An artistic display of a high-Q metasurface beam splitter. These “high-quality factor” as well as “high-Q” resonators could lead to new ways of manipulating and using light. Credit: Riley A. Suhar
Researchers have created ultrathin silicon nanoantennas that drop and divert light, for applications in Quantum Computing, LIDAR and even virus detection.
Light is notoriously fast. Speed is crucial for rapid information exchange, but as light is transmitted through materials, their chances of interaction and exciting atoms and molecules can become very small. If scientists could put the brakes on light particles, like photons, it would open the door to a host of new technology applications.
Now, in a paper published on August 17, 2020, in Nature nanotechnology, Stanford scientists demonstrate a new approach to slow light significantly, just as an echo chamber keeps on sound, and to direct it. Researchers in the lab of Jennifer Dionne, associate professor of materials science and engineering at Stanford, structured ultrathin silicon chips in nanoscale beams to capture light resonant and later release or deflect it. These “high-quality factor” as well as “high-Q” resonators could lead to new ways of manipulating and using light, including new applications for quantum processing, virtual reality, and augmented reality; light-based WiFi; and even the detection of viruses such as SARS-CoV-2.
“We are essentially trying to capture light in a small box through which light can still come in and go from many different directions,” said postdoctoral fellow Mark Lawrence, who is also lead author of the paper. “It’s easy to catch light in a box with many sides, but not as easy as the sides are transparent – as is the case with many Silicon-based applications.”
Make and produce
Before they can manipulate light, the resonators must be fabricated, and this poses a number of challenges.
A central component of the device is an extremely thin layer of silicon, which captures light very efficiently and has a low absorption in the near-infrared, the spectrum of light that scientists want to control. The silicon lies on top of a wafer with transparent material (sapphire, in this case) in which the researchers “pin” an electron microscope to etch their nano-antenna pattern. The pattern should be drawn as smoothly as possible, because these antennas serve as the walls in the echo-chamber analogy, and imperfections hinder the ability of light catcher.
“High-Q resonance requires the creation of very smooth sidewalls that do not let the light out,” said Dionne, who is also Senior Associate Vice Provost of research platforms / shared facilities. “That can be reasonably routine achieved with larger micron-scale structures, but is very challenging with nanostructures that scatter more light.”
Pattern design plays an important role in creating the high Q nanostructures. “On a computer, I can draw ultra-smooth lines and blocks of any given geometry, but manufacturing is limited,” Lawrence said. “Ultimately, we had to find a design that gave good-light trapping performance, but was within the realm of existing manufacturing methods.”
High quality applications (factor)
Thinking with the design has resulted in what Dionne and Lawrence describe as an important platform technology with various practical applications.
The devices demonstrate so-called quality factors up to 2500, which is two orders of magnitude (or 100 times) higher than what similar devices have previously achieved. Quality factors are a measure that describes resonant behavior, which in this case is proportional to the lifetime of the light. “By achieving quality factors in the thousands, we are already in a nice sweet spot from some very exciting technological applications,” said Dionne.
For example, biosensing. A single biomolecule is so small that it is essentially invisible. But transmitting light over a molecule hundreds or thousands of times can greatly increase the chance of producing a detectable scattering effect.
Dionne’s lab is working on applying this technique for detection COVID-19 antigens – molecules that trigger an immune response – and antibodies – proteins produced by the immune system in response. “Our technology would give an optical reading, as doctors and clinicians are accustomed to seeing,” Dionne said. “But we have the chance to detect a single virus or very low concentrations of a multitude of antibodies because of the strong interactions of light molecules.” The design of the high-Q nanoresonators also allows each antenna to function independently to detect different types of antibodies simultaneously.
Although the pandemic stimulated her interest in viral detection, Dionne is also excited about other applications, such as LIDAR – or Light Detection and Ranging, which is laser-based distance measurement technology commonly used in self-driving cars – that this new technology could contribute to. “A few years ago, I could not imagine the immense application space that this work would provide,” Dionne said. “For me, this project has reinforced the importance of fundamental research – you can not always predict where fundamental science is going or where it will lead, but it can provide critical solutions to future challenges.”
This innovation could also be useful in quantum science. For example, splitting photons to create familiar photons that remain connected at a quantum level, even when far apart, would require typical large-scale optical experiments at the table with large expensive precisely polished crystals. “If we can do that, but use our nanostructures to control and shape that scattered light, maybe one day we will have an entanglement generator that you can hold in your hand,” Lawrence said. “With our results, we are excited to look at the new science that is now being realized, but we are also trying to push the boundaries of what is possible.”
Reference: “Metaasurfaces for Factor Phase Gradient Gradients” by Mark Lawrence, David R. Barton III, Jefferson Dixon, Jung-Hwan Song, Jorik van de Groep, Mark L. Brongersma and Jennifer A. Dionne, 17 August 2020, Nature nanotechnology.
DOI: 10.1038 / s41565-020-0754-x
Additional Stanford co-authors include graduate students David Russell Barton III and Jefferson Dixon, research fellow Jung-Hwan Song, former research scientist Jorik van de Groep, and Mark Brongersma, professor of materials science and engineering. This work was funded by the DOE-EFRC, “Photonics at Thermodynamic Limits” as well as by the AFOSR. Jen is also a courtesy co-author of Radiology and a member of the Wu Tsai Neurosciences Institute and Bio-X.
