Speed of Light Reduced Using Nanoantennas

In 1905, Albert Einstein wrote his seminal paper on special relativity. In it, he established that light travels at a constant velocity relative to every observer (186,000 miles per second in a vacuum), no matter how fast that observer is moving. The implication of the theory is that nothing can travel as fast as light, which has caused light to be dubbed the universe’s “speed limit.”¹ Light’s speed is crucial for rapid information exchange, but as photons (the particles of light) travel through different materials, its chances of exciting and interacting with atoms diminishes. If scientists can slow down these photons, interactions between light and atoms could greatly increase, ushering in a host of new technological applications.²

In a paper published on August 17, 2020 in Nature Magazine, a team of Stanford University scientists led by Jennifer Dionne, associate professor of materials science and engineering, illuminated a new approach to significantly slow down light. The scientists arranged ultra-thin silicon chips into nanoscale bars (materials where at least one dimension is on the order of nanometers) in order to trap and redirect light at their will.³ This material is a type of “high-Q” resonator–one with a very low rate of energy loss.²

Manufacturing this device involved a number of meticulous and challenging steps. The central component of the device, the extremely thin layer of silicon, traps light extremely effectively and has low absorption in the near-infrared range of the electromagnetic spectrum. The silicon chip lies atop a thin wafer of sapphire into which a specific nanoantenna pattern is etched using an electron microscope “pen.” This is quite challenging, for the pattern must be drawn as smooth as possible. These nanoantennas act as the “walls” that trap and direct the light, and a single minor imperfection could inhibit the light-trapping ability. Pattern design is also important, as it plays a key role in the high-Q nanostructure’s performance. As Professor Dione states, “[the team]  had to find a design that gave good-light trapping performance but was within the realm of existing fabrication methods.” Ultimately, the design of their structure played out very well. Their material achieved Q-factors (the measure of how efficiently a resonator can trap energy) greater than 2,500, which is around 100 times higher than any similar device has achieved, and was very effective at steering light in specific directions.^2,4,5

The device designed by the Stanford team has numerous practical applications. For one, it could be useful for quantum physicists as a way to control entangled photons without the use of large optical devices. It could also be used to improve biosensing (the detection of biomolecules derived from living organisms), for the device may be used to focus light on a molecule multiple times, which increases the chance of detecting it. And of particular relevance right now, Dionne’s lab is working on applying this technique to detect SARS-CoV-2 antigens. The device offers the opportunity to detect single virus particles, and tinkering around with the design of the nanoantennas offers the possibility to detect multiple different types of antibodies simultaneously. In the race to diagnose COVID-19 and halt its spread, the device may prove a very valuable asset.²