Tunable Nanophotonic Interface Simplifies PIC Integration

A chiral nanophotonic interface developed by a research team at the University of Chicago could make photonic integrated circuits (PICs) easier to integrate into mapping systems, biosensors, and other technologies. The interface provides a way for PICs to direct light at the nanoscale.

PICs typically use optical isolators to control the direction of light. These devices prevent light from reentering and destabilizing a system, but they often require large components that make it challenging to create small-scale PICs.

The researchers fabricated titanium dioxide (TiO₂) waveguides directly onto the surface of boron nitride (BN)-encapsulated tungsten diselenide (WSe₂), integrating light confined in a nanophotonic waveguide with the atomically thin, 2D semiconductor material. The band structure of WSe₂ contains properties that enable it to interact with light in various ways, depending on the helicity of the light’s polarization.

In nanophotonic structures, where light is confined below its wavelength, circular polarization arises naturally, and the tracking of the polarization is based on the direction of the propagating light. Consequently, the light emitted from the WSe₂ can be guided through the photonic waveguide in a preferred direction.

“We’ve figured out a scalable method for putting photonics and 2D semiconductors together in a way that adds new control knobs and preserves the high quality of the sensitive material,” researcher Robert Shreiner said. “This interface opens new doors for designing ultracompact, one-way photonic devices.”

The researchers can switch the biased coupling of the photonic interface on and off by adding electrons to create a tunable emission router on a micron-size scale.

After the WSe₂ is coupled with the photonic waveguide, the photoluminescence from excitonic states going into the waveguide can be electrically switched between balanced and directionally biased emission. This capability leverages the doping-dependent valley polarization of excitonic states in WSe₂. The nanophotonic waveguide can also function as a near-field source for diffusive exciton fluxes that display valley and spin polarizations derived from the interface chirality.

Researchers at the University of Chicago have developed a way to guide light in one direction on a small scale. This would allow for smaller photonic integrated circuits that could be more easily integrated into modern technologies. Courtesy of istockphoto.com.

The nanoscale design of the interface and its tunability leads to smaller PICs that could be integrated more easily into optoelectronic technologies, such as computing systems and self-driving cars. Potentially, the photonic interface could be used to develop on-chip lasers for the lidar navigation systems in autonomous vehicles. In this case, the photonic element would be configured as a compact, on-chip optical isolator that would protect the laser system without the need for bulky components.

“We see this research as paving the way towards a whole new class of integrated photonic circuits,” said professor Alex High, who led the research.

Ultimately, the interface could make it easier to integrate PICs into optical computers that operate with light instead of electricity, thereby using less energy and creating less heat than conventional computers.

“We already use photonics to carry information throughout the country in fiber optic networks, but advancements like this could help fully control the flow of light on the nanoscale, thus realizing on-chip optical networks,” researcher Kai Hao said.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-00971-7).