Novel Method to Manipulate Phase of Light Through 2D Materials

Nanophotonics, or optical manipulation on the nanometer scale, has turned out to be an important field of research as an increasing number of scientists look for ways to fulfill the ubiquitous need for data processing and communications.

The potential to manage and exploit light on the nanoscale will result in many applications such as spectroscopy, imaging, sensing, ranging, data communication, and quantum and neural circuits; think light detection and ranging (LIDAR) for faster video-on-demand and self-driving cars, for instance.

Currently, silicon has become the integrated photonics platform of choice because of its transparency at telecommunication wavelengths, its compatibility with present semiconductor fabrication methods, and its potential for thermo-optic and electro-optic modulation.

However, while silicon nanophotonics has made immense advances in the areas of quantum and neural circuits, optical data communications, LIDAR, and phased arrays, two major problems are associated with the commercial incorporation of photonics within these systems: their ever-increasing demand for upgrading optical bandwidth and their high consumption of electricity.

While current bulk silicon phase modulators are capable of altering the phase of an optical signal, this process comes at the cost of high consumption of electrical power (thermo-optic modulation) or high optical loss (electro-optic modulation).

Researchers at Columbia University, headed by Michal Lipson, Eugene Higgins Professor of Electrical Engineering and Professor of Applied Physics at Columbia Engineering, reported that they have identified a novel method to manage the phase of light utilizing two-dimensional (2D) materials without altering its amplitude, at very low dissipation of electrical power.

2D materials are atomically thin materials that measure around 0.8 nm, or 1/100,000 the size of a single strand of human hair.

In this latest analysis, the researchers showed that when the thin material is placed on top of the passive silicon waveguides, the phase of light could be changed as intensely as prevalent silicon phase modulators, but with relatively lower power consumption and optical loss. The study was recently published by the Nature Photonics journal.

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

Michal Lipson, Eugene Higgins Professor of Departments of Electrical Engineering and Applied Physics, Columbia Engineering, Columbia University

It is known that the optical characteristics of semiconductor 2D materials, such as transition metal dichalcogenides (TMDs), vary considerably with free-carrier injection (doping) close to their excitonic resonances, or absorption peaks.

But not much is known about the impact of doping on the optical characteristics of TMDs at telecom wavelengths, which are far away from these excitonic resonances in which the material is see-through and thus can be manipulated in photonic circuits.

The researchers at Columbia University, including James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, and Dimitri Basov, professor of physics at Columbia University, explored the electro-optic reaction of the TMD by incorporating the monolayer of semiconductor over a low-loss silicon nitride optical cavity and then using an ionic liquid to dope the semiconductor monolayer.

While the researchers found a massive phase change with the doping approach, the optical loss varied only minimally in the transmission reaction of the ring cavity.

The team demonstrated that the phase change induced by doping in relation to the absorption change for monolayer TMDs is about 125. This phase change is considerably more than that seen in materials that are often used for silicon photonic modulators, such as Si and III-V on Si, and are usually accompanied by insignificant insertion loss.

We are the first to observe strong electro-refractive change in these thin monolayers. 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.

Ipshita Datta, Study Lead Author and PhD Student, Columbia University

Datta continued, “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 scientists are continuing to explore and better interpret the fundamental physical mechanism for the powerful electrorefractive effect. At present, the researchers are manipulating their low-power and low-loss phase modulators to substitute conventional phase shifters, and thus decrease the consumption of electrical power in commercial applications like neural and quantum circuits and optical phased arrays.