A Bragg beam splitter for hard x-ray free-electron lasers

Area of Science:

• Physics
• Materials Science
• Optics

Background:

• Hard X-ray free-electron lasers (XFELs) require advanced optical components for precise beam manipulation.
• Existing beam splitters may not meet the stringent requirements for reflectivity, transmissivity, and wavefront uniformity demanded by XFEL applications.

Purpose of the Study:

• To develop and characterize a Bragg beam splitter optimized for hard X-ray free-electron laser (XFEL) utilization.
• To achieve simultaneous high reflectivity and transmissivity using ultrathin silicon crystals in a symmetric Bragg geometry.

Main Methods:

• Fabrication of frame-shaped Si(511) and (110) crystals with thicknesses below 10 μm using reactive dry etching with atmospheric-pressure plasma.
• Characterization of crystal thickness uniformity (<300 nm peak-to-valley variation) and crystalline perfection using topographic and diffractometric measurements.
• Evaluation of crystal thickness via Pendellösung beats with a monochromatic and collimated X-ray probe.

Main Results:

• Successfully fabricated ultrathin silicon crystals with high crystalline perfection.
• Demonstrated simultaneous high reflectivity and transmissivity, crucial for XFEL beam splitting.
• Achieved two replica pulses with uniform wavefront (<1/50λ) and low spatial intensity variation (<5%).

Conclusions:

• The developed Bragg beam splitter meets the demanding specifications for hard X-ray free-electron laser applications.
• These ultrathin silicon crystal beam splitters are poised to innovate and enhance various XFEL-based research areas.
• The precise control over crystal thickness and uniformity is key to the splitter's high performance.