4-D Microscopy Films Photons

A new technique is being used to track and image nanoscale matter in real time, allowing researchers at the California Institute of Technology (Caltech) to image electrical fields produced by the interaction of electrons and photons.

This technique uses four-dimensional (4-D) microscopy developed at Caltech’s Physical Biology Center for Ultrafast Science and Technology. The center is directed by Ahmed Zewail, the Linus Pauling Professor of Chemistry and professor of physics at Caltech, and winner of the 1999 Nobel Prize in Chemistry.

Zewail was awarded the Nobel Prize for pioneering the science of femtochemistry, the use of ultrashort laser flashes to observe fundamental chemical reactions occurring at the timescale of the femtosecond (one-millionth of a billionth of a second). The work “captured atoms and molecules in motion,” Zewail says, but while snapshots of such molecules provide the “time dimension” of chemical reactions, they don’t give the structure or architecture of those reactions.

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

These are photons imaged in nanoscale structures (carbon nanotubes) using pulsed electrons at very high speed. Shown are the evanescent fields for two time frames and for two polarizations. (Images: Zewail/Caltech)

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.

Zewail and postdoctoral scholar Aycan Yurtsever addressed the blurring problem by using electron pulses instead of a steady electron beam. The sample is first heated by being struck with a short pulse of laser light. It is then hit with a femtosecond pulse of electrons, which bounce off the atoms, producing a diffraction pattern on a detector.

Because the electron pulses are so brief, the heated atoms don’t have time to move much, 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 explained, 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.”

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

The new diffraction method allows the structures of materials to be mapped out at an atomic scale – a technique discovered by postdoctoral scholars Brett Barwick and David Flannigan. This involves the interaction between electrons and photons. Photons generate an evanescent field in nanostructures, and electrons can gain energy from such fields, which makes them visible in the 4-D microscope.

In what is known as the photon-induced near-field electron microscopy (PINEM) effect, 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 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.”

The work was supported by the National Science Foundation, the Air Force Office of Scientific Research, and the Gordon and Betty Moore Foundation at the Center for Physical Biology at Caltech.

For more information, visit: www.caltech.edu