As scientists grow more capable of analyzing events that zip by at ultrafast timeframes, they would love to investigate complex systems, such as how all the molecules in photosynthesis or the components in quantum computers interact. Now researchers have developed antennae only nanometers or billionths of a meter in size to shape and steer quasiparticles known as plasmons, report findings detailed in the Proceedings of the National Academy of Sciences. These antennae could help them investigate events at nanometer levels at ultrafast scales.
In addition to objects commonly thought of particles, such as protons, electrons and neutrons, scientists have conceived of a zoo of “quasiparticles” that can in some respects be regarded as particles. One such quasiparticle is the plasmon, which is an oscillation in the density of electric charges in a material, like a ripple in the surface of a pond is an oscillation in the density of water there.
Plasmons can oscillate at very high frequencies, giving them correspondingly small wavelengths. Tiny plasmons can exist within equally tiny objects, and if probed the right way, they can yield details about the nature of these microscopic hosts. If one could develop a way to control plasmons at ultrafast timescales, such as femtoseconds, or millionths of a billionth of a second, one could get a view and a handle on complex and perhaps disorderly systems with components only nanometers large, for many potential applications in biology, physics, chemistry, electronics, microscopy and diagnostics.
For instance, ultrafast nanoplasmonics could help shed light on the behavior of many biomolecules on timeframes matching the ones they generally move in their natural environment, “such as cell membranes,” says researcher Daan Brinks, a physicist at the Institute of Photonic Sciences in Barcelona.
Despite advances with plasmonics, scientists have yet to control the response of plasmons on a nanometer level at femtosecond timescales at will. Now Brinks and his colleagues have discovered they could help enter this regime by coupling antennae only nanometers in size together to form plasmonic structures.
“The most surprising thing is that it worked at all,” Brinks says. “It was an open question for us whether we would be able to reach the required accuracy in fabrication and whether the scheme we devised would be robust enough to overcome the inevitable uncertainties imposed by the fabrication limits.”
The antennas in question — bars of gold 480 to 640 nanometers long — helped researchers shape the amplitude and phase responses of plasmons to excitations from ultrafast laser pulses, thereby guiding the dynamics of plasmons on specific times and spots. The researchers suggest nanoantennae could serve as a simple, reproducible and scalable approach to ultrafast plasmonics.
“It will be fun to see how far that can be pushed, using, for example, more crystalline particles, different materials,” Brinks says. Finding ways to actively change the properties of these structures on the fly is a possibility, he added.