The technological world of the 21st century owes a tremendous amount to advances in electrical engineering, specifically, the ability to finely control the flow of electrical charges using increasingly small and complicated circuits. And while those electrical advances continue to race ahead, researchers at the University of Pennsylvania are pushing circuitry forward in a different way, by replacing electricity with light.
"Looking at the success of electronics over the last century, I have always wondered why we should be limited to electric current in making circuits," said Nader Engheta , professor in the electrical and systems engineering department of Penn’s School of Engineering and Applied Science. "If we moved to shorter wavelengths in the electromagnetic spectrum -- like light -- we could make things smaller, faster and more efficient."
Different arrangements and combinations of electronic circuits have different functions, ranging from simple light switches to complex supercomputers. These circuits are in turn built of different arrangements of circuit elements, like resistors, inductors and capacitors, which manipulate the flow of electrons in a circuit in mathematically precise ways. And because both electric circuits and optics follow Maxwell’s equations -- the fundamental formulas that describe the behavior of electromagnetic fields -- Engheta’s dream of building circuits with light wasn’t just the stuff of imagination. In 2005, he and his students published a theoretical paper outlining how optical circuit elements could work.
Now, he and his group at Penn have made this dream a reality, creating the first physical demonstration of "lumped" optical circuit elements. This represents a milestone in a nascent field of science and engineering Engheta has dubbed "metatronics."
Engheta’s research, which was conducted with members of his group in the electrical and systems engineering department, Yong Sun, Brian Edwards and Andrea Alů, was published Materials.
In electronics, the "lumped" designation refers to elements that can be treated as a black box, something that turns a given input to a perfectly predictable output without an engineer having to worry about what exactly is going on inside the element every time he or she is designing a circuit.
"Optics has always had its own analogs of elements, things like lenses, waveguides and gratings," Engheta said, "but they were never lumped. Those elements are all much larger than the wavelength of light because that’s all that could be easily built in the old days. For electronics, the lumped circuit elements were always much smaller than the wavelength of operation, which is in the radio or microwave frequency range."
Nanotechnology has now opened that possibility for lumped optical circuit elements, allowing construction of structures that have dimensions measured in nanometers. In this experiment’s case, the structure was comb-like arrays of rectangular nanorods made of silicon nitrite.
The "meta" in "metatronics" refers to metamaterials, the relatively new field of research where nanoscale patterns and structures embedded in materials allow them to manipulate waves in ways that were previously impossible. Here, the cross-sections of the nanorods and the gaps between them form a pattern that replicates the function of resistors, inductors and capacitors, three of the most basic circuit elements, but in optical wavelengths.
"If we have the optical version of those lumped elements in our repertoire, we can actually make designs similar to what we do in electronics but now for operation with light," Engheta said. "We can build a circuit with light."