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  • Filming photons using electrons

Photonics Spectra
Jan 2010
Krista D. Zanolli,

PASADENA, Calif. – A new technique that tracks and images nanoscale matter in real time also enables researchers to image electrical fields produced by the interaction of electrons and photons.

The method, which uses four-dimensional microscopy, was developed by researchers at the Physical Biology Center for Ultrafast Science and Technology at California Institute of Technology (Caltech). The center is directed by Ahmed H. Zewail, the Linus Pauling professor of chemistry and professor of physics at Caltech.

Zewail was awarded the Nobel Prize in chemistry in 1999 for pioneering the science of femtochemistry, the use of ultrashort laser flashes to observe fundamental chemical reactions occurring at the timescale of the femtosecond. Although the work “captured atoms and molecules in motion,” Zewail said, snapshots of such molecules provide the “time dimension” of chemical reactions but not the structure or architecture of those reactions.

With 4-D microscopy, Zewail and his colleagues could see the architecture, which employs single electrons to introduce the dimension of time into traditional high-resolution electron microscopy, providing a way to see the changing structure of complex systems at the atomic scale.

In electron diffraction, an object is illuminated with a beam of electrons. The electrons bounce off the atoms in the object, then scatter and strike a detector. The patterns produced on the detector provide information about the arrangement of the atoms in the material. However, if the atoms are in motion, the patterns will be blurred, obscuring details about small-scale variations in the material.

In research published in the Oct. 30, 2009, issue of Science, Zewail and postdoctoral scholar Aycan Yurtsever addressed the blurring problem by using electron pulses instead of a steady electron beam. They first heated the sample by striking it with a short pulse of laser light, then with a femtosecond pulse of electrons – which bounce off the atoms – producing a diffraction pattern on a detector.

These photons were imaged in nanoscale structures (carbon nanotubes) using pulsed electrons at a very high speed. Shown are the evanescent fields for two time frames and for two polarizations. Images courtesy of Ahmed Zewail, California Institute of Technology.

The electron pulses are so brief that the heated atoms do not have time to move much, thus producing a sharper image. By adjusting the delay between when the sample is heated and when the image is taken, the scientists can gather a number of still images that can be strung together into a movie.

“Essentially all of the specimens we deal with are heterogeneous,” Zewail said, with varying compositions over very small areas. “This technique provides the means for examining local sites in materials and biological structures, with a spatial resolution of a nanometer or less and time resolution of femtoseconds.”

The new 4-D microscopy technique, reported by Zewail and postdoctoral scholars Brett Barwick and David J. Flannigan, was published in the Dec. 17, 2009, issue of Nature. The visualization process involves the interaction between electrons and photons. Photons generate an evanescent field in nanostructures, and electrons can gain energy from such fields, enabling them to be visible under the 4-D microscope.

This diffraction was obtained for silicon with 4-D electron microscopy. The nanoscale can be determined from the patterns in the structure.

In photon-induced near-field electron microscopy (PINEM), certain materials – after being hit with laser pulses – continue to “glow” for a short but measurable amount of time (on the order of tens to hundreds of femtoseconds). In their experiment, the latter researchers illuminated carbon nanotubes and silver nanowires with short pulses of laser light as electrons were being shot past.

The power of this technique lies in the ability to visualize the evanescent field when the electrons that have gained energy are selectively identified, and to image the nanostructures themselves when electrons that have not gained energy are selected.

“As noted by the reviewers of this paper, this technique of visualization opens new vistas of imaging, with the potential to impact fields such as plasmonics, photonics and related disciplines,” Zewail said. “What is interesting from a fundamental physics point of view is that we are able to image photons using electrons. Traditionally, because of the mismatch between the energy and momentum of electrons and photons, we did not expect the strength of the PINEM effect or the ability to visualize it in space and time.”

As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form various diffraction patterns.  
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