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.
Different arrangements and combinations of electronic circuits have
different functions, ranging from simple light switches to complex
supercomputers. These circuits are built of different arrangements of circuit
elements, for example resistors, inductors and capacitors, which manipulate the
flow of electrons in a circuit in mathematically precise ways.
Now, researchers at Penn have created the first physical demonstration
of lumped optical circuit elements. This represents a milestone in a nascent
field of science and engineering. 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. Optics has always had its
own set of elements, things like lenses, waveguides and gratings, 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.
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.
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, 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.
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. Another reason for success in electronics has to do
with its modularity.
