Hybrid Microscope Combines Optics, Ultrafast Laser

A new hybrid device that combines an optical microscope with an ultrafast laser could be used to simultaneously image both the electronic and physical patterns in devices such as nanotransistors or to identify the chemicals or elements that comprise them.

The new form of microscopy, called scanning photoionization microscopy (SPIM), can reveal the physical and electronic profiles of metal nanostructures. It is described in the paper "Imaging Nanostructures with Scanning Photoionization Microscopy" by researchers at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and University of Colorado at Boulder. The paper appears in the Oct. 21 issue of Journal of Chemical Physics.

JILA's scanning photoionization microscope (SPIM) includes an optical microscope (in vacuum chamber, background) and an ultrafast laser (appears as blue, foreground).

The new hybrid microscope combines the high spatial resolution of optical microscopy with the high sensitivity to subtle electrical activity made possible by detecting the low-energy electrons emitted by a material as it is illuminated with laser pulses, the researchers said. The technique has the potential to make pictures of both electronic and physical patterns in devices such as nanostructured transistors or electrode sensors, or to identify chemicals or even elements in such structures, they said.

"You make images by virtue of how readily electrons are photoejected from a material," says NIST Fellow David Nesbitt, leader of the research group. "The method is in its infancy, but nevertheless it really does have the power to provide a new set of eyes for looking at nanostructured metals and semiconductors."

The JILA-built apparatus includes a moving optical microscopy stage in a vacuum, an ultrafast near-ultraviolet laser beam that provides sufficient peak power to inject two photons (particles of light) into a metal at virtually the same time, and equipment for measuring the numbers and energy of electrons ejected from the material.

A false color SPIM image (b) reveals the same physical structure of a gold pattern on glass as an atomic force microscope image (a), but the high intensity regions in the SPIM image indicate that electron ejection is much more efficient at metal edge discontinuities. (Images: O.L.A. Monti, T.A. Baker, and D.J. Nesbitt/JILA)

By comparing SPIM images of nanostructured gold films to scans using atomic force microscopy, which profiles surface topology, the researchers confirmed the correlations and physical mapping accuracy of the new technique. They also determined that lines in SPIM images correspond to spikes in electron energy, or current, and that contrast depends on the depth of electrons escaping from the metal as well as variations in material thickness.

Work is continuing to further develop the method, which may be able to make chemically specific images, for example, if the lasers are tuned to different colors to affect only one type of molecule at a time.

The research is supported by the Air Force Office of Scientific Research and National Science Foundation. For more information, visit: www.nist.gov