Changing Frequency Speeds Phosphor Light Emission

A technique for controlling light from phosphors at very high speeds could overcome the materials' slow optical performance, potentially enabling their use in optical communications.

Phosphors are common light emitters used in light bulbs, LEDs and elsewhere. While they are extremely efficient, they can't be turned on and off very quickly, which has essentially prohibited their use in applications where rapid optical modulation is required.

Using phase-change materials, researchers from Brown and Harvard universities have found a way to modulate light from phosphor emitters significantly faster than under typical conditions. Courtesy of the Zia Lab/Brown University.

Now researchers from Brown and Harvard universities have demonstrated an approach to modulation using phosphors that is much faster. By leveraging the phase-change of a vanadium dioxide (VO₂) nanolayer, the team demonstrated broadband all-optical direct modulation of 1.5-μm emission from trivalent erbium ions more than three orders of magnitude faster than their excited-state lifetime.

"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," said Rashid Zia, associate professor of engineering and physics at Brown. The work was led by Sebastien Cueff, a postdoctoral researcher in Zia's lab.

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, alters how nearby erbium ions emit light.

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.

Optical modulation is used in silicon-based nanoscale devices such as computer chips and other optoelectronic components. Prototype on-chip networks have used semiconductor lasers as light emitters, which can modulate very quickly but can't be grown directly on a silicon chip, making fabrication difficult. Semiconductor lasers are also not particularly efficient, producing a lot of heat along with light, which can be problematic on a silicon chip.

Erbium and other phosphors, on the other hand, can be deposited directly on silicon and produce far less heat.

While the phosphor emitters are still too slow for on-chip applications, Zia and his team are examining faster means of changing the VO₂ phase. In this initial experiment, the researchers used a laser to zap the VO₂, but future experiments may employ electricity instead.

The work was published in Nature Communications (doi: 10.1038/ncomms9636).