Taking Advantage of (Plasmonic) Loss

Plasmonic ring resonator and accompanying chip

Artist’s rendering of the plasmon-assisted electro-optic switch developed by a joint Swiss-U.S. research team. [Image: Virginia Commonwealth University image/Nathaniel Kinsey]

For decades, plasmonics—the confinement of light energy at subwavelength scales at metal-dielectric interfaces—has tantalized engineers with the possibility of ultraminiaturized devices for applications ranging from information technology to sensing. But plasmonics has an Achilles’ heel: the tiny metal structures essential for plasmonic photon-electron interactions inevitably lead to absorption and ohmic losses of optical energy. That’s made it tricky to design efficient, practical devices leveraging the ultra-compact length scales, field enhancement and rapid operation possible through plasmonic effects.

Now, a team of researchers in Switzerland and the United States has designed a micron-scale, plasmon-assisted electro-optic modulator that reportedly gets around the loss conundrum (Nature, doi: 10.1038/s41586-018-0031-4). The researchers pulled off the feat not by attempting to minimize the device’s plasmonic losses, but by incorporating those losses into the device design itself.

The loss problem

In a plasmonic device, the electrical component of a light wave hitting a surface dotted with metal nanostructures can excite a subwavelength-scale electromagnetic wave, known as a surface plasmon polariton (SPP), that propagates along the metal-dielectric surface. By confining and channeling the light energy at the nanoscale, plasmonic devices can break the diffraction limit and locally (and substantially) boost a relatively weak incident light field.

These advantages have already found niches for plasmonics in areas such as biosensors based on detecting surface plasmon resonances. But wider application of plasmonics, in areas such as communications and photonic-electronic circuits, has often run aground on the problem of loss. That’s because as the SPP propagates across the metal surface, energy is inevitably absorbed by the metal and given off as heat.

Thus, while plasmonic devices can deliver substantial modulation effects at micrometer length scales, they also suffer from propagation losses on the order of dB/μm, versus dB/cm for silicon photonics. As a result, for on-chip technologies, dropping a plasmonic device into the mix can bring on an unacceptable “insertion loss”—a reduction of signal power in the circuit attributable to addition of the lossy plasmonic component.

From bug to feature

The Swiss-U.S. team, including researchers from ETH Zürich, Switzerland, and the University of Washington, Purdue University (Ind.), and Virginia Commonwealth University, USA, has reportedly found a way around the loss dilemma in its new design for an electro-optic modulator, an on-chip switch for converting between electrical and photonic energy. And the researchers did so by treating plasmonic loss not as a bug, but as a feature.

The modulator consists of a gold metal-insulator-metal slot waveguide ring resonator, around 3 microns in diameter and tens of nanometers thick, and filled with an organic electro-optic material that’s used to control the resonance state of the ring under an applied voltage bias. The ring resonator sits on an SiO2 substrate around 70 nm above a buried silicon bus waveguide.

The result is an ingenious notch filter that uses plasmonic loss in the ring to control light transmission through the silicon bus waveguide below. When the ring is tweaked into its resonant state—the “off” state of the switch—the SPPs in the ring interfere constructively, leading to strong plasmonic coupling and absorption of the light traveling through the bus waveguide, and effectively shutting down transmission of light. When the ring is in the nonresonant state—the switch’s “on” position—the SPPs in the ring interfere destructively; almost all of the light in the bus waveguide escapes plasmonic coupling and passes through the waveguide unimpeded.

Ultracompact electronic-photonic gateway

The Swiss-U.S. team reports that experiments with its proof-of-principle resonator “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.” The researchers believe that the scheme they’ve outlined—which, they note, is CMOS-compatible—could prove particularly useful in developing ultracompact gateways between electronics and photonics in emerging hybrid chips for communications and I.T., and also in on-chip sensor applications.

The work was conducted by the research groups led by OSA Fellows Juerg Leuthold at ETH Zürich, Larry Dalton at the University of Washington, and Vladimir Shalaev and Alexandra Boltasseva at Purdue, and co-conceived by OSA members Christian Haffner of ETH Zürich and Nathaniel Kinsey of Virginia Commonwealth.

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