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May 2015

Could Computers Reach Light Speed?

Trapped light waves go farther than expected, giving insights on designing computer circuit interconnects that work at nearly the speed of light

Artistic view of plasmon speed test
Specially designed extremely small metal structures can trap light. Once trapped, the light becomes a confined wave known as a surface plasmon. The plasmons propagate from the source to locations several hundred microns away, almost as fast as light through the air. Copyright 2015: American Chemical Society zoom Enlarge Image.

Results: Light waves trapped on a metal's surface travel nearly as fast as light through the air, and new research at Pacific Northwest National Laboratory shows these waves, called surface plasmons, travel far enough to possibly be useful for ultra-fast electronic circuit interconnects. The PNNL team captured, on video, surface plasmons moving at least 250 microns (or about 1/100th of an inch) across the surface.

Why It Matters: Because circuit interconnects based on surface plasmons could be much faster than current interconnects, this basic research could lead to faster computer circuits and provide significant advances in the chemical, biological, and health fields. Also, the results give insights about these trapped light waves to the scientific communities. The study experimentally confirms the linear relationship between the input light waves and generated surface plasmons. It also indicates the plasmons have a long life and low dissipation, critical fundamental information needed to use the waves in circuits and other applications.

Methods: When a surface plasmon is generated on a metal surface, it can be observed by using laser light to emit electrons. By detecting these photo-electrons, with a special instrument called a photoemission electron microscope (PEEM), the scientists explored the nature of surface plasmons.

In their experiments, the team applied two laser pulses to the sample: one is called the pump, used to generate the surface plasmon; the other is called the probe, used to detect the plasmon. The probe pulse strikes the sample and detects the plasmon at different time delays. By continuously tuning the time delay between the pump and probe pulses, the team monitored the motion of the plasmon on the gold surface, finding that the wave traveled up to 250 microns on the metal surface.

Microscopy image of plasmon movement
This image, taken with a photoemission electron microscope, shows the spatially separated pump and probe pulse. Copyright 2015: American Chemical Society

"The distance is surprisingly long because plasmon waves don't propagate like a normal free space wave," said Dr. Yu Gong, a scientist at PNNL and the lead author on this study. "In our case, the plasmons travel unexpectedly long distances in metal films."

The team applied numerical simulations to further confirm their experimental results.

What's Next? Now, the team is exploring how to control the propagation of the surface plasmon. For example, how efficiently can the surface plasmon be generated? How can it be guided? How can it be stopped? The scientists are using the PEEM and other resources, including those in DOE's EMSL, to answer these and other questions. The results are crucial to making circuits that operate at light speed a reality.

Acknowledgments

Funding: The researchers acknowledge support from the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. PZE acknowledges support from the Laboratory Directed Research and Development Program through a Linus Pauling Fellowship at PNNL.

User Facility: EMSL

Research Area: Chemical Sciences

Research Team: Yu Gong, Alan G. Joly, Dehong Hu, Patrick Z. El-Khoury, and Wayne Hess, Pacific Northwest National Laboratory

Reference: Gong Y, AG Joly, D Hu, PZ El-Khoury, and W Hess. 2015. "Ultrafast Imaging of Surface Plasmons Propagating on a Gold Surface." Nano Letters 15(5):3472-3478.  DOI: 10.1021/acs.nanolett.5b00803


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