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Quantitative x-ray phase imaging at the nanoscale by multilayer Laue lenses

Significance Statement

For an x-ray microscope, achieving spatial resolution in the nanometer range is a challenging task even though wavelength of x-ray is in the order of angstroms. The main challenge is due to an extremely small refractive power of medium when interacting with x-rays. For most materials, refractive indices deviate from unity (the refractive index in vacuum) only by an extremely small amount of 10-5 – 10-6. As a result, conventional lenses that work well for visible light are not efficient for nanofocusing of x-rays. Multilayer Laue lens (MLL) is a novel x-ray lens developed to overcome the difficulty of focusing x-rays by utilizing the dynamical diffraction effect, a phenomenon usually observed in crystal diffraction. It is a an artificially constructed “crystal”, consisting of thousands of alternating low- and high-density thin layers with nanometer thickness stacked together with an extremely high precision. In our paper entitled “Quantitative x-ray phase imaging at the nanoscale by multilayer Laue lens” (Sci. Repts. 3, 1307 (2013)), we used scanning x-ray microscope equipped with Multilayer Laue lens nanofocusing optics to achieve not only high spatial resolution but also performed quantitative phase imaging. Imaging the phase of a light wave is always a nontrivial problem since a detector only records the intensity. The phase, however, is encoded in the intensity variation. Here we proposed a way to measure the phase gradients by analyzing how much the x-ray nanobeam is bent when it propagates through a specimen, similar to light bending through a prism. A fitting procedure was developed to unambiguously determine phase gradients and minimize the noise effects. The phase is then obtained by an integration of phase gradients. One advantage of this method pertains to the fact that it does not suffer from the phase-wrapping problem that is commonly encountered in iterative phase-retrieval methods due to inability to distinguish two phases with multi-2p difference, since we measure the phase gradient here, not the phase itself. Phase is much more sensitive to structural or compositional variations through the sample when compared to absorption; therefore it can reveal small features providing much higher contrast, technique especially useful for imaging of x-ray transparent specimens.

At the National Synchrotron Light Source-II of Brookhaven National Laboratory, a new third-generation synchrotron source with ultra-high brightness, the Hard X-ray Nanoprobe (HXN) is underway to deliver x-ray microscopy capabilities at an initial resolution of 10 nm using Multilayer Laue lens. Scheduled for commissioning in 2014, the HXN will offer a variety of imaging methods including absorption-, fluorescence- and phase-contrast imaging, nanodiffraction for strain measurement and spectroscopy for chemical state mapping.

 

Quantitative x-ray phase imaging at the nanoscale by multilayer Laue lenses.

Yan H, Chu YS, Maser J, Nazaretski E, Kim J, Kang HC, Lombardo JJ, Chiu WK.

Sci Rep. 2013 Feb 19;3:1307.
National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973, USA. [email protected]

 

Abstract

For scanning x-ray microscopy, many attempts have been made to image the phase contrast based on a concept of the beam being deflected by a specimen, the so-called differential phase contrast imaging (DPC). Despite the successful demonstration in a number of representative cases at moderate spatial resolutions, these methods suffer from various limitations that preclude applications of DPC for ultra-high spatial resolution imaging, where the emerging wave field from the focusing optic tends to be significantly more complicated. In this work, we propose a highly robust and generic approach based on a Fourier-shift fitting process and demonstrate quantitative phase imaging of a solid oxide fuel cell (SOFC) anode by multilayer Laue lenses (MLLs). The high sensitivity of the phase to structural and compositional variations makes our technique extremely powerful in correlating the electrode performance with its buried nanoscale interfacial structures that may be invisible to the absorption and fluorescence contrasts.

 

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