Single-crystal diamond refractive lens for focusing X-rays in two dimensions

At a Glance

Metadata	Details
Publication Date	2015-12-15
Journal	Journal of Synchrotron Radiation
Authors	Sergey Antipov, Sergey V. Baryshev, J. E. Butler, Olga Antipova, Z. Liu
Institutions	Illinois Institute of Technology, Argonne National Laboratory
Citations	26
Analysis	Full AI Review Included

Technical Documentation & Analysis: Single-Crystal Diamond Refractive Lenses

6CCVD Material Analysis Reference: Antipov et al. (2016). Single-crystal diamond refractive lens for focusing X-rays in two dimensions. J. Synchrotron Rad. 23, 163-168.

Executive Summary

This research validates the use of single-crystal chemical vapor deposition (SCD CVD) diamond as the superior material for next-generation X-ray Compound Refractive Lenses (CRLs), essential for high-brilliance sources like XFELs and diffraction-limited storage rings.

Material Superiority: SCD diamond’s exceptional thermal conductivity, high radiation hardness, and low thermal expansion coefficient make it indispensable for wavefront-preserving optics operating under extreme peak and average power densities.
Two-Dimensional Focusing: Successful fabrication and testing of SCD lenses with paraboloids of revolution surfaces, enabling simultaneous X-ray focusing in two dimensions.
Fabrication Method: Femtosecond laser micromachining was employed to minimize thermal fatigue and achieve the required parabolic profiles, demonstrating a viable path for complex diamond optics manufacturing.
High Performance: A single lens achieved an effective transmission (T^eff) of 0.87 (87%) at 13.6 keV, confirming excellent material quality and minimal attenuation losses.
CRL Achievement: A stack of three lenses formed a CRL prototype, achieving a high gain of 53.5 and focusing an 11.85 keV undulator beam into a tight spot size of 52.6 µm x 21.4 µm.
Future Optimization: The results indicate that performance can be further enhanced through improved surface polishing (reducing roughness from the measured ~1 µm r.m.s.) and precise lens stacking techniques.

Technical Specifications

Parameter	Value	Unit	Context
Material Type	Single-Crystal CVD Diamond	N/A	Mechanical, Optical, and Electronic grades tested
Lens Thickness	~500	µm	Thickness of starting SCD plates
Lens Aperture (Single)	450	µm	Largest diameter machined on the surface
Radius of Curvature (R)	100 ± 5.0 and 109 ± 5.5	µm	Measured by white-light interferometry (WLI)
Waist Separation (d)	50	µm	Measured distance between paraboloids
Surface Roughness (σ)	~1	µm r.m.s.	Estimated from SEM measurements
Optimal Photon Energy (Single Lens)	13.6	keV	Used for bending-magnet source refocusing
Effective Transmission (T^eff)	0.87	N/A	Single lens performance at 13.6 keV
Single Lens Gain	2.83	N/A	Measured gain at 13.6 keV
CRL Stack Size	3	Lenses	Used for Compound Refractive Lens prototype
CRL Total Length	1.5	mm	Total length of the 3-lens stack
CRL Test Energy	11.85	keV	Used for undulator radiation testing
CRL Focal Length	3.36	m	Calculated focal length at 11.85 keV
CRL Gain	53.5	N/A	Corrected gain for the Gaussian portion of the beam
CRL Focused Spot Size	52.6 x 21.4	µm	FWHM spot size achieved at 11.85 keV

Key Methodologies

The fabrication and testing of the two-dimensional diamond refractive lenses relied on specialized material processing and high-precision synchrotron testing protocols.

Material Selection: Single-crystal CVD diamond plates, approximately 500 µm thick, were sourced. Various grades (mechanical, optical, electronic, distinguished by nitrogen content: <1 p.p.b. to ~1 p.p.m.) were tested, showing no substantial difference in performance.
Micromachining: Femtosecond laser micromachining was used to ablate the diamond material. This ultra-short pulse duration minimized pulsed heating effects and thermal fatigue, which are detrimental when using conventional nanosecond-pulse laser cutters.
Profile Generation: The femtosecond laser beam was steered by a galvo mirror to ablate circle patterns, gradually reducing the circle diameter with depth to generate the required paraboloids of revolution.
Dual Paraboloid Machining: Identical, matching paraboloids were machined on opposite sides of the diamond plate. Laser power was scaled for the second paraboloid to maintain a minimum separation (waist) of 40 µm, with a measured waist of 50 µm in the tested lenses.
Post-Processing: After micromachining, the diamond lenses were cleaned in a hot mineral acid bath containing an oxidizing agent to remove residual carbon or debris.
Metrology: White-light interferometry (WLI) and optical profilometry were used to measure the radii of curvature (R). Scanning Electron Microscopy (SEM) confirmed surface roughness (σ) to be approximately 1 µm r.m.s.
X-ray Testing: Lenses were tested using synchrotron radiation at the Advanced Photon Source (APS). Single lenses were tested at 13.6 keV (bending magnet source), and the 3-lens CRL stack was tested at 11.85 keV (undulator source).

6CCVD Solutions & Capabilities

The successful fabrication of high-performance diamond CRLs hinges on the availability of ultra-high quality, thick SCD material and precise post-processing. 6CCVD is uniquely positioned to supply the foundational materials and engineering services required to replicate and advance this critical X-ray optics research.

Applicable Materials for X-ray Optics

The research confirmed that high-purity SCD is necessary for wavefront preservation and high radiation tolerance. 6CCVD provides the following materials suitable for X-ray optics applications:

6CCVD Material	Purity/Grade	Key Application Relevance
Electronic Grade SCD	Nitrogen <1 p.p.b.	Ideal for minimizing absorption and maximizing wavefront quality in high-brilliance XFEL and Synchrotron applications.
Optical Grade SCD	Nitrogen <200 p.p.b.	Excellent thermal properties and high transparency, suitable for high-heat-load monochromators and CRLs.
High-Purity PCD	Customizable Grain Size	Cost-effective alternative for applications where grain boundaries are acceptable or where large-area optics (up to 125mm) are required.

Customization Potential for CRL Fabrication

The paper utilized 500 µm thick SCD plates with a 450 µm aperture. 6CCVD’s manufacturing capabilities directly address the dimensional and quality requirements for replicating and scaling up this research:

Custom Dimensions and Thickness:
- 6CCVD supplies SCD plates with thicknesses ranging from 0.1 µm up to 500 µm, matching the starting material thickness used in the study.
- We offer custom plate sizes and wafers up to 125 mm (PCD) and large-area SCD, enabling the fabrication of lenses with significantly larger lateral apertures than the 450 µm demonstrated, accommodating typical high-brightness X-ray beams.
Surface Quality Improvement:
- The paper noted that the measured surface roughness (σ ≈ 1 µm r.m.s.) contributed to reduced gain and broadened focal spots.
- 6CCVD specializes in ultra-low roughness polishing, achieving Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD. Utilizing 6CCVD polished substrates would significantly reduce scattering losses and improve the effective transmission and gain of the resulting CRLs.
Metalization Services:
- While not explicitly required for the refractive lens itself, 6CCVD offers in-house metalization (Au, Pt, Pd, Ti, W, Cu) for integration of diamond optics into complex mounting or cooling systems, or for creating alignment fiducials.
Precision Cutting:
- We provide high-precision laser cutting and dicing services to achieve the exact lateral dimensions and stacking tolerances required for advanced CRL prototypes.

Engineering Support

6CCVD’s in-house PhD team possesses deep expertise in the material science and application of diamond in extreme environments. We can assist researchers and engineers with:

Material Selection: Optimizing the SCD grade (e.g., Electronic vs. Optical) based on specific X-ray energy, flux, and thermal load requirements for similar X-ray Optics and Compound Refractive Lens (CRL) projects.
Tolerance Specification: Defining optimal thickness uniformity and surface roughness specifications to ensure maximum wavefront preservation and minimal scattering, crucial for achieving theoretical gain limits.
Integration Support: Consulting on mounting and thermal management strategies for diamond optics in high-heat-load environments, leveraging diamond’s record high thermal conductivity.

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

View Original Abstract

The fabrication and performance evaluation of single-crystal diamond refractive X-ray lenses of which the surfaces are paraboloids of revolution for focusing X-rays in two dimensions simultaneously are reported. The lenses were manufactured using a femtosecond laser micromachining process and tested using X-ray synchrotron radiation. Such lenses were stacked together to form a standard compound refractive lens (CRL). Owing to the superior physical properties of the material, diamond CRLs could become indispensable wavefront-preserving primary focusing optics for X-ray free-electron lasers and the next-generation synchrotron storage rings. They can be used for highly efficient refocusing of the extremely bright X-ray sources for secondary optical schemes with limited aperture such as nanofocusing Fresnel zone plates and multilayer Laue lenses.

Tech Support

Original Source

DOI: https://doi.org/10.1107/s1600577515020639