Bringing interferometric imaging into the X-ray regime

Bringing interferometric imaging into the X-ray regime

    Phay J. Ho

    • Division of Chemical Sciences and Engineering, Argonne National Laboratory, Lemont, IL, USA

Physics 16, 66

The experimental realization of a recently proposed technique points to new possibilities for x-ray imaging of molecules.

Stacy Huang

Figure 1: An intense X-ray free electron pulse is diffracted to ionize atoms at two distinct points on a copper film, resulting in the generation of X-ray fluorescence photon pairs. Each photon pair has two indistinguishable paths to reach a pair of pixels on a detector, thus producing a two-photon interference effect associated with Hanbury Brown and Twiss interferometry.An intense X-ray free electron pulse is diffracted to ionize atoms at two distinct points on a copper film, resulting in the generation of X-ray fluorescence photon pairs. Each photon pair has two indistinguishable paths to reach a pair of pixels or… Show more

Hanbury Brown and Twiss interferometry (HBT). [1] it is a versatile technique widely used in various fields of physics, such as astronomy, quantum optics and particle physics. By measuring the correlation of photon arrival times at two detectors as a function of the spatial separation of the photons, HBT interferometry makes it possible to determine the size and spatial distribution of a light source. Recently, a new X-ray imaging technique based on the HBT method has been proposed to visualize the spatial arrangement of heavy elements in a crystal or molecule by inducing those elements to fluoresce at X-ray wavelengths [2]. Now Fabian Trost of the German Electron Synchrotron (DESY) and colleagues, including some of the early proponents of the scheme, have implemented this technique, successfully demonstrating that the time correlation of fluorescence photons on a detector can be used to visualize the emitter structure on copper film [3]. This achievement marks a significant milestone towards extending HBT interferometry into high-resolution X-ray imaging, with the potential to visualize the structure and dynamics of isolated biomolecules without the need for crystallization [4].

The HBT effect is a two-photon interference phenomenon that occurs when two indistinguishable photons emitted from different points within a source reach two different detectors. The condition of indistinguishability is satisfied when the time interval between the two photons is included in the coherence time of the light source. The interference effect, constructive or destructive, can be quantified using the second order correlation function, or g(2), which describes the probability of detecting two photons simultaneously as a function of their spatial separation. If the arrival time is longer than the coherence time, this will lead to a reduction in contrast in the interference fringes g(2).

Extending the HBT technique to the X-ray regime has been a challenge due to the low probability of X-ray excitation and the short coherence time of X-ray fluorescence, especially for heavy elements. Here the coherence time is given by the duration of the fluorescence states. For example, the electronic state lifetime of a copper atom with a K-shell vacancy is less than 1 fs. The emission of two copper atoms will be coherent or indistinguishable only if the atoms are excited within that range. The development of X-ray free electron lasers (XFELs) has helped in this regard, making it possible to generate high-intensity X-ray pulses with durations of femtoseconds or even shorter [5]. These pulses can excite the K-shell electrons of heavy elements with a high probability and increase the possibility of producing indistinguishable pairs of photons through X-ray fluorescence.

Trost and colleagues used these pulses to measure the intensity correlation of fluorescence photons emitted from a copper film. Conducting their experiment at the European XFEL facility in Germany, the researchers used a phase grating to diffract the incoming X-ray pulse and focus it to two points on a micron-sized copper foil. The incident X-ray photons had an energy of 9 keV, which was sufficient to ionize the K-shell electron of the illuminated copper atoms on the film. This ionization process created a short-lived excited state that decayed mainly via fluorescence emission, which the researchers measured using a specially developed detector at the XFEL facility. With one million pixels, each capable of detecting a single photon, this detector can measure 1012 correlations between pairs of pixels.

The team faced several challenges to implement X-ray imaging using this technique. One challenge was that the detector used in the experiment does not resolve photon energy, meaning that it cannot exclude contributions from other sources of radiation than the desired one


emission. This non-selectivity would lead to a low signal-to-noise ratio. To address this problem, Trost and colleagues used a nickel filter to block elastically scattered radiation and copper



Another challenge was that the X-ray pulse duration was 10 times longer than the coherence duration of the


emission, which reduces the contrast of the interference fringes. To improve signal quality, the researchers recorded 58 million 2D fluorescence images in about 5 hours, made possible by the detector’s high readout speed and high repetition rate of the XFEL pulses. To ensure that the sample was not damaged by the pulses, the copper foil was rotated so that each pulse illuminated a new area. Although the combined fluorescence image was isotropic and contained no structural information, the constructed g(2) revealed interference fringes reaching third-order peaks, which is an improvement over previous studies that only measured the zero-order peak in g(2) [6, 7]. Using an iterative algorithm, the researchers were able to reconstruct the size (300 nm) and separation (860 nm) of the two excited dots on the copper film.

Given a substantial improvement in spatial resolution, the technique could eventually allow for the imaging of single particles of biomolecules and atomic-scale catalysts and, with sufficient temporal resolution, the characterization of their reaction dynamics. Continuing advances in XFEL technology could also mean that the photon-hungry process of fluorescence excitation can be achieved with sub-femtosecond pulses. For example, using intense sub-femtosecond X-ray pulses, the temporal fluorescence profile can be tailored to be shorter than the duration of the fluorescence states [8]. To harness the power of HBT interferometry for chemical imaging with elemental specificity, it is highly desirable to develop multicolor imaging using multiple detectors or detectors with energy discrimination capabilities. Further development in this direction has the potential to revolutionize the characterization of important catalytic functions and associated structural changes in their native environments, such as metal-containing clusters in metalloproteins [9]. Such a characterization would provide unprecedented insight into the structure and activity of these catalysts, which is crucial for the development of renewable energy sources.


  1. R. Hanbury Brown and RQ Twiss, Correlation between photons in two coherent light beams, Nature 17727 (1956).
  2. A. Class et al.Inconsistent diffractive imaging via hard x-ray intensity correlations, Phys. Rev. Lett. 119053401 (2017).
  3. F. Trost et al.X-ray fluorescence photon correlation imaging, Phys. Rev. Lett. 130173201 (2023).
  4. R. Neuze et al.Potential for biomolecular imaging with femtosecond X-ray pulses, Nature 406752 (2000).
  5. J.Duris et al.Tunable isolated certosecond X-ray pulses with peak power in gigawatts from a free electron laser, Nat. Photonics 1430 (2019).
  6. I. Inoue et al.Determination of X-ray pulse duration by X-ray fluorescence intensity correlation measurements, J. Synchrotron radiation. 262050 (2019).
  7. N.Nakamura et al.Characterization of the focus of an X-ray free electron laser by measuring the intensity correlation of X-ray fluorescence, J. Synchrotron radiation. 271366 (2020).
  8. PJ Ho et al.Fluorescence intensity correlation imaging with high spatial resolution and elemental contrast using intense X-ray pulses, Structure. dyn. 8044101 (2021).
  9. J. Yano and V. Yachandra, Mn4Cluster CA in photosynthesis: where and how water is oxidized to dioxygen, chem. rev. 1144175 (2014).

About the author

Image by Phay J. Ho

Phay J. Ho is a physicist in the Atomic, Molecular, and Optical Physics Group in the Division of Chemical Sciences and Engineering at Argonne National Laboratory in Illinois. He holds degrees in physics and chemistry from Louisiana State University and a PhD from the University of Rochester, New York. His research focuses mainly on the study of fundamental interactions between atomic, molecular and nanoscale systems with optical and X-ray radiation, with the aim of characterizing and controlling their dynamics for potential applications in imaging and spectroscopy.

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