Box 1. Lensless imaging

From the following article

High-harmonic generation: Ultrafast lasers yield X-rays

Iain McKinnie & Henry Kapteyn

Nature Photonics 4, 149 - 151 (2010)

doi:10.1038/nphoton.2010.20

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The optical microscope has contributed greatly to our understanding of the world around us. Unfortunately, the smallest object that can be imaged is determined — and limited — by the wavelength of the light used. To visualize much smaller objects at the nanoscale, X-ray microscopes are needed. Facility X-ray synchrotron sources already provide this capability to a limited number of researchers, but the challenges in fabricating X-ray lenses limit the resolution to around 15 nm.

A team led by the Kapteyn–Murnane research group at JILA has recently demonstrated a table-top, lensless, soft-X-ray microscope with a resolution that is very close to the wavelength of the EUV light used. The object is illuminated by a XUUS-like source, and a lensless microscope uses a computer algorithm to analyse the scatter patterns produced from the illuminated sample. The algorithm iterates to reproduce the actual image, in essence 'shrink wrapping' the image until it retrieves the original object. Avoiding the use of a lens also avoids possible aberrations, so the resolution of this microscope can ultimately be as good as the wavelength of the light used to illuminate the sample.

Figure B1 shows imaging of a test sample using 13 nm coherent light. A resolution of 92 nm is obtained, limited only by the flux of the EUV source. Higher repetition rate ultrafast lasers currently under development will significantly reduce image capture time and thus improve resolution towards the wavelength-limited value.

High-harmonic generationUltrafast lasers yield X-rays GREG KUEBLER, JILA

Figure B1 | Lensless diffractive imaging combined with multiple-reference fast Fourier transform (FFT) holography. A coherent high-harmonic beam illuminates a test pattern surrounded by five reference holes. The scattered light interferes and is recorded on a CCD camera in the far-field as a hologram. The spatial autocorrelation of the object can be retrieved by taking the squared magnitude of the Fourier transform of the hologram. Further refinement of the image to a resolution of 50 nm is possible using phase-retrieval algorithms to recover the spatial frequency information scattered at high angles.

This table-top soft-X-ray diffraction microscope should find applications in biology, medicine, nanoscience and materials science, owing 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. It is possible that in 10–20 years, every hospital may have such a microscope for diagnosing disease using high-resolution images of single cells.