A landmark discovery has launched a slab waveguide made of hexagonal boron nitride (hBN), demonstrating the potential of two-dimensional materials to advance optical device technology. This innovation highlights the unique properties of hexagonal boron nitride as an optical waveguide and its broad impact on enhancing devices based on 2D materials.

The U.S. Naval Research Laboratory (NRL), in collaboration with Kansas State University, announced the discovery of a plate waveguide made of the two-dimensional material hexagonal boron nitride. This landmark discovery has been published in the journal Advanced Materials.

Two-dimensional (2D) materials are a class of materials that can be reduced to the single-layer limit by mechanically peeling off layers, using weak interlayer or van der Waals attractions to separate the layers through the so-called "Scotch tape" method. The best-known two-dimensional material, graphene, is a semi-metallic material composed of a single layer of carbon atoms. Recently, other 2D materials including semiconducting transition metal di-dopings (TMDs) and insulating hexagonal boron nitride (hBN) have also attracted attention. When reduced to near the single-layer limit, 2D materials possess unique nanoscale properties that are attractive for fabricating atomically thin electronic and optical devices.

Dr. Samuel Lagasse from the Novel Materials and Applications Department said: "We knew that using hexagonal boron nitride would give our samples excellent optical properties, but none of us expected it to also function as a waveguide. Since boron nitride is so widely used in devices based on two-dimensional materials, this new use as an optical waveguide could have wide-ranging implications."

Confocal microscopy image of waveguide photoluminescence in a hexagonal boron nitride waveguide with a leaf-like pattern at the edges reminiscent of a koi carp swimming around a pond. Image taken by Samuel LaGasse in April 2023. Image source: U.S. Naval Research Laboratory/Samuel LaGasse

Both graphene and TMD single-layer materials are extremely sensitive to their surrounding environment. Therefore, researchers try to protect these materials by encapsulating them in passivation layers. This is where boron hydrobromide comes in: The boron hydrobromide layer is able to "screen" impurities near the graphene or TMD layer, resulting in fantastic properties. In recent work led by NRL, the hBN thickness around the luminescent TMD layer was carefully tuned to support optical waveguide modes.

Advances in Waveguide Technology

NRL researchers carefully stacked two-dimensional materials called "van der Waals heterostructures." These heterostructures have special properties due to their layering. hBN sheets are placed around a single layer of TMDs such as molybdenum diselenide or tungsten diselenide, which emit visible and near-infrared light. The thickness of the hBN plate is carefully adjusted so that the emitted light is trapped within the hBN and waveguided. When the light waveguide reaches the edge of boron hydride, it is scattered and detected by a microscope.

Real-space (left) and Fourier-space (right) photoluminescence images of a hexagonal boron nitride waveguide. The real-space image shows where within the sample the photoluminescence is emitted, while the Fourier-space image describes the angle at which the light is emitted. Image taken in April 2023 by Nicholas Proscia. Image source: U.S. Naval Research Laboratory/Nicholas Proscia

This research was motivated by the challenges faced by optical measurements of 2D TMDs. When laser light is focused on the TMD, particles called excitons are produced. Most excitons emit light outside the plane of the TMD, but in some TMDs there exists an elusive type of exciton, called a "dark" exciton, which emits light within the plane of the TMD. NRL's slab waveguide can capture the light emitted by dark excitons, providing a way to study them optically.

"Two-dimensional materials have peculiar optoelectronic properties that would be very useful for the navy," Lagasse said. "A big challenge is to connect these materials to existing platforms without damaging them - these boron nitride waveguides are a step towards that goal."

NRL researchers used two special types of optical microscopy to characterize boron hydride waveguides. One device allowed the researchers to spectroscopically resolve the photoluminescence emitted at different points in the waveguide. Another device allowed them to observe the angular distribution of the emitted light.

NRL researchers also developed a three-dimensional electromagnetic model of the waveguide. The modeling results provide a toolkit for designing future 2D devices using sheet waveguides.

Compiled source: ScitechDaily