Polished diamond X-ray lenses

At a Glance

Metadata	Details
Publication Date	2022-03-15
Journal	Journal of Synchrotron Radiation
Authors	Rafael Celestre, Sergey Antipov, Edgar Gomez, Thomas Zinn, Raymond Barrett
Institutions	Euclid Techlabs (United States), European Synchrotron Radiation Facility
Citations	15
Analysis	Full AI Review Included

Executive Summary

This research presents the successful fabrication and characterization of high-quality, bi-concave 2D focusing Compound Refractive Lenses (CRLs) made from single-crystalline High-Pressure High-Temperature (HPHT) diamond.

Core Achievement: Production of polished diamond X-ray lenses (R = 100 µm) via femtosecond laser ablation followed by chemical-mechanical polishing.
Performance Parity: The focusing capabilities (focal spot size) of the polished diamond lens stacks are comparable to commercial R = 50 µm Beryllium (Be) lens stacks of equivalent focusing strength.
Superior Scattering: Polishing reduced the surface roughness (from 300-500 nm S_a to ~20 nm S_a), resulting in a Small-Angle X-ray Scattering (SAXS) signal almost two orders of magnitude lower than that of commercial O30-H grade Be lenses.
Figure Error: The 10x polished diamond stack achieved an RMS figure error (σ) of 4.36 µm, comparing favorably to the 11x Be stack (σ = 5.60 µm).
Material Advantage: Diamond offers superior thermal conductivity and a high refraction-to-absorption ratio (δ/µ), making it the material of choice for high-energy (above 30 keV) and high-power X-ray applications.
Standardized Packaging: Lenses are housed in 12 mm-diameter precision-machined bronze disks, ensuring compatibility with existing synchrotron hardware (transfocators, v-blocks).

Technical Specifications

Parameter	Value	Unit	Context
Lens Material	HPHT (100)-oriented diamond	-	Low dislocation density synthetic diamond
Target Apex Radius (R)	100	µm	Diamond lenses
Equivalent Apex Radius (R)	50	µm	Commercial Be lenses
Refraction Ratio (δ_C*/δ_Be)	~2.14	-	Mean ratio across 5 keV to 50 keV
Physical Aperture (A_phys)	~440	µm	Similar for both diamond and Be lenses
Crystal Thickness (L_c)	~500	µm	Diamond lens thickness
Web Thickness (t)	~20	µm	Distance between paraboloidal apices (diamond)
As-Ablated Roughness (S_a)	300-500	nm	Measured by confocal microscopy
Polished Roughness (S_a)	~20	nm	Achieved via chemical-mechanical polishing
RMS Figure Error (σ)	0.91	µm	Single polished diamond lens
RMS Figure Error (σ)	4.36	µm	10x polished diamond stack (full aperture)
Focal Length (f)	694	mm	10x unpolished diamond stack (at 10 keV)
Focal Length (f)	674	mm	11x Be stack (at 10 keV)
FWHM Beam Size (H)	1.61	µm	10x polished diamond stack (at focus, 10 keV)
FWHM Beam Size (V)	0.98	µm	10x polished diamond stack (at focus, 10 keV)
SAXS Reduction	~2	orders of magnitude	Polished diamond vs. Be (O30-H grade)
Laser Wavelength	515	nm	Green femtosecond laser ablation
Laser Pulse Duration	200	fs	Used for micro-machining

Key Methodologies

The fabrication and characterization process involved three main stages: raw material preparation, laser ablation, and post-processing/metrology.

Raw Material Selection:
- Used low dislocation density HPHT (100)-oriented synthetic diamond plates (3 mm x 3 mm x 0.5 mm).
- Plates were truncated cone-shaped and pressed into 12 mm-diameter bronze support disks for standardized packaging and alignment.
Femtosecond Laser Ablation (Micro-machining):
- A green 515 nm laser with a 200 fs pulse duration was used.
- The bi-concave paraboloid profile was achieved by decomposing the shape into circular layers, with the laser beam steered to cover the circular area uniformly.
- Fiducial markings were used on the support disk to maintain azimuthal orientation during ablation.
Post-Processing (Chemical-Mechanical Polishing):
- A chemical-mechanical polishing procedure was developed to reduce the as-ablated roughness (300-500 nm S_a).
- A conformal needle (polishing bit) was spun inside the lens cavity for several hours.
- A 0.1 µm diamond slurry was used as the abrasive.
- Periodic sideways pressure was applied to maintain a quasi-uniform removal rate across the curved surface.
At-Wavelength Metrology (XSVT):
- X-ray Speckle Vector Tracking (XSVT) was performed at 17 keV (single lenses) and 30 keV (stacks) to measure figure errors and projected thickness.
- Random modulators (cellulose acetate filters or silicon carbide abrasive paper) were used to generate speckle patterns.
- XSVT allowed quantification of RMS figure errors and Optical Path Difference (OPD) for both individual lenses and stacked CRLs.
Performance Characterization:
- Beam Caustics: Measured 2D beam profiles along the beam path using a CCD detector (0.62 µm effective pixel size) at 10 keV.
- Focal Spot Size: Measured minimum beam size at focus using a tungsten wire scan (0.25 µm step size) coupled to a p-i-n diode to overcome the detector resolution limit.
- SAXS: Small-Angle X-ray Scattering measurements were performed at 12.23 keV to compare background scattering intensity between unpolished, polished diamond, and Be lenses.

Commercial Applications

The development of high-quality, low-scattering diamond X-ray lenses is critical for advanced X-ray optics, particularly in high-flux and high-energy environments where traditional materials like Beryllium suffer from thermal load and increased background noise.

Synchrotron and Free-Electron Lasers (FELs):
- High-Energy Focusing: Diamond lenses are preferred for X-ray energies above 30 keV due to their superior transmission and refraction-to-absorption ratio compared to Be.
- High-Heat Load Optics: Diamond’s unrivaled thermal conductivity allows these lenses to withstand intense, short-duration X-ray pulses or high-power white beams without significant thermal deformation, maintaining focus stability.
Nanofocusing and Microscopy:
- High-Resolution Imaging: The low SAXS background achieved by polishing enables cleaner, higher-contrast images, crucial for techniques like coherent diffraction imaging (CDI) and ptychography.
- Micro- and Nanoprobe Beamlines: Used to achieve the smallest possible focal spots for elemental analysis (XRF) and structural studies (XRD) at the nanoscale.
Wavefront Correction:
- The strong rotational symmetry of the figure errors in the polished stacks (spherical aberration) makes them highly amenable to correction using azimuthally symmetric refractive phase plates, leading to near-ideal focusing performance.
Industrial and Research X-ray Sources:
- Applications requiring compact, robust, and high-performance focusing optics for laboratory-based X-ray systems.

View Original Abstract

High-quality bi-concave 2D focusing diamond X-ray lenses of apex-radius R = 100 µm produced via laser-ablation and improved via mechanical polishing are presented here. Both for polished and unpolished individual lenses and for stacks of ten lenses, the remaining figure errors determined using X-ray speckle tracking are shown and these results are compared with those of commercial R = 50 µm beryllium lenses that have similar focusing strength and physical aperture. For two stacks of ten diamond lenses (polished and unpolished) and a stack of eleven beryllium lenses, this paper presents measured 2D beam profiles out of focus and wire scans to obtain the beam size in the focal plane. These results are complemented with small-angle X-ray scattering (SAXS) measurements of a polished and an unpolished diamond lens. Again, this is compared with the SAXS of a beryllium lens. The polished X-ray lenses show similar figure errors to commercially available beryllium lenses. While the beam size in the focal plane is comparable to that of the beryllium lenses, the SAXS signal of the polished diamond lenses is considerably lower.

Tech Support

Original Source

DOI: https://doi.org/10.1107/s1600577522001795