2D-Material Emits Ultrafast Pulses

Light strikes a molecular lattice deposited on a metal substrate. The molecules can quickly exchange energy with the metal below, a mechanism that leads to a much faster response time for the emission of fluorescent light from the lattice. (Source: MIT)

Two-dimen­sional materials are very effective light emitters that work on a different principle than typical organic light-emitting diodes (OLEDs) or quantum dots. But their potential as components for new kinds of opto­electronic devices has been limited by their relatively slow response time. Now, researchers at MIT, the University of Cali­fornia at Berkeley, and North­eastern University have found a way to overcome that limi­tation, poten­tially opening up a variety of appli­cations for these materials.

The key to enhancing the response time of these 2-D molecular aggre­gates (2DMA), Nicholas X. Fang and his team found, is to couple that material with a thin layer of a metal such as silver. The interaction between the 2DMA and the metal that is just a few nano­meters away boosts the speed of the material’s light pulses more than tenfold. These 2DMA materials exhibit a number of unusual proper­ties and have been used to create Bose-Einstein conden­sates at room temperature, while other approaches required extreme cooling. They have also been applied in tech­nologies such as solar cells and light-har­vesting organic antennas. But the new work for the first time identi­fies the strong influence that a very close sheet of metal can have on the way these materials emit light.

In order for these materials to be useful in devices such as photonic chips, “the challenge is to be able to switch them on and off quickly,” which had not been possible before, Fang says. With the metal substrate nearby, the response time for the light emission dropped from 60 pico­seconds to just 2 pico­seconds, Fang says: “This is pretty exciting, because we observed this effect even when the material is 5 to 10 nano­meters away from the surface,” with a spacing layer of polymer in between. That’s enough of a sepa­ration that fabricating such paired materials in quantity should not be an overly demanding process. “This is something we think could be adapted to roll-to-roll printing,” he says.

If used for signal proces­sing, such as sending data by light rather than radio waves, Fang says, this advance could lead to a data trans­mission rate of about 40 gigahertz, which is eight times faster than such devices can currently deliver. This is “a very promising step, but it’s still very early” as far as trans­lating that into practical, manu­facturable devices, he cautions. The team studied only one of the many kinds of molecular aggre­gates that have been developed, so there may still be oppor­tunities to find even better variations. “This is actually a very rich family of luminous materials,” Fang says.

Because the respon­siveness of the material is so strongly influenced by the exact proximity of the nearby metal substrate, such systems could also be used for very precise measuring tools. “The inter­action is reduced as a function of the gap size, so it could now be used if we want to measure the proximity of a surface,” Fang says. As the team continues its studies of these materials, one next step is to study the effects that patter­ning of the metal surface might have, since the tests so far only used flat surfaces. Other questions to be addressed include deter­mining the useful lifetimes of these materials and how they might be extended. Fang says a first proto­type of a device using this system might be produced “within a year or so.” (Source: MIT)

Reference: Q. Hu et al.: Ultrafast fluorescent decay induced by metal-mediated dipole–dipole interaction in two-dimensional molecular aggregates, Proc. Nat. Ac. Sc. 114, 10017 (2017). DOI: 10.1073/pnas.1703000114

Link: Nanophotonics, Dept. of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, USA

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