Multilayer reflecting x-ray optical systems: chromatic vignetting by narrow reflection bands.

Area of Science:

• Optics
• Materials Science
• X-ray Optics

Background:

• Multilayer mirrors can achieve unlimited numerical aperture for point-to-point focusing.
• Axially symmetric systems achieve this by radially varying multilayer Bragg spacing.
• Short wavelengths (<75 A) require many layer pairs, leading to narrow reflection bandwidths and phase shifts.

Purpose of the Study:

• Investigate the impact of narrow bandwidths on multilayer mirror performance.
• Address chromatic vignetting in multilayer reflecting optical systems.
• Determine optimal multilayer spacing strategies for image quality.

Main Methods:

• Analysis of optical phase changes across narrow reflection bandwidths.
• Evaluation of reflectance and phase coherence for off-axis points.
• Comparison of uniform vs. corrected Bragg spacing for image formation.

Main Results:

• Narrow bandwidths (<1/40) cause reduced reflectance and phase coherence off-axis.
• Chromatic vignetting affects both axially symmetric and non-symmetric systems.
• Uniform multilayer spacing provides consistent resolution and intensity over the image, albeit with limitations.

Conclusions:

• For moderate image sizes and mirror sizes, uniform multilayer spacing is preferable.
• This approach limits numerical aperture and resolution but ensures uniformity.
• Shorter wavelengths (<75 A) paradoxically decrease resolution in such systems due to increased layer requirements.