Engineers at the University of California, San Diego, have developed a waveguide that is more than 10,000 times thinner than a typical optical fiber and about 500 times thinner than on-chip optical waveguides in integrated photonic circuits. The atomically thin waveguide, which is able to channel light in the visible spectrum, could facilitate the scaling down of optical devices and the development of higher-capacity photonic chips. The researchers used advanced micro- and nanofabrication techniques to create the waveguide from material that is just three layers of atoms thick, suspending the material on a silicon frame so they could work with precision and without risk of the material breaking. Illustration of a monolayer of tungsten disulfide crystal suspended in air and patterned with a square array of nanoholes. Upon laser excitation, the monolayer crystal emits photoluminescence. A portion of this light couples into the monolayer crystal and is guided along the material. At the nanohole array, periodic modulation in the refractive index causes a small portion of the light to decay out of the plane of the material, allowing the light to be observed as guided mode resonance. Courtesy of the Cubukcu lab. A thin silicon nitride membrane supported by the silicon frame is the substrate upon which the waveguide is built. The researchers patterned an array of nanosize holes into the membrane to create a template. Next, they stamped a monolayer of tungsten disulfide crystal onto the membrane. They sent ions through the membrane to etch the same pattern of nanosize holes into the crystal. In the last step, the silicon nitride membrane was etched away, leaving the crystal suspended on the silicon frame. The photonic crystal supports electron-hole pairs, or excitons, at room temperature. These excitons generate a strong optical response, giving the crystal a refractive index that is about four times greater than that of the air that surrounds its surfaces. The result is an optical waveguide in which the core consists of a monolayer tungsten disulfide photonic crystal surrounded by a material (air) with a lower refractive index. An image of the waveguide structure taken with scanning electron microscopy (SEM): a suspended tungsten disulfide monoloayer patterned with nanosize holes. Courtesy of the Cubukcu lab. When light is sent through the crystal, it is trapped inside and guided along the plane by total internal reflection, with only a small portion of the light diffracted to the far field. The nanosize holes etched into the crystal allow some light to scatter perpendicular to the plane so that it can be observed and probed. This array of holes produces a periodic structure that makes the crystal work as a resonator as well as an optical waveguide. “This also makes it the thinnest optical resonator for visible light ever to be demonstrated experimentally,” researcher Xingwang Zhang said. “This system not only resonantly enhances the light-matter interaction, but also serves as a second-order grating coupler to couple the light into the optical waveguide.” Researcher Chawina De-Eknamkul prepares to transfer monolayer tungsten disulfide onto a photonic crystal/nanohole array template. Courtesy of Liezel Labios/UC San Diego Jacobs School of Engineering. The waveguide, which measures about 6 Å, is able to channel light in the visible spectrum. “This is challenging to do in a material this thin,” professor Ertugrul Cubukcu said. “Waveguiding has previously been demonstrated with graphene, which is also atomically thin, but at infrared wavelengths.” The team will continue to explore the fundamental properties and physics pertaining to the atomically thin waveguide. The ability to guide visible light at an angstrom thickness limit opens possibilities to miniaturize optoelectronic devices and test fundamental physical concepts. The research was published in Nature Nanotechnology (https://doi.org/10.1038/s41565-019-0519-6).
