Speckle contrast of interfering fluorescence X-rays

Area of Science:

• Atomic and Molecular Physics
• X-ray Science
• Coherent X-ray Optics

Background:

• X-ray free-electron lasers (XFELs) enable femtosecond X-ray pulses, allowing observation of intensity-intensity correlations from interfering atomic emissions.
• Previous applications include measuring X-ray pulse durations and focused X-ray beam sizes.
• Speckle contrast analysis is typically used with X-ray diffraction from elastic scattering, not fluorescence patterns.

Purpose of the Study:

• To demonstrate the observation of fluorescence photon interference via speckle contrast in angle-resolved fluorescence patterns.
• To address limitations of existing speckle contrast estimation methods when applied to XFEL fluorescence.
• To introduce a novel, reliable method for estimating speckle contrast under challenging XFEL conditions.

Main Methods:

• Utilized angle-resolved fluorescence patterns to observe interference phenomena.
• Investigated limitations of standard speckle contrast estimation techniques with low photon counts and shot-to-shot pulse energy variations.
• Developed and applied a new speckle contrast estimation method incorporating a weighting scheme.

Main Results:

• Successfully observed interference of fluorescence photons through speckle contrast measurements.
• Identified that conventional speckle contrast methods are unreliable for XFEL fluorescence data.
• Demonstrated the efficacy of the new weighted method by comparing speckle contrast for X-ray pulses ranging from 3 fs to 15 fs.

Conclusions:

• Fluorescence photon interference can be reliably measured using speckle contrast analysis.
• A new weighting-based method provides accurate speckle contrast estimation for XFEL fluorescence, even with low photon counts and variable pulse energies.
• The developed method enables precise characterization of X-ray pulse durations through fluorescence speckle patterns.