Recording a free space optical light for high speed WiFi

Visible and infrared light can carry more data than radio waves, but has always been limited to a fixed, fiber optic cable. A Duke research team, along with Facebook’s Connectivity Lab, has made great strides with the dream of dumping the fiber into the optical fiber.

While working to create a high-speed, high-speed wireless Internet-based optical communication system, the researchers also show that the speed and efficiency characteristics previously shown on small single-unit plasmonic antennas are also larger. scales can be achieved.

The research appears in the journal on February 11 Optics.

In 2016, researchers from Internet.org’s Connectivity Lab – a subsidiary of Facebook – outlined a new type of light detector that could potentially be used for optical communication in space. Traditionally, hard-wired optical fiber connections can be much faster than wireless wireless connections. This is because visible and near-infrared light frequencies can contain much more information than radio waves (WiFi, Bluetooth, etc.).

But it is difficult to use these higher frequencies in wireless devices. Current settings use LEDs or lasers aimed at detectors that can reorient themselves to optimize the connection. However, it will be much more efficient if a detector can simultaneously capture light from different directions. The advantage is that increasing the size of an optical receiver also slows it down.

This was also the case for the design of the Connectivity Lab. A spherical bundle of fluorescent fibers captured blue laser light from any direction and again green light that could be directed at a small receiver. Although the prototype could reach two gigabits per second, most fiber optic ISPs offer up to 10 Gb, and higher-end systems can cost thousands.

In search of a way to speed up their optical communication designs in free space, the Connectivity Lab turned to Maiken Mikkelsen, the James N. and Elizabeth H. Barton, associate professor of Electrical and Computer Engineering and Physics at Duke. Over the past decade, Mikkelsen has been a leading researcher in the field of plasmonics, capturing light on the surface of small nanocubes to increase the speed and efficiency of light transmission more than a thousand times.

“The prototype of the Connectivity Lab is limited by the emission life of the fluorescent dye they use, which makes it inefficient and slow,” Mikkelsen said. “They wanted to increase efficiency and came across my work that shows ultra-fast response times in fluorescent systems. My research has only proven that these efficiency rates are possible on some nanoscale systems, so we do not know if it can increase to a centimeter-scale detector. . “

Mikkelsen explains that all the previous work was demonstrations with a single antenna. These systems usually involve metal nanocubes with a distance of ten to hundreds of nanometers and place only a handful of nanometers above a metal film. While an experiment could use tens of thousands of nanocubes over a large area, research showing the potential for superfast properties has historically chosen only one cube to measure.

In the new article, Mikkelsen and Andrew Traverso, a postdoctoral researcher working in her laboratory, brought a more purposeful and optimized design to a large area with plasmonic devices. Silver nanocubes of only 60 nanometers wide are located about 200 nanometers apart and cover 17% of the surface of the device. These nanocubes sit just seven nanometers above a thin layer of silver, with a layer of polymer covered with four layers of fluorescent dye.

The nanocubes interact with the silver base in a way that enhances the photonic abilities of the fluorescent dye, increasing a 910-fold increase in overall fluorescence and a 133-fold emission. The super-fast antenna can also capture light from a 120-degree field of view and convert it to a directional source with a record-high overall efficiency of 30%.

“Plasmonic effects have always been known to lose a lot of efficiency over a large area,” Traverso said. “But we have shown that you can take attractive ultra-fast emission properties of a nanoscale device and recreate it on a macroscopic scale. And our method is very easy to transfer to manufacturing facilities. We can make these large-scale plasmonic surfaces in less than one hour with pipettes and petri dishes, just simple liquid deposits on metal films. ‘

The overall effect of the demonstration is the ability to capture light from a large field of view and divert it into a narrow cone without losing speed. To continue with this technology, researchers need to assemble several plasmonic devices to cover a 360-degree field of view and once again include a separate indoor detector. While there is still work to be done, the researchers see a viable path forward.

“In this demonstration, our structure works to effectively transfer the photons from a wide angle to a narrow angle without losing speed,” Mikkelsen said. “We do not yet have a standard fast photodetector like the Connectivity Lab integrated into their original article. But we have solved the biggest bottleneck in the design, and the future applications are very exciting!”