The U.S. Naval Research Laboratory (NRL), in collaboration with Kansas State University, announce the discovery of slab waveguides based on the two-dimensional material hexagonal boron nitride. Two-dimen­sional 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 “Scotch tape” method. 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. “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 transition metal dichalcoge­nides (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 fantastic properties. In recent NRL-led work, the thickness of hBN surrounding a light-emitting TMD layer was carefully tuned in order to support optical waveguide modes.

The researchers carefully assembled stacks of 2D materials. These van der Waals heterostructures can 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, 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, 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,” said Lagasse. “A large challenge is interfacing these materials with existing platforms without damaging them – these boron nitride waveguides are a step towards that realization.” The 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 let them observe the angular distri­bution of the emitted light. They also developed 3D electromagnetic models of the waveguides. The modeling results provide a toolkit for designing future 2D devices that use slab waveguides. (Source: NRL)

Reference: S. W. LaGasse et al.: Hexagonal Boron Nitride Slab Waveguides for Enhanced Spectroscopy of Encapsulated 2D Materials, Adv. Mat. 36, 2309777 (2024); DOI: 10.1002/adma.202309777

Link: Electronics Science and Technology Division, US Naval Research Laboratory, Washington, DC, USA