Optical waveguide is atomically-thin

August 14, 2019 //By Julien Happich
waveguide
Researchers at the University of California - San Diego, have developed what they claim to be the thinnest optical device in the world, a waveguide only three layers of atoms thin.

The work is a proof of concept for scaling down optical devices to sizes that are orders of magnitude smaller than today's devices. It could lead to the development of higher density, higher capacity photonic chips.

The new waveguide measures about six angstroms thin—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. It consists of a tungsten disulfide monolayer (made up of one layer of tungsten atoms sandwiched between two layers of sulfur atoms) suspended on a silicon frame. The monolayer is also patterned with an array of nanosized holes forming a photonic crystal.

"Fundamentally, we demonstrate the ultimate limit for how thin an optical waveguide can be built," said senior author Ertugrul Cubukcu, a professor of nanoengineering and electrical engineering at UC San Diego.

What's special about this monolayer crystal is that it supports electron-hole pairs, known as 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 air, which surrounds its surfaces. By comparison, another material with the same thickness would not have as high of a refractive index. When light is sent through the crystal, it is trapped inside and guided along the plane by total internal reflection. This is the basic mechanism for how an optical waveguide works.


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. Credit: Cubukcu lab.

Another special feature is that the waveguide channels light in the visible spectrum. "This is challenging to do in a material this thin," Cubukcu said. "Waveguiding has previously been demonstrated with graphene, which is also atomically thin, but at infrared wavelengths. We've demonstrated for the first time waveguiding in the visible region."


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