August 2nd, 2008
Massively parallel x-ray holography
An international group of scientists has produced some of the sharpest x-ray holograms of microscopic objects ever made. According to one of them, they improved the efficiency of holography by a factor of 2,500. In order to achieve these spectacular results, they put a uniformly redundant array next to the object to image. And they found that this parallel approach multiplied ‘the efficiency of X-ray Fourier transform holography by more than three orders of magnitude, approaching that of a perfect lens.’ Besides these impressive achievements, it’s worth noting that this technology has been inspired by the pinhole camera, a technique used by ancient Greeks. But read more…
You can see above a “x-ray hologram of Leonardo da Vinci’s famous drawing, ‘Vitruvian Man,’ a lithographic reproduction less than two micrometers square, etched with an electron-beam nanowriter. The hologram required a five-second exposure and had a resolution of 50 nanometers.” (Credit: Lawrence Berkeley National Laboratory)
This hologram was made at the Advanced Light Source (ALS) at the U.S. DoE’s Lawrence Berkeley National Laboratory (Berkeley Lab) on its beamline 9.0.1. You’ll find more details about imaging techniques developed there by reading Coherent X-ray Imaging Advanced Light Source (PDF format, 36 pages, 16.81 MB). The picture above, “Massively Parallel Holography With Coded Apertures,” is featured on page 15 of this document.
The other hologram shown in the Berkeley Lab news release has been made at the Deutsches Elektronen-Synchrotron (DESY), Germany, in its FLASH facility — FLASH meaning “‘F’reie-Elektronen-’LAS’er in ‘H’amburg.” This hologram of a single bacterium, Spiroplasma milliferum, was made at a 150-nanometer resolution and computer-refined to 75 nanometers, but it required an exposure to the beam of just 15 femtoseconds (quadrillionths of a second). Pretty amazing!
Now, let’s look at how the pinhole camera inspired this group of scientists. “‘Our purpose was to explore methods of making images of nanoscale objects on the time scale of atomic motions, a length and time regime that promises to become accessible with advances in free-electron lasers,’ says Stefano Marchesini of the ALS, who led the research. ‘The technique we used is called massively parallel x-ray Fourier-transform holography, with ‘coded apertures.’ What inspired me to try this approach was the pinhole camera.’”
Here are some additional details. “The ancient Greeks made note of pinhole-camera effects without understanding them; later, pinhole cameras were used by Chinese, Arab, and European scholars. Renaissance painters learned the principals of perspective using the camera obscura, literally a ‘dark room,’ with a pinhole in one wall that projected the outside scene onto the opposite wall. ‘The room had to be dark for the good reason that a sharp image requires a small pinhole, but a small pinhole also produces a dim image,’ says Marchesini. ‘To get a brighter image without lenses you have to use many pinholes. The problem then becomes how to assemble the information, including depth information, from the overlapping shadow images. This is where ‘coded apertures’ come in.’”
So the team decided to use not one pinhole, but a large array of pinholes. “By knowing the precise layout of a pinhole array, including the different sizes of the different pinholes, a computer can recover a bright, high-resolution image numerically. Random pinhole arrays were first used in x-ray astronomy but soon evolved into regular rows and columns of tiny square apertures of varying dimension. These coded apertures are called uniformly redundant arrays, or URAs. Marchesini knew that colleagues at Livermore were using URAs in gamma-ray detectors. He asked himself, ‘What would happen if we put a URA right next to an object we were imaging with the x-ray beamline? It should allow us to create a holographic image — one with orders of magnitude more intensity than a standard hologram.’” And it worked well.
This research work is available from Nature Photonics as an advance online publication under the title “Massively parallel X-ray holography.” Here is the beginning of the abstract. “Advances in the development of free-electron lasers offer the realistic prospect of nanoscale imaging on the timescale of atomic motions. We identify X-ray Fourier-transform holography as a promising but, so far, inefficient scheme to do this. We show that a uniformly redundant array placed next to the sample, multiplies the efficiency of X-ray Fourier transform holography by more than three orders of magnitude, approaching that of a perfect lens, and provides holographic images with both amplitude- and phase-contrast information.”
Sources: Lawrence Berkeley National Laboratory news release, August 1, 2008; and various websites
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