Switch controls light on a nanoscale for faster information processing

It's well known that photons, or units of light, are faster than electrons and could, therefore, process information faster from smaller chip structures. A switch designed 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 so-called surface plasmons, or light coupled to oscillations of free electron clouds, to help confine light to a nanoscale.

"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.

In a study publishing April 26 in Nature, researchers addressed this problem through the development of a switch, called 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.

Purdue 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 -- including Christian Haffner and other group members -- and in opto-electronic switching materials from Larry Dalton's group at the University of Washington. Haffner and Nathaniel Kinsey, former Purdue student and now a professor of electrical and computer engineering at Virginia Commonwealth University, along with Leuthold, Shalaev and Boltasseva, conceived the idea of a low-loss plasmon assisted electro-optic modulator for subwavelength optical devices, including compact on-chip sensing and communications technologies.