Engineers Develop World’s Thinnest Optical Resonator for Visible Light

A waveguide with a thickness of three layers of atoms—which is the world’s thinnest optical device—has been developed by engineers from the University of California San Diego.

The study is a proof of concept for reducing the size of optical devices to several orders of magnitude smaller than that of existing devices. The work could pave the way for developing photonic chips with higher capacity and higher density. The study outcomes were published in Nature Nanotechnology on August 12^th, 2019.

Fundamentally, we demonstrate the ultimate limit for how thin an optical waveguide can be built.

Ertugrul Cubukcu, Study Senior Author, Professor of Nanoengineering and Electrical Engineering, UC San Diego

The thickness of the new waveguide is around six angstroms—that is, over 10,000 times thinner than a standard optical fiber and nearly 500 times thinner when compared to on-chip optical waveguides in integrated photonic circuits.

The waveguide includes a tungsten disulfide monolayer (formed of a single layer of tungsten atoms interposed between two sulfur atom layers) suspended on a silicon frame. In addition, the monolayer is patterned with an array of nanosized holes that form a photonic crystal.

The unique feature of this monolayer crystal is that it supports electron-hole pairs, called excitons, at ambient temperature. The excitons produce a powerful optical response, providing the crystal with a refractive index nearly four times greater than that of air surrounding its surfaces.

In comparison, the refractive index of another material with the same thickness would not be so high. Light irradiated through the crystal is trapped within and guided over the plane by total internal reflection. This is the mechanism fundamental to the working of an optical waveguide.

Another unique aspect is that the waveguide channels light in the visible spectrum.

This is challenging to do in a material this thin. 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.

Ertugrul Cubukcu, Study Senior Author, Professor of Nanoengineering and Electrical Engineering, UC San Diego

Through nanosized holes etched into the crystal, some portion of the light scatters perpendicular to the plane, enabling it to be observed and investigated. This array of holes forms a periodic structure that makes the crystal operate as a resonator as well.

“This also makes it the thinnest optical resonator for visible light ever to be demonstrated experimentally,” stated Xingwang Zhang, study first author who worked on this project as a postdoctoral researcher in Cubukcu’s lab at UC San Diego. “This system does not only resonantly enhance the light-matter interaction, but also serves as a second-order grating coupler to couple the light into the optical waveguide.”

The team employed sophisticated micro- and nanofabrication methods to develop the waveguide. According to Chawina De-Eknamkul, a nanoengineering PhD student at UC San Diego and a co-author of the study, the structure was specifically difficult to create.

The material is atomically thin, so we had to devise a process to suspend it on a silicon frame and pattern it precisely without breaking it.

Chawina De-Eknamkul, Nanoengineering PhD Student, UC San Diego

The process commences with a thin silicon nitride membrane that is supported by a silicon frame. The waveguide is formed on this substrate. A template is developed by patterning an array of nanosized holes into the membrane. Then, a monolayer of tungsten disulfide crystal is shifted onto the membrane. Subsequently, the same pattern of holes is etched into the crystal by passing ions through the membrane.

The last step is to gently etch away the silicon nitride membrane, which leaves the crystal attached to the silicon frame. The outcome is an optical waveguide where the core includes a monolayer tungsten disulfide photonic crystal that is surrounded by a lower refractive index material (air).

In the future, the researchers will further investigate the basic properties and physics behind the waveguide.

The co-authors of the paper titled “Guiding of visible photons at the ångström thickness limit” are Jie Gu, Alexandra L. Boehmke, and Vinod M. Menon from the City University of New York; and Jacob Khurgin from Johns Hopkins University.

This study was partially supported by the National Science Foundation under the NSF 2-DARE Program (EFMA-1542879).