Materials Integrated with Silicon Photonics Manipulate Light Phase at Low Loss

A Columbia University team used a 2D material from the transition metal dichalcogenide (TMD) class to control the phase of light without changing its amplitude or depleting its power. By placing the atomically thin material on top of passive silicon waveguides, the researchers were able to change the phase of light as strongly as if they had used silicon phase modulators, but with much lower optical loss and power consumption.

The researchers probed the electro-optic response of tungsten disulfide (WS₂) by integrating the semiconductor monolayer on top of a low-loss silicon nitride optical cavity and doping the monolayer using an ionic liquid. They observed a large phase change with doping and minimal optical loss in the transmission response of the ring cavity. The doping-induced phase change, relative to change in absorption for monolayer TMDs, was approximately 125 — significantly higher than that observed in materials commonly employed for silicon photonic modulators — and was accompanied by negligible insertion loss.

Illustration of an integrated microring resonator based low-loss optical cavity with semiconductor 2D material on top of the waveguide. Courtesy of Ipshita Datta and Aseema Mohanty, Lipson Nanophotonics Group/Columbia Engineering.

The optical properties of TMDs are known to change with doping near their excitonic resonances, but little is known about the effect of doping on the optical properties of TMDs at wavelengths far from these resonances, where the material is transparent and could be used in photonic circuits.

“We showed pure optical phase modulation by utilizing a low-loss silicon nitride (SiN)-TMD composite waveguide platform in which the optical mode of the waveguide interacts with the monolayer. So now, by simply placing these monolayers on silicon waveguides, we can change the phase by the same order of magnitude, but at 10,000 times lower electrical power dissipation,” researcher Ipshita Datta said. “This is extremely encouraging for the scaling of photonic circuits and for low-power lidar.”

Illustration of an integrated optical interferometer with semiconductor monolayers such as TMDs on both the arms of the silicon nitride (SiN) interferometer. One can probe the electro-optic properties of the monolayer with high precision using these on-chip optical interferometers. Courtesy of Ipshita Datta and Aseema Mohanty, Lipson Nanophotonics Group/Columbia Engineering.

Datta said that the Columbia team is the first to observe strong electro-refractive change in thin monolayers. The researchers continue to explore the underlying physical mechanism responsible for the strong electrorefractive effect. By replacing traditional phase shifters with their low-loss and low-power phase modulators, they hope to reduce the electrical power consumption in large-scale applications such as optical phased arrays and neural and quantum circuits. The ability to better control light at the nanoscale also could lead to applications in data communication, imaging, ranging, sensing, and spectroscopy.

“Phase modulation in optical coherent communication has remained a challenge to scale, due to the high optical loss that was associated with phase change,” professor Michal Lipson said. “Now we’ve found a material that can change the phase only, providing us another avenue to expand the bandwidth of optical technologies.”

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-0590-4).