It takes a relatively long time for phosphors to return to the ground state after being excited. As a result, phosphors can't be turned on and off very quickly. Glow-in-the-dark toys, for example, take advantage of this property. (Credit: Paul Hocksenar/Flickr)


Phase-change materials make phosphors more useful

Researchers have developed a new way to control light from phosphorescent emitters at very high speeds. The technique provides a new approach to modulation that could be useful in all kinds of silicon-based nanoscale devices, including computer chips and other optoelectronic components.

“Our results demonstrate relatively fast modulation from fundamentally slow phosphorescent light emitters,” says Rashid Zia, associate professor of engineering and physics at Brown University and senior author of a new paper in Nature Communications describing the work.

“We think this could help make phosphors useful in a variety of new systems and settings.”

Phosphors are common light emitters used in light bulbs, LEDs, and elsewhere. They are extremely efficient because much of the energy pumped into them is converted to light as opposed to heat. But they have a slow optical lifetime, meaning it takes a relatively long time for them to return to the ground state after being excited. As a result, phosphors can’t be turned on and off very quickly. Glow-in-the-dark toys, for example, take advantage of this property.

That property is bad, however, for optical modulation, a process that often involves flipping the light on and off to encode information. Because of their slow lifetimes, phosphors have traditionally been a non-starter for applications that require high-speed modulation.

Change the environment, not the emitter

But in this latest work, Zia and collaborators, including researchers from Shriram Ramanathan’s group at Harvard University, took a different approach to modulation.

“Instead of changing how much light is coming out, which can only be done slowly in phosphor emitters, we came up with a system that changes another quality of that light, namely the color or spectrum of light emission, by rapidly changing the environment around the emitter,” Zia says.

Sébastien Cueff, a postdoctoral researcher in Zia’s lab, led the research. Cueff started with an emitter made of erbium ions, an important phosphor that is widely used in fiber-optic telecommunication networks. He combined that with a material called vanadium dioxide (VO2). VO2 is a phase-change material that, when pumped with energy, changes very quickly from a transparent insulating state to a reflective metallic state. This change in reflectivity, in turn, switches how nearby erbium ions emit light.

[For better antennas, add vanadium dioxide]

As the VO2 changes phase, the erbium emissions go from being generated mostly by magnetic dipole transitions (the rotational torque push and pull of magnetic forces), to being generated mostly by electric dipole transitions (the linear push and pull of electric forces). Those two emission pathways have distinct spectra, and the modulation back and forth between the two can be used as a means to encode information.

The researchers show that this direct modulation of light emission could be done as quickly as the VO2 phase could be changed, which is much faster than the speed at which erbium can be turned on and off. The test system used in these initial experiments showed that the system could be switched three orders of magnitude faster than the optical lifetime of erbium.

On-chip networks

“Phosphorescent emitters have been considered impractical for high speed applications because of their intrinsically long lifetimes,” Zia says. “Our results provide a simple way to circumvent this limitation and modulate their emission at high speeds.”

And that could enable the use of phosphors in new applications. One example could be optical communications networks on computer chips.

[Tiny optical ‘tuning fork’ fits on a chip]

Prototype on-chip networks have used semiconductor lasers as light emitters. They can modulate very quickly, but they have downsides. Semiconductors can’t be grown directly on a silicon chip, so fabrication can be difficult. Using indirect means of modulation—interferometers, for example—makes for bulky systems that take up a lot of real estate on a chip. What’s more, semiconductor lasers are not particularly efficient. They produce a lot of heat along with light, which is a problem on a silicon chip.

Erbium and other phosphors, on the other hand, can be deposited directly on silicon, making fabrication easier. And phosphors are highly efficient, so heat is less of a concern. There’s still more work to be done to get such a system up to a speed that would be useful on a chip, but Zia and his colleagues think it’s possible.

In this initial experiment, the researchers used a laser to zap the VO2 and cause it to change phase. A faster means of changing the VO2 phase—perhaps using electricity instead of a laser—could make the system much faster still.

Zia and his group plan to continue to refine the technique, but they describe this first set of experiments as an important proof of concept.

The Air Force Office of Scientific Army Research Office, the Department of Education, and the National Science Foundation supported the work.

Source: Brown University

Related Articles