Millisecond-response phosphor is made to modulate with microsecond response

Resulting emitter could serve as a light source for communication networks on computer chips.

Millisecond-response phosphor is made to modulate with microsecond response
Millisecond-response phosphor is made to modulate with microsecond response
Experimental data for an erbium-based phosphor compares traditional lifetime (left) with new modulation approach (right); note the very different time scales. (Image: Brown University)

Researchers from Brown University (Providence, RI), in collaboration with colleagues from Harvard University (Cambridge, MA), have developed a new way to control light from phosphorescent emitters at very high speeds.1 The technique provides a new approach to modulation that could be useful in all kinds of silicon-based nanoscale devices, including photonic computing chips.

Instead of directly modulating the erbium-ion-based phosphor, the phase change of an adjacent vanadium dioxide (VO2) nanolayer controls the modulation speed.

VO2 is a phase-change material that, when pumped with energy (in this case, optical), 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 the VO2 changes phase, the erbium emissions go from being generated mostly by magnetic dipole transitions, to being generated mostly by electric dipole transitions. 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.

1000-fold speedup
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.

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

The result could enable the use of phosphors in new applications. One example could be optical communications networks on computer chips. Erbium and other phosphors can be deposited directly on silicon, making fabrication of emitters for silicon photonics easier than the conventional semiconductor lasers. And phosphors are highly efficient, so heat is less of a concern. More work needs to be done to get such a system up to a speed that would be useful on a chip, but Rashid Zia of Brown University and his colleagues think it's possible.



1. Sébastien Cueff et al., Nature Communications (2015); doi: 10.1038/ncomms9636.

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