This illustration shows an integrated micro-ring resonator-based low-loss optical cavity with semiconductor 2D material on top of the waveguide. Image: Ipshita Datta and Aseema Mohanty, Lipson Nanophotonics Group/Columbia Engineering.
This illustration shows an integrated micro-ring resonator-based low-loss optical cavity with semiconductor 2D material on top of the waveguide. Image: Ipshita Datta and Aseema Mohanty, Lipson Nanophotonics Group/Columbia Engineering.

Optical manipulation at the nanoscale, or nanophotonics, has become a critical research area, as researchers seek ways to meet the ever-increasing demand for information processing and communications. The ability to control and manipulate light at the nanometer scale will lead to numerous applications, including data communication, imaging, ranging, sensing, spectroscopy, and quantum and neural circuits.

Today, silicon has become the preferred integrated photonics platform due to its transparency at telecommunication wavelengths, ability for electro-optic and thermo-optic modulation, and its compatibility with existing semiconductor fabrication techniques. But while silicon nanophotonics has made great strides in the fields of optical data communications, phased arrays, LIDAR (light detection and ranging), and quantum and neural circuits, there are two major concerns regarding the large-scale integration of photonics into these systems. These are their ever-expanding need for scaling optical bandwidth and their high electrical power consumption.

Existing bulk silicon phase modulators can change the phase of an optical signal, but this process comes at the expense of either high optical loss (electro-optic modulation) or high electrical power consumption (thermo-optic modulation). Now, a team from Columbia University, led by Michal Lipson, professor of electrical engineering and professor of applied physics at Columbia Engineering, has discovered a new way to control the phase of light with 2D materials. Not only does this happen with extremely low electrical power dissipation, but also without changing the light’s amplitude.

In a paper in Nature Photonics, the researchers demonstrated that, when placed on top of passive silicon waveguides, 2D materials can change the phase of light as strongly as existing silicon phase modulators, but with much lower optical loss and power consumption.

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

The optical properties of semiconductor 2D materials such as transition metal dichalcogenides (TMDs) are known to change dramatically with free-carrier injection (doping) near their excitonic resonances (absorption peaks). However, very little is known about the effect of doping on the optical properties of TMDs at telecom wavelengths, far away from these excitonic resonances, where the material is transparent and therefore can be leveraged in photonic circuits.

The Columbia team probed the electro-optic response of TMD 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 after doping, while the optical loss changed minimally in the transmission response of the ring cavity.

The researchers showed that the doping-induced phase change relative to the change in absorption for monolayer TMDs is approximately 125. This is significantly higher than observed in materials commonly employed as silicon photonic modulators, while simultaneously accompanied by negligible insertion loss.

"We are the first to observe strong electro-refractive change in these thin monolayers," says the paper's lead author Ipshita Datta, a PhD student with Lipson. "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. This is extremely encouraging for the scaling of photonic circuits and for low-power LIDAR."

The researchers are continuing to probe and better understand the physical mechanism responsible for this strong electro-refractive effect. They are currently leveraging their low-loss and low-power phase modulators to replace traditional phase shifters, and therefore reduce electrical power consumption in large-scale applications such as optical phased arrays, and neural and quantum circuits.

This story is adapted from material from Columbia Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.