Experimental "metatronic" circuits imitate electronics, but at optical frequencies

Philadelphia, PA and Austin, TX--Researchers at the University of Pennsylvania and the University of Texas at Austin have experimentally demonstrated "metatronic" circuits, in which optical nanostructures behave as nanoscale lumped circuit elements at optical frequencies.¹ Silicon nitrite nanorods form a 2D optical nanocircuit operating at mid-IR wavelengths; the connections between the nanocircuit elements—whether they are in series or in parallel—is controlled by the polarization of the optical signal striking the circuit.

“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 of the University of Pennsylvania. Electrical circuits are built of elements such as resistors, inductors and capacitors, which manipulate the flow of electrons. Because both electric circuits and optics follow Maxwell’s equations, Engheta’s dream of building circuits with light is not just the stuff of imagination. In 2005, he and his students published a theoretical paper outlining how optical-circuit elements could work.

First experimental metatronics

Now, he and his collaborators 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.” In electronics, the “lumped” designation refers to elements that can be treated as a black box, something that turns a given input to a predictable output without an engineer having to worry about what exactly is going on inside the element.

“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 cross-sections of the nanorods and the gaps between them form a pattern that replicates the function of resistors, inductors, and capacitors, but at 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.”

In their experiment, the researchers illuminated the nanorods with an optical signal, using spectroscopy to measure the wave as it passed through the comb. Running the experiment using nanorods with nine different combinations of widths and heights, the researchers showed that the optical “current” and optical “voltage” were altered by the optical resistors, inductors, and capacitors with parameters corresponding to those differences in size.“ A section of the nanorod acts as both an inductor and resistor, and the air gap acts as a capacitor,” Engheta said.

Changing connections by changing polarization

Beyond changing the dimensions and the material the nanorods are made of, the function of these optical circuits can be altered by changing the orientation of the light, giving metatronic circuits access to configurations that would be impossible in traditional electronics. When the plane of the polarization is in line with the nanorods, the circuit is wired in parallel and the current passes through the elements simultaneously. When the plane of the electric field crosses both the nanorods and the gaps, the circuit is wired in series and the current passes through the elements sequentially.

“The orientation gives us two different circuits, which is why we call this ‘stereo-circuitry,’” Engheta said. “We could even have the wave hit the rods obliquely and get something we don’t have in regular electronics: a circuit that’s neither in series or in parallel but a mixture of the two.”

This principle could be taken to an even higher level of complexity by building nanorod arrays in three dimensions. An optical signal hitting such a structure’s top would encounter a different circuit than a signal hitting its side.

SOURCES:

1. http://www.upenn.edu/pennnews/news/penn-researchers-build-first-physical-metatronic-circuit

2. Nature Materials, see below.

REFERENCE:

1. Yong Sun et al., Nature Materials 11, p. 208 (2012); doi:10.1038/nmat3230.