility of using a grating-based lateral shearing interferometer with quantitative wavefront feedback for further improvement of bendable optics.

Automated suppression of errors in LTP-II slope measurements with x-ray optics

Systematic error and instrumental drift are the major limiting factors of sub-microradian slope metrology with state-of-the-art x-ray optics. Significant suppression of the errors can be achieved by using an optimal measurement strategy suggested in [Rev. Sci. Instrum. 80, 115101 (2009)]. Here, we report on development of an automated, kinematic, rotational system that provides fully controlled flipping, tilting, and shifting of a surface under test. The system is to be integrated into the Advanced Light Source long trace profiler, LTP-II, allowing for complete realization of the advantages of the optimal measurement strategy method. We describe in detail the systems specification, design operational control and data acquisition. The performance of the system is demonstrated via the results of high precision measurements with a number of super-polished mirrors.

Correlation analysis of surface slope metrology measurements of high quality x-ray optics

We discuss an application of correlation analysis to surface metrology of high quality x-ray optics with the aim of elicitation and, when possible, suppression of the instrumental systematic errors in the final metrology results. We describe and present the mathematical foundation for a novel method consisting of the randomization of the systematic error by the averaging of multiple measurements, specially arranged to mutually anti-correlate. We also discuss the possibility to apply correlation analysis to the entire residual surface slope distribution in order to find anti-correlation parameters of the distribution. In this case, repeated measurements with the corresponding change of the experimental arrangement (position of the surface and/or its overall tilt) can be used to identify the origin of the observed anticorrelation features by analyzing the difference between the measurements. If the corresponding minimum of the autocorrelation function is due to a systematic error, averaging over the repeated measurements will provide an efficient suppression of the systematic error. If the observed anti-correlation properties are due to the polishing process, and therefore belong to the surface itself, we suggest that the possibility of re-polishing the surface based on the correlation analysis be considered. Throughout the present work we have discussed correlation analysis of surface slope metrology data. However, a similar consideration can be applied to surface topography in the height domain measured with other metrology instrumentation, for example: interferometers and interferometric microscopes.

Proton transfer in histidine-tryptophan heterodimers embedded in helium droplets

We used cold helium droplets as nano-scale reactors to form and ionize, by electron bombardment and charge transfer, aromatic amino acid heterodimers of histidine with tryptophan, methyl-tryptophan, and indole. The molecular interaction occurring through an N-H···N hydrogen bond leads to a proton transfer from the indole group of tryptophan to the imidazole group of histidine in a radical cationic environment.

Experimental methods for optimal tuning of bendable mirrors for diffraction-limited soft x-ray focusing

We report on hands-on experimental methods developed at the Advanced Light Source (ALS) for optimal tuning of mechanically bendable x-ray mirrors for diffraction-limited soft x-ray nano-focusing. For ex situ tuning of the benders for optimal beam-line performance, we use a revised version of the method of characteristic functions recently developed at the ALS optical metrology laboratory. At-wavelength optimal tuning of bendable optics consists of a series of wavefront-sensing tests with increasing accuracy and sensitivity, including modified scanning-slit Hartmann tests. The methods have been experimentally validated at ALS test beamline 5.3.1 and the micro-diffraction beamline 12.3.2 in applications to optimally set bendable Kirkpatrick-Baez mirrors designed for sub-micron focusing.

An experimental apparatus for diffraction-limited soft x-ray nano-focusing

Realizing the experimental potential of high-brightness, next generation synchrotron and free-electron laser light sources requires the development of reflecting x-ray optics capable of wavefront preservation and high-resolution nano-focusing. At the Advanced Light Source (ALS) beamline 5.3.1, we are developing broadly applicable, high-accuracy, in situ, at-wavelength wavefront measurement techniques to surpass 100-nrad slope measurement accuracy for diffraction-limited Kirkpatrick-Baez (KB) mirrors. The at-wavelength methodology we are developing relies on a series of wavefront-sensing tests with increasing accuracy and sensitivity, including scanning-slit Hartmann tests, grating-based lateral shearing interferometry, and quantitative knife-edge testing. We describe the original experimental techniques and alignment methodology that have enabled us to optimally set a bendable KB mirror to achieve a focused, FWHM spot size of 150 nm, with 1 nm (1.24 keV) photons at 3.7 mrad numerical aperture. The predictions of wavefront measurement are confirmed by the knife-edge testing. The side-profiled elliptically bent mirror used in these one-dimensional focusing experiments was originally designed for a much different glancing angle and conjugate distances. Visible-light long-trace profilometry was used to pre-align the mirror before installation at the beamline. This work demonstrates that high-accuracy, at-wavelength wavefront-slope feedback can be used to optimize the pitch, roll, and mirror-bending forces in situ, using procedures that are deterministic and repeatable.

Design optimization of bendable x-ray mirrors

Convenience and cost often lead to synchrotron beamlines where the final bendable Kirkpatrick-Baez focusing pair must relay the final image to different samples at different image distances e.g., [Proc. FEL2009, 246-249 (2009)] either for different experimental chambers, or diagnostics. We present an initial analytical approach, starting from, and extending the work of Howells et al. [OE 39(10), 2748-62 (2000)] to analyze the trade-offs between choice of mirror, bending couples and the given, shaped sagittal width of the optic. Both experimentally and in simulation, we have found that after an appropriate re-bending, sagittally shaped optics can perform with high quality at significantly different incidence angles and conjugate distances. We present one successful demonstration from the ALS Optical Metrology Beamline 5.3.1, and review some new closed form analytical solutions with a view towards understanding our results.

Ex situ tuning of bendable x-ray mirrors for optimal beamline performance

We extend analytical and numerical methods recently developed at the Advanced Light Source (ALS) optical metrology laboratory (OML) for optimal tuning and calibration of bendable x-ray optics based on ex situ measurements with surface slope profilers [Opt. Eng. 48(8), 083601 (2009); Proc. SPIE 8141, 8141-19 (2011)]. We minimize the rms variation of residual slope deviations from ideal surface figure. Previously, our adjustment assumed the deviations were weighted equally across the optic. In this work, we analyze the case when the mirror length is significant with respect to the imaging conjugate. This corresponds, for example, to high de-magnification by bendable Kirkpatrick Baez mirror pairs, used near the ends of synchrotron and free electron laser beamlines for micro- and nano-focusing that often results in a very short mirror to image distance, of the same order of magnitude as the mirror’s length. In this case, contributions to focal distortion of residual errors of mirror surface figure (appearing due to mechanical alignment tolerances, sagittal shaping errors, and the limited number of adjustable parameters inherent in a two-couple bender) strongly depend on position across the optic. Specifically, the downstream deviations from exact shape should be weighted less because the rays have a shorter path to travel to the image. Here, we derive an analytical expression for the weighting function and present a mathematical background for the bending adjustment procedure for optimization of the mirror’s beamline performance. The efficacy of the optimization is demonstrated for a short-focus mirror used for diffraction limited focusing at ALS beamline 12.3.2.

Transparency of graphene and other direct-gap two-dimensional materials

The optical properties of graphene are among its remarkable characteristics. Measurements have found that the opacity of a single graphene layer for infrared and visible light is simply equal to

Metrology for the Advancement of X-ray Optics at the ALS

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