Shack–Hartmann wavefront sensors based on 2D refractive lens arrays and super-resolution multi-contrast X-ray imaging
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-04-22 |
| Journal | Journal of Synchrotron Radiation |
| Authors | Andrey Mikhaylov, Stefan Reich, Margarita Zakharova, Vitor Vlnieska, Roman Laptev |
| Institutions | National Research Tomsk State University, Tomsk Polytechnic University |
| Citations | 21 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research details the development and characterization of Shack-Hartmann wavefront sensors (SHSX) for hard X-rays, focusing on polymer refractive lens arrays fabricated via 3D Direct Laser Writing (DLW).
- Core Achievement: Demonstrated the first combination of SHSX with a super-resolution imaging approach to enable fast, single-shot multi-contrast X-ray imaging (absorption, phase, dark-field).
- Performance Metrics: Achieved high angular resolution (0.29 µrad) and enhanced spatial resolution (21 µm) using a fourfold interleaving measurement technique.
- Design Optimization: Parabolic lens designs (SHSX v2.0 and v2.1) significantly outperformed the initial cylindrical design (v1.0), achieving a peak average gain of 10.2.
- Resolution Enhancement: The interleaving method successfully overcomes the Field-of-View (FoV) and spatial resolution limitations imposed by the current 3D DLW fabrication technology.
- Durability Assessment: Prototypes showed good operational stability for several hours (tested up to 15 h) under continuous hard X-ray white beam exposure, despite measurable polymer shrinkage.
- Fabrication Challenge: The larger SHSX v2.1 prototype (2 x 2 mm2) exhibited manufacturing artifacts (low gain areas) due to the stitching process required for the 3D DLW technique.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Angular Resolution (DPC) | 0.29 | µrad | SHSX v2.1 (Differential Phase Contrast) |
| Spatial Resolution (Interleaved) | 21 | µm | Achieved via fourfold sample shifting |
| Peak Average Gain | 10.2 | N/A | SHSX v2.1 (at 40.7 cm) |
| Minimum Spot Width (Fx) | 5.9 ± 0.4 | µm | SHSX v1.0 (at 29.7 cm) |
| Astigmatism Parameter (Δ) | 1.31 | % | SHSX v2.0 (lowest astigmatism) |
| Astigmatism Parameter (Δ) | 6.31 | % | SHSX v1.0 (highest astigmatism) |
| Experimental Focal Length | 40.7 | cm | SHSX v2.1 (distance of maximum gain) |
| X-ray Energy (Monochromatic) | 8.5 | keV | Characterization measurements |
| Lens Array Material | IP-S | N/A | Photoresist (Photonic Professional GT2) |
| SHSX v2.1 Total Volume | 2 x 2 x 1 | mm3 | Largest prototype FoV |
| SHSX v2.1 Initial Spot Pitch | ~87.5 | µm | Before radiation exposure |
| Radiation Damage Test Duration | ~15 | h | Continuous white beam exposure time |
| Detector Pixel Size (Effective) | 5.3 | µm | CMOS camera (Phantom v2640) with 50 µm LYSO scintillator |
Key Methodologies
Section titled “Key Methodologies”The SHSX prototypes were developed and characterized using the following procedures:
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Fabrication Method:
- 2D refractive lens arrays were manufactured using 3D Direct Laser Writing (DLW) technology based on two-photon polymerization (TPL).
- The material used was IP-S photoresist (Photonic Professional GT2).
- Three designs were compared: v1.0 (cylindrical lenses), v2.0 (parabolic, 1 x 1 mm2), and v2.1 (parabolic, 2 x 2 mm2).
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X-ray Characterization Setup:
- Experiments were conducted at the TOPO-TOMO beamline of the KARA synchrotron facility (KIT).
- Monochromatic beam (8.5 keV) was used for focal distance and gain measurements.
- White beam (filtered by 0.2 mm Al) was used for multi-contrast imaging tests.
- Detection utilized a CMOS camera coupled to a 50 µm LYSO scintillator, yielding an effective pixel size of 5.3 µm.
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Performance Quantification:
- Gain and Focus: Focal length was defined as the distance maximizing the average gain value (ratio of on-axis intensity with/without the lens).
- Aberration: Astigmatic aberration (Δ) was quantified by measuring the difference between focal planes in the x and y directions (Fx and Fy).
- Data Processing: Multi-contrast retrieval (absorption, phase, dark-field) was performed using a Gaussian beamlet fitting procedure to analyze the spot pattern.
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Super-Resolution Imaging (Interleaving):
- The sample (a diamond parabolic X-ray lens) was measured interleaved with sub-pitch shifts.
- A fourfold interleaving measurement was performed in both X and Y directions to achieve a nominal spatial resolution of 21 µm.
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Radiation Damage Study:
- The SHSX v2.1 prototype was subjected to continuous exposure under a hard X-ray white beam for approximately 15 hours.
- Spot pitch was monitored over time to quantify negative photoresist shrinkage, which was observed to be less severe on the side fixed to the holder.
Commercial Applications
Section titled “Commercial Applications”This technology is critical for advanced X-ray optics and high-speed imaging systems, particularly where high spatial and angular resolution are required simultaneously.
- Advanced X-ray Microscopy: Enabling high-speed, multi-contrast imaging (phase, absorption, dark-field) for biological samples and soft materials where traditional absorption contrast is insufficient.
- Non-Destructive Evaluation (NDE/NDT): High-resolution inspection of micro-scale defects and internal strain fields in complex manufactured components (e.g., microelectronics, aerospace materials).
- Synchrotron Beamline Diagnostics: Real-time monitoring and precise alignment of X-ray wavefronts and complex optical components (e.g., Compound Refractive Lenses, mirrors) in high-flux environments.
- Dynamic Process Analysis: Utilizing the fast, single-shot capability for studying transient phenomena and material dynamics (e.g., crystallization, phase transitions) with micrometre spatial resolution.
- Micro-Optics Metrology: Quality control and characterization of 3D-printed or micro-fabricated X-ray optics, verifying lens shape and performance parameters like focal length and astigmatism.
View Original Abstract
Different approaches of 2D lens arrays as Shack-Hartmann sensors for hard X-rays are compared. For the first time, a combination of Shack-Hartmann sensors for hard X-rays (SHSX) with a super-resolution imaging approach to perform multi-contrast imaging is demonstrated. A diamond lens is employed as a well known test object. The interleaving approach has great potential to overcome the 2D lens array limitation given by the two-photon polymerization lithography. Finally, the radiation damage induced by continuous exposure of an SHSX prototype with a white beam was studied showing a good performance of several hours. The shape modification and influence in the final image quality are presented.