|Lensless X-Ray Imaging in Reflection|
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.
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.
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