Researchers at Rice University’s Laboratory for Nanophotonics (LANP) say they have uncovered a new way to make ultrasensitive conductivity measurements at optical frequencies on high-speed nanoscale electronic components. They linked pairs of puck-shaped metal nanodisks with metallic nanowires and demonstrated that the flow of current at optical frequencies through the nanowires produced “charge transfer plasmons” with unique optical signatures.

Fig. 1 – Linked pairs of nanodisks as seen with a scanning electron microscope. (Credit: Fangfang Wen, Rice University)
“The push to continually increase the speed of microchip components has researchers looking at nanoscale devices and components that operate at optical frequencies for next-generation electronics,” said LANP Director Naomi Halas, the lead scientist on the study. “It is not well-known how these materials and components operate at extremely high frequencies of light, and LANP’s new technique provides a way to measure the electrical transport properties of nanomaterials and structures at these extremely high frequencies.”

Halas, the Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering, explains that some metallic nanoparticles convert light into plasmons—waves of electrons that flow like a fluid across the particle’s surface. LANP researchers have long explored the basic physics of plasmonics and shown how plasmonic interactions can be harnessed for applications as diverse as medical diagnostics and cancer treatment.

Plasmonic coupling is one type of plasmonic interaction that the Rice team is studying, an interaction when two or more plasmonic particles are located near one another. For example, when two puck-shaped plasmonic nanodisks are located nearby, they act like a tiny, light-activated capacitor. When a conducting wire bridges the two, their plasmon energies change and a new resonance called a “charge transfer” plasmon, appears at a distinct frequency.

In a new study, researchers examined the optical properties of pairs of bridged nanodisks. By observing the charge flowing back and forth along the wires at optical frequencies in bridged pairs, they discovered that the electrical current flowing across the junction introduced a characteristic optical signature.

“In the case where a conducting wire was present in the junction, we saw an optical signature that was very different from the case without a wire,” Fangfang Wen, lead author of the study, said. In a series of experiments, Wen varied the width and shape of the bridging nanowires and repeated these measurements for nanowires of gold and aluminum.

These experiments revealed two key findings. First, at the low end of the conductance scale, even the slightest changes in conductivity resulted in notable optical shifts, which could be particularly useful for molecular-electronics researchers who want to measure conductivity in structures as small as a single molecule.

“We also found that our platform gave a different optical signature in cases where the level of conductance was the same but the junction material was different,” Wen said. “If we had nanowires with the same conductance that were made of different materials, we saw a different optical signature. If we used the same material, with different geometries, we saw the same signature.”

This specificity and repeatability could be useful to researchers who might want to use this approach to identify the conductance of nanowires, or other nanoscale electronic components, at optical frequencies.

“To reduce the size of electronics even beyond today’s limits, scientists want to study electron transfer through a single molecule, particularly at extremely high, even optical frequencies,” Wen said. “Such changes cannot be measured using standard electronic devices or instruments that operate at microwave frequencies. Our research provides a new platform for the measurement of nanoscale conductance at optical frequencies.”

For more information, visit www.news.rice.edu .