Squeezing Light Into Infinitesimally Thin Lines

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Researchers have demonstrated a new mode of electromagnetic wave, called a "line wave," which travels along an infinitesimally thin line along the interface between two adjacent surfaces with different electromagnetic properties. The scientists expect that line waves will be useful for the efficient routing and concentration of electromagnetic energy, such as light, with potential applications in areas ranging from integrated photonics, sensing and quantum processes to future vacuum electronics.

"Line waves are the first electromagnetic waves that are found to be guided by an infinitesimal, one-dimensional object," said Daniel Sievenpiper, a professor of electrical and computer engineering at the University of California San Diego and the study’s senior author. “The waveguide is nothing more than a line, which leads to a very high field concentration. This can be used to enhance nonlinear processes, leading to things like higher speed modulators for optical communication, or more sensitive chemical detectors.”

“What’s remarkable is how squeezing waves into a simple line could give rise to essentially infinite energy concentration and near-perfect energy transmission,” said Dia’aaldin Bisharat, a visiting graduate researcher from the City University of Hong Kong who is the first author of the study and conducted the work at the UC San Diego Jacobs School of Engineering. The researchers published their work in a recent issue of Physical Review Letters.

The new electromagnetic line waves are analogous to what are known as electromagnetic surface waves, which occur at the surface interface between two different materials laid one on top of the other. Surface waves can be used to strongly confine and guide light, making them useful for energy transmission and communication applications.

Line waves, on the other hand, are confined to the interface between two surfaces that are laid next to each other. Line waves are confined to a one-dimensional space while surface waves are confined to a two-dimensional space. Imagine that the two planes have zero thickness and are laid side by side, restricting the interface to a line—hence the name line wave. This extra confinement enables line waves to be made into complex circuits, making them even better for energy transmission and communication applications because they add lateral confinement that is not available with surface waves.

The key to realizing line waves is that one of the surfaces is inductive while the other is capacitive. The inductive surface supports transverse magnetic polarized waves, while the capacitive surface supports transverse electric polarized waves. When the two surface types are placed side by side, these two different boundaries support line waves at the interface.

Another important feature of line waves is that they naturally prevent backscattering. This means that defects in the line cannot scatter waves back toward the source, so this type of waveguide naturally prevents unwanted reflections, Sievenpiper explained. “This is similar to the recently developed photonic topological insulators, but line waves have some advantages such as broader bandwidth, and they allow simpler fabrication," Sievenpiper said.

“Our system can be potentially used for simple quantum information processes, where the signals are routed at different directions depending on the specific quantum state,” Bisharat noted.

The researchers demonstrated line waves in experiments and simulations by using periodic metasurfaces, and they expect that they could further increase the operating range by using other materials. One possibility is graphene, which can be designed to be either an inductive surface or a capacitive surface depending on its doping level. Using graphene sheets, one could alter the line waves’ speed and confinement, and guide them along different pathways, leading to electrically field-programmable circuits at terahertz frequencies, Bisharat explained.

To demonstrate the control of line waves, researchers showed in simulations how line waves can be guided along curved paths and routed to make sharp turns. This ability to confine and transport electromagnetic energy in a controlled way will likely be useful for building network devices and integrated photonics applications, which the researchers plan to further investigate in the future.

"We are also starting a project to extend this concept from the electromagnetic domain to acoustic or phonon waves, to enable materials with new properties for controlling vibration, sound propagation and heat transport,” Sievenpiper said.

Paper title: “Guiding Waves Along an Infinitesimal Line between Impedance Surfaces” by Dia’aldin J. Bisharat and Daniel F. Sievenpiper.

This work was supported in part by the Air Force Office of Scientific Research (AFOSR grant no. FA9550-16-1-0093).