A group of scientists have produced two of the brightest, sharpest x-ray holograms of microscopic objects ever made. Working at both the Advanced Light Source (ALS) at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, and at FLASH, the free-electron laser in Hamburg, Germany, this group is boasting a method that is thousands of times more efficient than previous x-ray holographic methods.
Inspired by an ancient technique known as the pinhole camera, the x-ray hologram (made at ALS beamline 9.0.1) was of Leonardo da Vinci’s “Vitruvian Man.” This lithographic reproduction of less than two micrometers (millionths of a meter) square, was etched with an electron-beam nanowriter. The hologram required a five-second exposure and had a resolution of 50 nanometers (billionths of a meter).
The other hologram, made at FLASH, was of a single bacterium, Spiroplasma milliferum, made at 150-nanometer resolution and computer-refined to 75 nanometers, but requiring an exposure to the beam of just 15 femtoseconds (quadrillionths of a second).
The values for these two holograms are among the best ever reported for micron-sized objects. With already established technologies, resolutions obtained by these methods could be pushed to only a few nanometers, or, using computer refinement, even better.
Holography was invented over 60 years ago by the physicist Dennis Gabor, but its use has long been limited by technology. Whereas a pinhole camera employs ray optics, in which the photons travel like a stream of particles, holography depends on the wave-like properties of light.
The principle is straightforward: a beam of light illuminates an object, which scatters the light onto a detector such as a photographic plate, while a second, identical beam of light shines directly on the detector. The scattered light waves from the object beam form interference patterns with the unscattered light waves from the reference beam.
This interference pattern serves to reconstruct an image of the object. One easy way to do so, if the detector is a photo transparency, is for the observer to look through the transparency in the direction of the (now absent) object; if only the reference beam is shining on the detector, the interference pattern serves to “unscatter” (diffract) the wavefront and reconstruct the object’s image.
Lasers, which produce coherent light, were the first invention that made holography practical; it is now possible to make small holograms using just a laser pointer. FLASH is a powerful free-electron laser (FEL); a new generation of FELs of much shorter wavelength will be capable of producing coherent light pulses so short they’ll be able to freeze atomic motion in the midst of chemical reactions.
Soft x-rays like those from ALS beamline 9.0.1 can also be made coherent, or laser-like, using a pair of pinholes. (The beam is conditioned by these pinholes, but they are not directly involved in imaging, except to make the beam laser-like.) To make a hologram, the beam issuing from the synchrotron scatters from the target object and is collected on a CCD detector. Meanwhile, the same beam simultaneously passes through the multiple-“pinhole” URA, mounted on the same plate as the target object, and produces a bright reference beam.
The scattered image of the object and the many overlapping reference beams from the URA combine to make an interference pattern which contains all the information, including the relative depth of individual features, needed to mathematically reconstruct a three-dimensional image of the object.
The hologram of the Spiroplasma bacterium was made in precisely the same way, with much brighter x-ray beams and a much shorter pulse of light. So bright was the flash of light that the sample was vaporized, but not before both the scattered object beam and the reference beams from the URA had been recorded.
Together, the two experiments demonstrate that holographic x-ray images with nanometer-scale resolution can be made of objects measured in microns, in times as brief as femtoseconds. Moreover, sample preparation time is fast and easily repeated for high throughput during repetitive experiments.
Citation: "Massively parallel x-ray holography," by Stefano Marchesini, Sébastien Boutet, Anne E. Sakdinawat, Michael J. Bogan, Sǎsa Bajt, Anton Barty, Henry N. Chapman, Matthias Frank, Stefan P. Hau-Riege, Abraham Szöke, Congwu Cui, David Shapiro, Malcolm Howells, John Spence, Joshua Shaevitz, Joanna Lee, Janos Hajdu, and Marvin M. Siebert, appears in advanced online publication of Nature Photonics and is available online to subscribers at http://dx.doi.org/10.1038/nphoton.2008.154 .