Silicon Metamaterials Hold Promise for Photonic Circuits, Chips

As data transmission technology moves towards miniaturization, all-diaelectric, silicon-based metamaterials may offer the necessary control of light through achievement of total internal reflectance.

Transparent metamaterials under development could enable computer chips and interconnecting circuits that use light instead of electrons to process and transmit data, representing a potential leap in performance. Courtesy of Purdue University/Zubin Jacob.

Researchers from Purdue University seek to engineer total internal reflection in optical fibers surrounded by a transparent silicon metamaterial, which could enable computer chips and interconnecting circuits that use light instead of electrons to process and transmit data.

Although optical fibers are now used to transmit large amounts of data over great distances, the technology cannot easily be miniaturized because the wavelength of light is too large to fit within the miniscule dimensions of microcircuits.

"The role of optical fibers is to guide light from point A to point B, in fact, across continents," said professor Zubin Jacob. "The biggest advantage of doing this compared to copper cables is that it has a very high bandwidth, so large amounts of data can pass through these optical cables as opposed to copper wires. However, on our computers and consumer electronics we still use copper wires between different parts of the chip. The reason is that you can't confine light to the same size as a nanoscale copper wire."

Transparent metamaterials — nanostructured artificial media with transparent building blocks — allow unprecedented control of light and may enable the wavelength of light to be shrunk, the researchers said. Very high-bandwidth communication on a chip and interconnecting circuits between chips is necessary for faster clock and, therefore, data processing speeds.

Unlike some of the metamaterials under development, which rely on the use of noble metals such as gold and silver, the Purdue researchers said their metamaterials are made entirely of dielectric materials, i.e., insulators and nonmetals. The use of metals results in too much loss of light to be practical in terms of output and energy required to operate; in metal-based systems, much of the light is converted into heat. Basing such devices on the silicon platform would enable integration of electronic and photonic devices on the same chip.

A critical detail is the material's anisotropic velocity, meaning light is transmitted much faster in one direction through the material than in another. Conventional materials transmit light at almost the same speed no matter which direction it is traveling through the material.

"So in one direction, light travels almost as fast as it would in a vacuum, and in the other direction it travels as it would in silicon, which is around four times slower,” said Jacob.

The innovation could make it possible to modify total internal reflection, the principle currently used to guide light in fiber optics. The researchers are working to engineer total internal reflection in optical fibers surrounded by the new silicon-based metamaterial; adapting total internal reflectance down to the nanoscale, which the researchers said they were the first to achieve, is the first step.

Because the materials are transparent, they are suitable for transmitting light, and miniaturized data-processing units are one potential application.

"Another fascinating application for these transparent metamaterials is in enhancing light-matter coupling for single quantum light emitters," Jacob said. "The size of light waves inside a fiber are too large to effectively interact with tiny atoms and molecules. The transparent metamaterial cladding can compress the light waves to sub-wavelength values thus allowing light to effectively interact with quantum objects. This can pave the way for light sources at the single photon level."

The researchers have obtained a U.S. patent on their design. The work was published in Nature Nanotechnology (doi: 10.1038/nnano.2015.304).