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Fig. 1. Setup of the tabletop soft x-ray diffraction microscope. A coherent
soft x-ray beam is produced by high-harmonic generation or by spatially
filtering the emission from a capillary discharge laser in an Ar-filled hollow
waveguide. The beam is gently focused by a multilayer mirror pair onto the
sample, and the diffraction pattern is collected on an x-ray CCD. A moveable
beam block allows for brighter portions of the diffraction pattern to be
blocked to avoid saturating the CCD when acquiring the highest angle diffracted
light. ROC, radius of curvature. PNAS 105, 24 (2008). | Full Text |
Microscopy has been a critical enabling technology for understanding materials and biological systems since its invention. One of the most promising alternative approaches for high-resolution, high-contrast, imaging of thick samples is to use short wavelength light, in the soft-x-ray region of the spectrum. Lensless imaging is a relatively new coherent imaging technique that requires spatially coherent beams. This technique eliminates lenses by replacing them with a computerized phase retrieval algorithm. By obviating the need for lenses, lensless imaging is well-suited to the x-ray region, where optical elements are very limited and can introduce aberrations. This technique was first demonstrated in 1999 by John Miao and co-workers using spatially-filtered light from a synchrotron source.
High harmonic generation in gas-filled waveguide generates spatially coherent EUV beams and is ideally suited for lensless imaging. In recent work, we performed the first experimental demonstration of lensless imaging using a tabletop source of coherent soft-x-rays. By taking multiple exposures while blocking small-angle scattered light using beam blocks of varying size, we obtained very high dynamic range diffraction patterns which successfully reconstruct to images with resolution below 100 nm. More recent work demonstrated lensless imaging with EUV laser and high harmonic sources, with a resolution around 70 nm. Since no aberrations are introduced by a lens, the resolution of this microscope can in-theory be as good as the wavelength of the light used to illuminate the sample. A tabletop soft x-ray diffraction microscope should find broad applications in biology, medicine, nanoscience, and materials science due to its simple optical design, broad accessibility, high spatial resolution, large depth of field, insensitivity to vibrations, 3D imaging capability, scalability to shorter wavelengths, and ultrafast temporal resolution.
Nature News and Views: Harmonic pictures in a flash.
Nature 449, 553 (2007)
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Fig. 2. A tabletop lensless soft X-Ray microscope. A beam of coherent soft X-Rays is focused onto a sample. Instead of using a lens, a computer algorithm processes the resulting scatter patterns, retrieving information about the phase of the X-Ray beam, in order to reconstruct the image. (image courtesy of J. Miao, UCLA).
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