Engineers at the University of California, San Diego have demonstrated a
new and more efficient way to trap light, using a phenomenon called
bound states in the continuum (BIC) that was first proposed in the early
days of quantum wave mechanics.
Boubacar Kanté, an assistant professor in electrical and computer
engineering at UC San Diego Jacobs School of Engineering, and his
postdoctoral researcher Thomas Lepetit described their BIC experiment
online in the rapid communication section of journal Physical Review B.
The study directly addresses one of the major challenges currently
facing nanophotonics, as researchers look for ways to trap and use light
for optical computing circuits and other devices such as tiny switches.
"The goal in the future is to make a computer that performs all kinds
of operations using light, not electronics, because electronic circuits
are relatively slow. We expect that an optical computer would be faster
by three to four orders of magnitude." Kanté said. "But to do this, we
have to be able to stop light and store it in some kind of cavity for an
extensive amount of time."
To slow down and eventually localize light, researchers rely on
cavities that trap light in the same way that sound is trapped in a
cave. Waves continuously bounce off the walls of the cavity and only
manage to escape after finding the narrow passage out. However, most
current cavities are quite leaky, and have not one but multiple ways
out. A cavity's capacity to retain light is measured by the quality
factor Q -- the higher the Q, the less leaky the cavity.
Lepetit and Kanté sought a way around the leak problem by designing a
metamaterials BIC device consisting of a rectangular metal waveguide
and ceramic light scatterer. Instead of limiting the size and number of
passages where light can escape the cavity, the cavity's design produces
destructive interferences for the light waves. Light is allowed to
escape, but the multiple waves that do so through the different passages
end up cancelling each other.
"In a nutshell, BICs can enhance your high-Q," the researchers joked.
Other researchers have worked on ways to trap light with BIC, but the
cavities have been constructed out of things like photonic crystals,
which are relatively large and designed to scale to the same wavelength
as light. The device tested by the UC San Diego researchers marks the
first time BIC has been observed in metamaterials, and contains even
smaller cavities, Kanté said.
The difference is important, he explains, "because if you want to
make compact photonic devices in the future, you need to be able to
store light in this subwavelength system."
Moreover, earlier researchers had reported observing only one BIC
within their systems. Lepetit and Kanté observed multiple bound states
in their system, which make the light trap more robust and less
vulnerable to outside disruptions.
The researchers say trapping light via BIC will likely have a variety
of other applications beyond circuitry and data storage. Since the
system can hold light for an extended time, it may enhance certain
nonlinear interactions between light and matter. These types of
interactions can be important in applications such as biosensors that
screen small molecules, or compact solar cells.
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