Hard X-ray wavefront correction via refractive phase plates made by additive and subtractive fabrication techniques
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-07-30 |
| Journal | Journal of Synchrotron Radiation |
| Authors | Frank Seiboth, Dennis BrĂŒckner, Maik Kahnt, Mikhail Lyubomirskiy, Felix Wittwer |
| Institutions | Friedrich Schiller University Jena, UniversitÀt Hamburg |
| Citations | 24 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates the successful correction of spherical aberration in Beryllium (Be) Compound Refractive Lenses (CRLs) using custom-designed refractive phase plates fabricated via complementary additive and subtractive manufacturing techniques.
- Core Achievement: Diffraction-limited nanofocusing was achieved at high X-ray energies (35 keV) using a polymer phase plate, increasing the Strehl ratio from 0.15 to 0.89.
- Low Energy Correction (8.2 keV): A diamond phase plate, fabricated via femtosecond laser ablation, corrected a 50-lens Be CRL stack, improving the Strehl ratio from 0.10 to 0.70.
- High Energy Correction (35 keV): A polymer phase plate, fabricated via two-photon polymerization (3D printing), corrected a 149-lens Be CRL stack, achieving a 95 nm FWHM coherent focal spot size.
- Manufacturing Methods: The study validates both subtractive (laser ablation of diamond for high radiation resistance) and additive (polymer printing for high shape accuracy and smooth surfaces) methods for producing complex X-ray optics.
- Engineering Significance: The ability to correct inherent manufacturing defects in CRLs allows large-aperture optics to operate at their theoretical diffraction limit, crucial for utilizing the high coherence of fourth-generation synchrotron sources and XFELs.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Low Energy Experiment | 8.2 | keV | DLS I13-1 Beamline |
| High Energy Experiment | 35 | keV | PETRA III P06 Beamline |
| Be CRL Stack (8.2 keV) | 50 | Lenses | Total N; LCRL = 55 mm |
| Be CRL Stack (35 keV) | 149 | Lenses | Total N; LCRL = 298 mm |
| Numerical Aperture (8.2 keV) | 0.88 x 10-3 | NA | Effective aperture Deff = 191 ”m |
| Numerical Aperture (35 keV) | 0.18 x 10-3 | NA | Effective aperture Deff = 236 ”m |
| Strehl Ratio (Diamond Corrected, 8.2 keV) | 0.70 | Ratio | Improvement from 0.10 |
| Strehl Ratio (Polymer Corrected, 35 keV) | 0.89 | Ratio | Improvement from 0.15 (Diffraction-limited) |
| Corrected Focal Spot Size (8.2 keV) | 76 | nm (FWHM) | Coherent focus size |
| Corrected Focal Spot Size (35 keV) | 95 | nm (FWHM) | Coherent focus size |
| Diamond Phase Plate Material | Single-crystal CVD Diamond | Material | Subtractive fabrication |
| Polymer Phase Plate Material | IP-S Resist | Material | Additive fabrication (Nanoscribe) |
| Diamond Surface Roughness (Sa) | 0.32 | ”m | Measured via LSM |
| Polymer Refractive Index Decrement (Estimated) | 2.124 x 10-7 | ÎŽpp | Used for modeling at 35 keV |
| Diamond Phase Plate Transmission | 75 | % | Total transmission of the plate itself |
| Polymer Phase Plate Transmission | > 99 | % | Highly transparent at 35 keV |
Key Methodologies
Section titled âKey MethodologiesâThe experimental methodology centered on fabricating custom phase plates based on wavefront errors measured via ptychography, followed by integration and re-characterization.
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Wavefield Characterization (Ptychography):
- The complex illumination wavefield (probe) was retrieved using ptychography, a scanning coherent diffraction imaging technique, at the plane of the lens exit aperture.
- The residual wavefront error (ΚΔ) was calculated by backpropagating the measured wavefield and subtracting a fitted ideal spherical wave (Ïs).
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Phase Plate Design:
- The required thickness profile zpp(x, y) was calculated to introduce a phase shift (Ïpp) that precisely compensates the measured wavefront error (arg[ΚΔ] = -Ïpp).
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Subtractive Fabrication (Diamond, 8.2 keV):
- Process: Femtosecond laser ablation was used on a CVD diamond substrate (300 fs pulses, 515 nm wavelength).
- Procedure: The desired profile was achieved by sequentially ablating 70 layers, using high-precision motion control to define the structure based on the design goal. Diamond was chosen for its high thermal conductivity and low absorption, suitable for high-intensity XFEL applications.
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Additive Fabrication (Polymer, 35 keV):
- Process: Two-photon polymerization (3D printing) was performed using a Nanoscribe Photonic Professional GT in dip-in lithography mode (IP-S resist).
- Procedure: The structure was sliced and hatched with 200 nm spacing, utilizing a galvo scanner for fast writing. This method provided superior shape accuracy (error < 200 nm) and smooth surfaces, ideal for storage-ring applications.
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Integration and Alignment:
- The fabricated phase plate was placed immediately behind the Be CRL stack casing (10.5 mm downstream at 8.2 keV; 20 mm downstream at 35 keV).
- Fine alignment (typically 2 ”m or less) was achieved by monitoring the wavefront correction via real-time ptychography until the best focus was reached.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology is critical for advancing high-resolution X-ray optics, particularly in fields requiring high flux density and diffraction-limited performance.
- Advanced X-ray Microscopy: Enables diffraction-limited nanofocusing (sub-100 nm) necessary for high-resolution scanning techniques like fluorescence imaging and ptychography, especially when investigating thick samples.
- Fourth-Generation Synchrotron Sources: Provides the necessary aberration correction for large-aperture refractive optics, allowing facilities (like PETRA IV or future upgrades) to fully exploit the increased lateral coherence and brightness from ultra-low-emittance storage rings.
- X-ray Free-Electron Lasers (XFELs): Diamond phase plates offer high radiation resistance and thermal stability, making them suitable for wavefront conditioning and focusing ultra-intense XFEL pulses.
- Custom Optical Element Manufacturing: Validates advanced manufacturing techniques (femtosecond ablation and two-photon polymerization) for producing complex, non-parabolic refractive X-ray optics, opening avenues for correcting other types of aberrations (e.g., those found in X-ray mirrors).
- Beam Transport and Conditioning: Improves the overall quality and Strehl ratio of focused X-ray beams, enhancing experimental throughput and signal-to-noise ratio in demanding experiments.
View Original Abstract
Modern subtractive and additive manufacturing techniques present new avenues for X-ray optics with complex shapes and patterns. Refractive phase plates acting as glasses for X-ray optics have been fabricated, and spherical aberration in refractive X-ray lenses made from beryllium has been successfully corrected. A diamond phase plate made by femtosecond laser ablation was found to improve the Strehl ratio of a lens stack with a numerical aperture (NA) of 0.88 Ă 10 â3 at 8.2 keV from 0.1 to 0.7. A polymer phase plate made by additive printing achieved an increase in the Strehl ratio of a lens stack at 35 keV with NA of 0.18 Ă 10 â3 from 0.15 to 0.89, demonstrating diffraction-limited nanofocusing at high X-ray energies.