Phosphors are efficient light emitters but they’re not optimal for high-speed communications because they turn on and off slowly. Researchers from Brown and Harvard have now found a way to modulate light from phosphor emitters three orders of magnitude faster using phase-change materials (VO2, in this case), which could make phosphors useful in a range of new optoelectronic applications. Credit: Zia Lab / Brown University
The technique provides a new approach to modulation useful in silicon-based nanoscale devices, including computer chips and other optoelectronic components.
Phosphors are efficient light emitters (eg light bulbs, LEDs) but they’re not optimal for high-speed communications because they turn on and off slowly. Researchers can now modulate light from phosphor emitters 3 orders of magnitude faster using phase-change materials.
“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 said.
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. As 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 2 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 showed 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.
Erbium and other phosphors can be deposited directly on silicon, making fabrication easier than semiconductor lasers. 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. https://news.brown.edu/articles/2015/10/phosphormod
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