Switching Light Faster on a Nanoscale

It’s well known that photons are faster than electrons and could, therefore, process infor­mation faster from smaller chip structures. A switch designed at Purdue Uni­versity in colla­boration with researchers from ETH Zürich, the Univer­sity of Washington, and Virginia Common­wealth University bypasses a tendency for the unwanted absorp­tion of light when using surface plasmons – light coupled to oscil­lations of free electron clouds – to help confine light to a nano­scale.

This artistic rendering magnifies a switch researchers have developed within a computer chip to control for loss of photons when light is reduced to a nanoscale. (Source: N. Kinsey, Virginia Commonwealth U.)

“The big idea behind this is going from elec­tronic cir­cuitry to photonic cir­cuitry,” said Vladimir Shalaev, Purdue’s Bob and Anne Burnett Distin­guished Professor of Electrical and Computer Engi­neering. “From elec­tronics to photonics, you need some structures that confine light to be put into very small areas. And plas­monics seems to be the solution.” Even though plas­monics downsizes light, photons also get lost, or absorbed, rather than trans­ferred to other parts of the computer chip when they interact with plasmons.

Now, the researchers addressed this problem through the develop­ment of a switch made of a ring modulator, that uses resonance to control whether light couples with plasmons. When on, or out of reso­nance, light travels through silicon wave­guides to other parts of the chip. When off, or in reso­nance, light couples with plasmons and is absorbed. “When you have a purely plasmonic device, light can be lossy, but in this case it’s a gain for us because it reduces a signal when necessary,” said Soham Saha, a graduate research assistant in Purdue’s school of elec­trical and computer engi­neering. “The idea is to select when you want loss and when you don’t.”

The loss creates a contrast between on and off states, thus better enabling control over the direction of light where appro­priate for proces­sing bits of information. A plasmon-assisted ring modu­lator also results in a smaller foot­print because plasmons enable confine­ment of light down to nanoscale chip structures, Shalaev said. The researchers plan to make this modu­lator fully compatible with comple­mentary metal-oxide-semi­conductor tran­sistors, paving the way to truly hybrid photonic and elec­tronic nano­circuitry for computer chips.

“Super­computers already contain both electronic and optical components to do massive calcu­lations very fast,” said Alexandra Bol­tasseva, Purdue pro­fessor of elec­trical and computer engineering, whose lab specializes in plas­monic materials. “What we’re working on would fit very well into this hybrid model, so we don’t have to wait to use it when computer chips go all-optical.” Develop­ment of the plasmon assisted electro-optic modu­lator required expertise in not only plas­monics, but also integrated circuitry and nano­photonics from the leading group of Juerg Leuthold at ETH Zürich and in opto-elec­tronic switching materials from Larry Dalton’s group at the Univer­sity of Washington. The scientists conceived the idea of a low-loss plasmon assisted electro-optic modu­lator for subwave­length optical devices, including compact on-chip sensing and communi­cations techno­logies. (Source: Purdue U.)

Reference: C. Haffner et al.: Low-loss plasmon-assisted electro-optic modulator, Nature 556, 483 (2018); DOI: 10.1038/s41586-018-0031-4

Link: Birck Nanotechnology Center, Purdue University, West Lafayette, USA

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