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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 Alù, was published
in the journal Nature Materials.
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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.”
In their experiment, the
researchers illuminated the nanorods with an optical signal, a wave of light in
the mid-infrared range. They then used spectroscopy to measure the wave as it
passed through the comb. Repeating 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.
An illustration of an array of silicon nitrite nanorods. The entire array is about half a millimeter long. Image: University of Pennsylvania |
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.
This is because a light
wave has polarizations; the electric field that oscillates in the wave has a definable
orientation in space. In metatronics, it is that electric field that interacts
and is changed by elements, so changing the field’s orientation can be like
rewiring an electric circuit.
When the plane of the
field is in line with the nanorods, as in Figure A, 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, as in
Figure B, 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. Building off their success
with basic optical elements, Engheta and his group are laying the foundation
for this kind of complex metatronics.
“Another reason for
success in electronics has to do with its modularity,” he said. “We can make an
infinite number of circuits depending on how we arrange different circuit
elements, just like we can arrange the alphabet into different words, sentences
and paragraphs.”
“We’re now working on
designs for more complicated optical elements,” Engheta said. “We’re on a quest
to build these new letters one by one.”