Hard X-ray wavefront correction via refractive phase plates made by additive and subtractive fabrication techniques

At a Glance

Metadata	Details
Publication Date	2020-07-30
Journal	Journal of Synchrotron Radiation
Authors	Frank Seiboth, Dennis Brückner, Maik Kahnt, Mikhail Lyubomirskiy, Felix Wittwer
Institutions	Deutsches Elektronen-Synchrotron DESY, Friedrich Schiller University Jena
Citations	24
Analysis	Full AI Review Included

Technical Documentation & Analysis: Hard X-ray Wavefront Correction using MPCVD Diamond Phase Plates

Reference: Seiboth et al. (2020). Hard X-ray wavefront correction via refractive phase plates made by additive and subtractive fabrication techniques. J. Synchrotron Rad. 27, 1121-1130.

Executive Summary

This research validates the use of diamond phase plates for high-performance X-ray optics, directly aligning with 6CCVD’s core material expertise.

Aberration Correction Success: Spherical aberration in Beryllium Compound Refractive Lenses (CRLs) was successfully corrected using refractive phase plates fabricated from single-crystal CVD diamond and polymer.
Performance Improvement: The diamond phase plate improved the Strehl ratio of a lens stack at 8.2 keV from 0.10 to 0.70, demonstrating a seven-fold increase in central speckle intensity.
Material Superiority for XFELs: Single-crystal diamond (SCD) is confirmed as the material of choice for high-intensity applications due to its high thermal conductivity and low X-ray absorption.
High-Energy Capability: Correction was successfully demonstrated at high X-ray energy (35 keV), achieving a diffraction-limited Strehl ratio of 0.89 using a polymer plate.
Fabrication Challenge Identified: Femtosecond laser ablation of diamond resulted in a high surface roughness (S_a = 0.32 µm), which limits ultimate performance and necessitates post-processing (polishing).
6CCVD Value Proposition: 6CCVD’s ultra-low roughness SCD material (Ra < 1 nm) directly addresses the primary limitation (surface roughness) encountered in the diamond fabrication process described.

Technical Specifications

The following hard data points were extracted from the experimental results comparing aberrated and corrected lens systems.

Parameter	Value	Unit	Context
Low X-ray Energy	8.2	keV	DLS I13-1 Beamline
High X-ray Energy	35.0	keV	PETRA III P06 Beamline
Diamond Substrate Thickness	300	µm	CVD Single-Crystal Diamond
Diamond Phase Plate Roughness (S_a)	0.32	µm	Post-ablation measurement
Strehl Ratio (Aberrated, 8.2 keV)	0.10	N/A	50 Be CRLs, without phase plate
Strehl Ratio (Corrected, 8.2 keV)	0.70	N/A	Diamond phase plate correction
Corrected Focal Spot Size (8.2 keV)	76	nm	FWHM
Strehl Ratio (Aberrated, 35 keV)	0.15	N/A	149 Be CRLs, without phase plate
Strehl Ratio (Corrected, 35 keV)	0.89	N/A	Polymer phase plate correction
Corrected Focal Spot Size (35 keV)	95	nm	FWHM (Coherent spot size)
Diamond Transmission (System Total)	12.4	%	Total optical system transmission
Polymer Transmission (System Total)	35	%	Total optical system transmission
Diamond PV Wavefront Error Reduction	3.5 λ to 0.75 λ	N/A	Peak-to-Valley error

Key Methodologies

The experiment utilized complementary fabrication techniques and advanced X-ray imaging methods to characterize and correct wavefront errors.

Material Selection: Single-crystal CVD diamond (2.6 mm x 2.6 mm x 0.3 mm, (100) orientation) was selected for its superior radiation resistance and thermal properties, essential for high-intensity X-ray sources.
Diamond Fabrication (Subtractive): Phase plates were manufactured using femtosecond laser ablation.
- The diamond substrate was initially thinned by 76 µm (12 kHz repetition rate, 160 nJ pulse energy).
- The phase plate structure was subsequently ablated in 70 layers (6 kHz repetition rate, 50 nJ pulse energy).
Polymer Fabrication (Additive): Phase plates for 35 keV correction were manufactured using two-photon polymerization (TPP) in a Nanoscribe IP-S resist.
Wavefront Characterization: Ptychography, a scanning coherent diffraction imaging method, was used to retrieve the complex wavefield and quantitatively determine the performance of the Be CRLs.
Phase Plate Design: The thickness profile $z_{pp}(x, y)$ was calculated based on the residual wavefront error ($\Psi_{\epsilon}$) derived from the ptychographic reconstruction, designed to introduce a compensating phase shift ($\phi_{pp}$).
Surface Metrology: Laser-scanning microscopy (LSM) was used to measure the surface profile and determine the roughness of the fabricated phase plates.

6CCVD Solutions & Capabilities

6CCVD provides the high-specification MPCVD diamond materials and precision processing required to replicate and extend this research, particularly addressing the surface roughness limitations inherent in the laser ablation method used in the paper.

Applicable Materials for X-ray Wavefront Correction

Requirement from Paper	6CCVD Solution	Technical Specification	Value Proposition
High Radiation Resistance	Optical Grade SCD	SCD (100) orientation, low nitrogen content	Ideal thermal management and low absorption for XFEL and high-flux synchrotron applications.
Low Surface Roughness	Precision Polished SCD	Ra < 1 nm (Standard)	Directly mitigates the S_a = 0.32 µm roughness reported from laser ablation, minimizing incoherent scattering and improving shape fidelity.
High-Energy Transparency	Optical Grade SCD	Thicknesses from 0.1 µm up to 500 µm	Optimized transmission for hard X-ray energies (8.2 keV to 35 keV and beyond).

Customization Potential for Advanced X-ray Optics

6CCVD’s manufacturing capabilities are perfectly suited to meet the demanding specifications of next-generation X-ray optics, including custom phase plates and CRL components.

Custom Dimensions: While the paper used small 2.6 mm x 2.6 mm plates, 6CCVD offers SCD plates up to 500 µm thick and large-area PCD wafers up to 125 mm in diameter, enabling the production of larger aperture optics necessary for collecting high lateral coherence beams.
Precision Thickness Control: 6CCVD offers SCD and PCD materials with thickness control down to 0.1 µm, crucial for achieving the precise phase shifts required for aberration correction across a broad energy range.
Metalization Services: For integration into complex optical setups, 6CCVD provides in-house metalization capabilities (Au, Pt, Pd, Ti, W, Cu). This allows researchers to specify precise metal layers for mounting, alignment, or electrical contacting of the diamond phase plates.
Substrate Preparation: 6CCVD can supply custom diamond substrates up to 10 mm thick, providing robust platforms for subsequent microfabrication techniques (like the laser ablation or ion beam etching mentioned in the paper).

Engineering Support

6CCVD’s in-house PhD team specializes in the application of MPCVD diamond for demanding scientific instruments. We offer expert consultation on:

Material Selection: Optimizing diamond grade (SCD vs. PCD, doping levels) based on specific X-ray energy, flux, and thermal load requirements.
Surface Finish Optimization: Assisting researchers in specifying the optimal polishing grade to ensure diffraction-limited performance (Ra < 1 nm) for X-ray wavefront correction projects.
Integration Support: Providing technical guidance on mounting and integrating custom diamond optics into ptychography and nanofocusing beamline setups.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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 × 10 −3 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 × 10 −3 from 0.15 to 0.89, demonstrating diffraction-limited nanofocusing at high X-ray energies.

Tech Support

Original Source

DOI: https://doi.org/10.1107/s1600577520007900