April 25, 2018
WEST LAFAYETTE, Ind. — Purdue researchers have helped design a compact switch that enables light to be more reliably confined to small computer chip components 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.
Writer: Kayla Wiles
Low-loss plasmon-assisted electro-optic modulator
Christian Haffner1, Daniel Chelladurai1, Yuriy Fedoryshyn1, Arne Josten1, Benedikt Baeuerle1, Wolfgang Heni1, Tatsuhiko Watanabe1, Tong Cui1, Bojun Cheng1, Soham Saha3, Delwin L. Elder2, Larry. R. Dalton2, Alexandra Boltasseva3, Vladimir M. Shalaev3, Nathaniel Kinsey4 and Juerg Leuthold1
1ETH Zürich, Zürich, Switzerland
2University of Washington, Seattle, WA, USA
3Purdue University, West Lafayette, IN, USA
4Virginia Commonwealth University, Richmond, VA, USA
For nearly two decades, researchers in the field of plasmonics— which studies the coupling of electromagnetic waves to the motion of free electrons near the surface of a metal—have sought to realize subwavelength optical devices for information technology, sensing, nonlinear optics, optical nanotweezers and biomedical applications. However, the electron motion generates heat through ohmic losses. Although this heat is desirable for some applications such as photo-thermal therapy, it is a disadvantage in plasmonic devices for sensing and information technology and has led to a widespread view that plasmonics is too lossy to be practical. Here we demonstrate that the ohmic losses can be bypassed by using ‘resonant switching’. In the proposed approach, light is coupled to the lossy surface plasmon polaritons only in the device’s off state (in resonance) in which attenuation is desired, to ensure large extinction ratios between the on and off states and allow subpicosecond switching. In the on state (out of resonance), destructive interference prevents the light from coupling to the lossy plasmonic section of a device. To validate the approach, we fabricated a plasmonic electro-optic ring modulator. The experiments confirm that low on-chip optical losses, operation at over 100 gigahertz, good energy efficiency, low thermal drift and a compact footprint can be combined in a single device. Our result illustrates that plasmonics has the potential to enable fast, compact on-chip sensing and communications technologies.