Researchers' 2D Waveguides Enable Dark Exciton Study

A team at the U.S. Naval Research Laboratory (NRL), in collaboration with Kansas State University, has developed slab waveguides based on the 2D material hexagonal boron nitride. NRL researchers also developed 3D electromagnetic models of the waveguides. The modeling results provide a toolkit for designing future 2D devices that use slab waveguides. The technology has applications in optoelectronics and enables the study of dark excitons.

2D materials are a class of materials which can be reduced to the monolayer limit by mechanically peeling the layers apart. The weak interlayer attractions, or van der Waals attraction, allows the layers to be separated via the so-called “Scotch tape” method. The most famous 2D material, graphene, is a semimetallic material consisting of a single layer of carbon atoms. Recently, other 2D materials, including semiconducting transition metal dichalcogenides (TMDs) and insulating hexagonal boron nitride (hBN), have also garnered attention. When reduced near the monolayer limit, 2D materials have unique nanoscale properties, which are appealing for creating atomically thin electronic and optical devices.

“We knew using hexagonal boron nitride would lead to outstanding optical properties in our samples, none of us expected that it would also act a waveguide,” said Samuel Lagasse of the NRL’s Novel Materials and Applications Division. “Since hBN is used so widely in 2D material-based devices, this novel usage as an optical waveguide potentially has wide-ranging impacts.”

Graphene and TMD monolayers are both extremely sensitive to the surrounding environment. Therefore, researchers have sought to protect these materials by encapsulating them in a passivating layer. This is where hBN comes in: layers of hBN are able to “screen” impurities near graphene or TMD layers, leading to better performance. In the current work, researchers tuned the thickness of hBN surrounding a light emitting TMD to enable optical waveguide modes.

A real-space (left) and Fourier space (right) photoluminescence image of a hexagonal boron nitride waveguide. The real-space image shows where photoluminescence is emitted from within the sample, while the Fourier space image depicts the angle of the emitted light. Courtesy of Nicholas Proscia/Naval Research Lab.

The researchers carefully assembled stacks of 2D materials, known as van der Waals heterostructures, which have specialized properties due to the layering. Slabs of hBN were placed around single layers of TMDs, such as molybdenum diselenide or tungsten diselenide, which can emit light in the visible and near-infrared. The slabs of hBN were carefully tuned in thickness so that the emitted light would be trapped within the hBN and waveguided. When the light waveguides to the edge of the hBN, it can scatter out and be detected by a microscope.

The research was motivated by the challenges of optical measurements of 2D TMDs. When laser light is focused on TMDs, particles known as excitons are generated. Most excitons emit light out of the plane of the TMD, however an elusive type of exciton known as a dark exciton exists in some TMDs and emits in the plane of the TMD. NRL’s slab waveguides capture the light from the dark excitons, providing a way to study them optically.

“2D materials have exotic optoelectronic properties that will be useful to the Navy,” Lagasse said. “A large challenge is interfacing these materials with existing platforms without damaging them — these boron nitride waveguides are a step toward that realization.”

NRL researchers used two special types of optical microscopes to characterize the hBN waveguides. One setup allows researchers to spectroscopically resolve photoluminescence emitting from different spots of the waveguide. The other setup allowed them observe the angular distribution of the emitted light.

The results provide a direct route for waveguide-based interrogation of layered materials, as well as a way to integrate layered materials into future photonic devices.

The study was published in Advanced Materials (www.doi.org/10.1002/adma.202309777).