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Lensless X-Ray Imaging in Reflection Print

The advent of x-ray free-electron laser (XFEL) light sources has led to an outburst of research activities in the field of lensless imaging. XFELs combine the advantages of sychrotron light sources (high brightness and x-ray wavelengths relevant to atomic and molecular phenomena) with the advantages of visible-light lasers (highly coherent beams). All of these characteristics are important for coherent x-ray diffraction imaging—lensless imaging techniques that are proving to be integral to single-shot, high-resolution imaging of both complex materials and biological samples. Existing techniques are typically designed for transmission geometry, however, and use isolated objects, requiring special sample fabrication and restricting the type of samples under investigation. Recently, researchers from the ALS and the University of Oregon have shown at ALS Beamline 12.0.2 that it is possible to form x-ray holograms in reflection geometry by using the light scattered from a sample, opening the door to lensless imaging of a wealth of new material samples.

X-Rays, Lasers, and Holograms

Taken individually, x-rays, lasers, and holograms can each produce some pretty nifty effects. X-rays are a short-wavelength form of light energetic enough to penetrate opaque solids. Lasers produce light rays that are extremely uniform in color, direction, and phase. Holograms are ghostly images produced by recording the interference patterns of scattered light waves. In the 1960s, the introduction of lasers to holographic techniques resulted in the first practical holograms of three-dimensional objects. In the 1970s, the invention of the free-electron laser would lead to the generation of x-rays with the characteristics of a laser beam. Today, these three elements—x-rays, lasers, and holograms—can be combined and employed simultaneously, resulting in a powerful tool for the lensless imaging of nanoscale phenomena. In this work, Roy et al. demonstrate for the first time a lensless x-ray imaging technique involving holographic principles and carried out in reflection (as opposed to transmission) geometry. The reflection geometry opens up the possibility of single-shot imaging of surfaces in thin films, buried interfaces in magnetic multilayers or electronic devices, and Bragg planes in single crystals.

Schematic of reflection geometry

Schematic of the geometry used to achieve lensless diffractive imaging in reflection geometry. The x-ray beam's wave information at the three apertures in the exit screen can be reconstructed and back-propagated to the sample.

Complex interplay between spin, lattice, and orbital degrees of freedom in materials often gives rise to multifunctional properties and exotic phases. A few examples among many include spin/orbital ordering in colossal magnetorersistive materials and helical ordering of spins that gives rise to chiral vortex phases ("skyrmions") in magnetic systems. The ability to directly image the Bragg planes or surfaces where such order emerges would be a great boon to understanding how these phases develop and how they affect material properties, but there has been no easy way to perform such measurements. While many specimens can be prepared to work in transmission, others—particularly in materials physics—will require samples that are both extended and nontransmissive. The development of an imaging technique suitable for extended objects in reflection geometry is therefore imperative.

In ordinary imaging, a lens produces an image of an object by manipulating and reconstructing the phase and amplitude relationships of the optical light rays scattered from the object. In lensless imaging, the x-rays scattered from an object form a diffraction pattern that retains information about the amplitudes, but not the phases. To obtain a reconstruction of the object, a solution to this "phase problem" is required. While difficult, recovering the missing phase in a suitably oversampled diffraction pattern can be accomplished through iterative computational techniques. Alternatively, introducing a planar reference wave into the scattering leads to a unique encoding of the phase, and the sample can then be reconstructed by a single Fourier transform; this is known as Fourier transform holography. The problem of getting sufficiently coherent x-rays onto and off of the sample in a manner consistent with either technique has prevented the use of coherent imaging techniques with reflective samples.

Researchers working at  Beamline 12.0.2 have solved this problem by placing a screen containing a set of apertures in the near field downstream of the scattering object. The screen splits the scattered signal into object and reference waves that interfere in the far field and are recorded on a detector (a holographic recording). When the reference apertures are made very small, the reference wave can be considered planar and the detected image can be inverted with a single Fourier transform. When the reference apertures are somewhat larger, the speckle pattern is inverted using iterative techniques. Once the phase and amplitude information of the light rays at the apertures has been recovered, it is numerically back-propagated to the sample surface, revealing the final reconstructed image—a two-dimensional hologram.

Actual test pattern and image

Left: The test pattern was a pseudo-random nanostructure, shown here from the perspective of the incident x-ray beam. Right: Reconstructed x-ray image of the nonredundant array. The distance from the sample to the exit aperture (z) was a variable determined in the reconstruction.

This new technique can use an extended sample as in a conventional scattering experiment without the need for special sample preparation and can give an image in a single shot, providing the framework to perform high-throughput imaging experiments. Combining holography with ultrafast dynamic experiments will open up the possibility of real-time direct observation of surface or interface dynamics.

 


 

Research conducted by S. Roy (ALS); D. Parks, K.A. Seu, and R. Su (University of Oregon and ALS); J.J. Turner (SLAC National Accelerator Laboratory); W. Chao, E.H. Anderson, and S. Cabrini (Berkeley Lab); and S.D. Kevan (University of Oregon).

Research funding: National Science Foundation and U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.

Publication about this research: S. Roy, D. Parks, K.A. Seu, E.H. Anderson, S. Cabrini, and S.D. Kevan, "Lenseless x-ray imaging in reflection," Nat. Photonics 5, 243 (2011).

ALS Science Highlight #237

 

ALSNews Vol. 325