Selective Light Transmission Could Advance Optical Computing

A new way to select and send light of a specific color over long distances has been applied to development of a nanoscale “light switch” for turning on and turning off the transmission of one color of light. The technique, which uses long silicon wires that are several hundred nanometers in diameter, could lead to a new platform for controlling light management when designing optical circuits.

The Encoded Nanowire Growth and Appearance through VLS and Etching (ENGRAVE) technique (VLS = vapor-liquid-solid), developed by a team at the University of North Carolina at Chapel Hill, can be used to achieve selective light transmission through precise diameter modulation.

Overlaid scanning electron microscopy (SEM) and optical images showing an Encoded Nanowire Growth and Appearance through VLS and Etching (ENGRAVE) nanowire that is periodically modulated at one end. Light of one select color (green) from a broad rainbow background enters at the modulated segment (false-colored red), traverses the uniform wire (false-colored blue), and is emitted at the end. Insets are magnified SEM images of the modulated segment and end of the nanowire, where the end contains a spherical gold particle that was used to grow the nanowire by the ENGRAVE process. Courtesy of UNC-Chapel Hill.

Researchers developed a nanowire geometric superlattice (GSL) that allowed narrowband guiding in silicon nanowires through coupling of a Mie resonance with a bound-guided state (BGS). Through periodic diameter modulation, they created a Mie-BGS-coupled excitation that manifested as a scattering dark state with a pronounced scattering dip in the Mie resonance. The frequency of the coupled mode, tunable from the visible to NIR, is determined by the pitch of the GSL.

The team believes this development to be the first report of direct use of a Mie resonance for guiding light in a nanowire.

Using the combined GSL-waveguide system, researchers demonstrated spectrally selective guiding and optical switching and sensing at telecommunication wavelengths.

Schematic illustration of the ENGRAVE technique. Courtesy of UNC-Chapel Hill.

Results of the study point to the potential use of nanowire GSLs for the design of on-chip optical components. The team's findings could enable downsizing of optical components, making them easier to integrate with the existing electronic components in computers.

Additionally, the color of light conducted by the nanowires is sensitive, changing as the environment changes. Thus, these nanostructures could be used as sensors, in which the color of the conducted light senses the environment of the wire.

“In the past there hasn’t been a controlled method for selectively sending light down nanoscale wires, so optical technology has either used much larger structures or wasted a lot of light in the process,” said professor James Cahoon. “We found a way to turn on and off the transmission of a specific color of light, and it represents an important step towards the more controlled, effective use of light that would enable optical computing."

The research was published in Nature Communications (doi:10.1038/s41467-018-05224-2).