Holding Light (Temporarily) in Place

New research describes potential ways to momentarily store light pulses

Illustration: CCNY
This illustration shows light localized in space inside a topological crystal, entangled by interaction and topology (schematically shown by ribbons).

Storing light beams—putting an ensemble of photons, traveling through a specially prepared material, into a virtual standstill—has come a step closer to reality with a new discovery involving microwaves.

The research finds that microwaves traveling through a particular configuration of ceramic aluminum oxide rods can be made to hold in place for several microseconds. If the optical or infrared equivalent of this technology can be fabricated, then Internet and computer communications networks (each carried by optical and infrared laser pulses) might gain a new versatile tool that enables temporary, stationary storage of a packet of photons.

For now, the researchers from the City College of New York and the Moscow Institute of Physics and Technology have developed this new technology for microwaves. Which represents an important stepping stone, says Alexander Khanikaev, associate professor of electrical engineering at City College. (The frequency of microwaves they’re working with falls right in the middle of the 802.11 Wi-Fi spectrum—a wireless local area network standard that IEEE sets and upholds.)

The group’s microwave “crystal”—a latticework of aluminum oxide ceramic columns sandwiched by aluminum plates in a triangle configuration some 20 centimeters on each side—is tuned to respond to specific wavelengths.

Send one frequency of microwaves (5.15 GHz) through this triangle-shaped course, and the waves will hold still at the triangle’s corners for some 1,000 cycles before being absorbed by the material or dissipated into the surrounding environment. Another frequency range (5.01 to 5.07 GHz) will propagate around the perimeter of the triangle.

In neither case will the microwaves penetrate inside of the triangle. This edge behavior of the medium makes it analogous to topological insulators—those materials that conduct electricity only at their outside edges, while inside behaving like a pure insulator.

The peculiar state of these microwave photons in which they hold in place at the corners of the triangular “crystal” is unique to electromagnetic waves, says Khanikaev. “You can think of it like they’re interfering destructively everywhere else,” he says. “And they’re interfering constructively at these corners.”

The geometric configuration of the aluminum oxide columns is crucial to the peculiar behavior the researchers observed, too. Khanikaev says they arranged the aluminum oxide columns in pairs (“dimers”) and groups of three (“trimers”). Using the dimers to trace out a smaller triangle in the middle of the 20-centimeter triangle, it turns out, made a kind of boundary inside of which the microwaves in the experiment did not venture.

The researchers describe their setup and results in a recent issue of the journal Nature Photonics.

However, Khanikaev also notes that they’ve begun to experiment with shrinking their findings down to a similar triangular crystal for infrared light. (Their new device measures 300 micrometers on a side.) They’ve reported their initial findings on the pre-print server arxiv.org.

“This geometric arrangement allows for the creation of a completely new class of electromagnetic modes,” he says. “Any new electromagnetic mode has potential for new applications, because it behaves differently.”

The ultimate idea, Khanikaev says, is to make the crystal tuneable. So a laser pulse might be bounced into one of these tiny resonator-like crystals. And then the crystal’s structure is altered so that the pulse now holds steady at the triangle’s corners—like a nanophotonic game of freeze tag.

The pulses couldn’t hold in place indefinitely, but crystals of higher quality factor (“Q factor”) might keep the photons steady for more cycles before being absorbed or dissipating into the surrounding environment.

The time spent in this photonic, semi-stationary state might only be microseconds or less. But in the world of laser pulses and digital communications, microseconds can still be long enough to, say, wait for another pulse to arrive while the trapped laser pulse idles in place.

“You can do signal manipulation,” Khanikaev says. “It has great potential. Because if you can do trapping and release on demand and control it in time, then you can control photonic signals.”

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