Controlling the Phase of Light Using 2D Materials

Illustration of an integrated micro-ring resonator based low loss optical cavity with semiconductor 2D material on top of the waveguide. (Source: I. Datta & A. Mohanty, Columbia U.)

Optical mani­pulation on the nano-scale, 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 on the nanometer scale will lead to numerous appli­cations including data communi­cation, imaging, ranging, sensing, spectroscopy, and quantum and neural circuits – think LIDAR for self-driving cars and faster video-on-demand, for example.

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, and quantum and neural circuits, there are two major concerns for large-scale integration of photonics into these systems: 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). A Columbia University team, led by Michal Lipson, Eugene Higgins Professor of Electrical Engi­neering, announced that they have disco­vered a new way to control the phase of light using 2D materials without changing its amplitude, at extremely low electrical power dissi­pation. The researchers demons­trated that by simply placing the thin material on top of passive silicon wave­guides, they could 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 near their excitonic resonances. 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, which included James Hone, Wang Fong-Jen Professor of Mechanical Engi­neering at Columbia Engi­neering, and Dimitri Basov, professor of physics at the Uni­versity, probed the electro-optic response of the 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 with doping, while the optical loss changed minimally in the trans­mission response of the ring cavity. They showed that the doping-induced phase change relative to change in absorption for monolayer TMDs is approxi­mately 125, which is signi­ficantly higher than that observed in materials commonly employed for silicon photonic modulators including Si and III-V on Si, while being simul­taneously accom­panied by negligible insertion loss.

“We are the first to observe strong electro-refractive change in these thin monolayers,” says 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 underlying physical mechanism for the strong electro­refractive effect. They are currently leveraging their low-loss and low-power phase modu­lators to replace tradi­tional phase shifters, and therefore reduce the electrical power consumption in large-scale appli­cations such as optical phased arrays, and neural and quantum circuits. (Source: Columbia U.)

Reference: I. Datta et al.: Low-loss composite photonic platform based on 2D semiconductor monolayers, Nat. Phot., online 24 February 2020; DOI: 10.1038/s41566-020-0590-4

Link: Dept. of Electrical Engineering, Columbia University, New York, USA

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