Photodetector IDs Individual Vortex Beams

A new device that simply adds a metallic pattern to the window of a commercially available, low-cost photodetector could contribute to a major increase in the rate of future optical communications.

Developed by applied physicists at Harvard School of Engineering and Applied Sciences (SEAS), the device enables a conventional optical detector, which would normally measure only light’s intensity, to pick up on a vortex beam’s rotation. Twisted light waves, also called optical vortices or vortex beams, rotate as they travel and have gained new attention for transmitting more data over limited bandwidth.

“Sophisticated optical detectors for vortex beams have been developed before, but they have always been complex, expensive and bulky,” said principal investigator Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. In contrast, the new device simply adds a metallic pattern to the photodetector that couples with a particular type of incoming vortex beam by matching its orbital angular momentum — the number of twists per wavelength in an optical vortex.

Federico Capasso and colleagues at Harvard SEAS have developed a device that enables a conventional optical detector to pick up on the rotation of vortex beams. It also has the potential to add capacity to future optical communication networks. Courtesy of Eliza Grinnell, SEAS Communications.

The detector, sensitive to the beam’s “twistiness,” can distinguish between various types of vortex beams. Existing communications systems maximize bandwidth by sending many messages simultaneously, each a fraction of a wavelength apart; this is known as wavelength division multiplexing. Vortex beams can add an additional level of multiplexing, expanding these systems’ capacity.

“In recent years, researchers have come to realize that there is a limit to the information transfer rate of about 100 terabits per second per fiber for communication systems that use wavelength division multiplexing to increase the capacity of single-mode optical fibers,” Capasso said. “In the future, this capacity could be greatly increased by using vortex beams transmitted on special multicore or multimode fibers. For a transmission system based on this ‘spatial division multiplexing’ to provide the extra capacity, special detectors capable of sorting out the type of vortex transmitted will be essential.”

The detector can differentiate one type of vortex beam from another based on its precise nanoscale patterning. When a vortex beam with the correct number of coils per wavelength strikes the gold plating on the detector’s surface, it encounters a holographic interference pattern that has been etched into the gold. This nanoscale patterning allows the light to excite the metal’s electrons precisely, producing a surface plasmon. The light component of this electromagnetic wave then shines through a series of perforations in the gold and lands on the photodetector below.

The plasmon beam fails to focus or converge if the incoming light does not match the interference pattern. If this happens, the plasmon is blocked from reaching the detector.

This illustration (not to scale) simulates the process by which an incoming complex wave can be identified and transmitted to a photodetector. Courtesy of Patrice Genevet.

Capasso’s team has demonstrated this process using vortex beams with orbital angular momentum of -1, 0 and 1.

“In principle, an array of many different couplers and detectors could be set up to read data transmitted on a very large number of channels,” said Patrice Genevet, a research associate in applied physics at SEAS and lead author of the paper, which appeared in Nature Communications (doi: 10.1038/ncomms2293). “With this approach, we transform detectors that were originally only sensitive to the intensity of light, so that they monitor the twist of the wavefronts. More than just detecting a specific twisted beam, our detectors gather additional information on the phase of the light beam.”

The device’s capabilities may extend beyond optical communications.

“Using the same holographic approach, the same device patterned in different ways should be able to couple any type of free-space light beam into any type of surface wave,” Genevet said.

Co-authors are Jiao Lin, a former postdoctoral fellow in Capasso’s lab (now at Singapore Institute of Manufacturing Technology), and Harvard graduate student Mikhail A. Kats.

For more information, visit: www.seas.harvard.edu

For more on twisted-light systems, see: Terabits transmitted by twisted light