Replacing Electricity With Light: First Physical 'Metatronic' Circuit Created
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Figure A. When the plane of the electric field is in line with the
nanorods the circuit is wired in parallel. Figure B. When the plane of
the electric field crosses both the nanorods and the gaps the circuit
is wired in series. (Credit: Image courtesy of University of
Pennsylvania)ScienceDaily (Feb. 23, 2012) — 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.
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.
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."
This work was supported in part by the U.S. Air Force Office of
Scientific Research.
Andrea Alù is now an assistant professor at the University of Texas at Austin.
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Story Source:
The above story is reprinted from materials provided by University of
Pennsylvania.
Note: Materials may be edited for content and length. For further
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Journal Reference:
1.Yong Sun, Brian Edwards, Andrea Alù, Nader Engheta. Experimental
realization of optical lumped nanocircuits at infrared wavelengths.
Nature Materials, 2012; 11 (3): 208 DOI: 10.1038/nmat3230
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