What are metamaterials and why do we need them?
A field of materials science known as metamaterials has recently captured the imagination of engineers hoping to create nanoscale optical devices. Jonathan Fan, an assistant professor of electrical engineering and director of the ExFab at the Stanford Nanofabrication Facility, is leading the way. He recently won the prestigious 2016 Packard Fellowship in Science and Engineering, which funds the most promising early-career professors in fields ranging from physics and chemistry to engineering. Fan is just the fourth Stanford electrical engineer to win the fellowship since 1988, and the financial support that comes with it will enable him to carry on work that is so innovative that it can otherwise prove difficult to fund through traditional means. We talked to Fan about his visions in metamaterial engineering and about his interdisciplinary collaborations with fellow Stanford professors Allison Okamura and Sean Follmer in projects such as integrating new types of electromagnetic systems with robots.
What are metamaterials?
At its most basic level, we are bringing the idea of an antenna down to the nanoscale. Back in the day before cable and satellite, TVs had metal antennas. If your picture wasn’t very good, you would get up and physically reconfigure the antenna geometry to change its performance. Those antennas were designed for radio waves that were centimeters to meters in length. We are working to create nanoscale antennas that would be able to respond to visible light with wavelengths of 400 to 700 nanometers, or infrared light, where wavelengths are on the order of a micron. By configuring the geometry of these antennas individually and in collections, we can engineer systems that can interact with and manipulate light in entirely new ways.
These tiny antennas are many orders of magnitude smaller than a TV antenna. Fortunately, the development of the modern electronic integrated circuit platform over the last half-century has produced mature technological processes that can help us define nanoscale features. We use those same patterning technologies to make these nanoscale antennas. That’s the very basic overview.
What is the derivation of the term “meta” in the name metamaterials?
When you think of a conventional lens, you think of glass – the material, right? The glass in your camera or your eyeglasses bends light in very predictable ways based on the intrinsic material response of glass. A lens made of a metamaterial will respond to light in ways that are no longer solely based on the properties of the material itself, but largely on the design and layout of these optical antennas. So the concept of “meta” comes from our ability to engineer artificial materials, consisting of a composite of nanoscale structures, which can respond to light in entirely new ways. It’s kind of neat to see an example in the case of a metal like gold. We usually think of gold as a bulk material that is reflective, yellowish and shiny. Even when you go down to the nanoscale, gold is still gold. But by specifying the geometry of nanoscale gold, we can change the color of gold from yellow to green or red, and it can support many other types of optical properties that we don’t associate with bulk gold. Those are properties engineers can use to make new devices.
What do metamaterials allow us to do that we couldn’t before?
Metamaterials are promising for a couple reasons. First, they enable the extreme miniaturization of existing optical devices. For example, we can take an eyeglass lens and we can make it 100 times thinner than a strand of hair. This allows us to translate traditionally bulky optical systems to extremely small form factors. Second, they can be customized to support novel properties that currently are not accessible with existing optical hardware, leading to entirely new optical systems.
What’s an example of a potential metamaterial device?
A major opportunity today arises from the fact that high-resolution cameras have miniaturized to sizes that can fit onto cellphones, making them accessible to audiences a million times larger than before. Part of my larger research question is: Is there something more we can do with imaging systems with form factors of a cellphone camera? There is so much information in the incoming light field that is not currently captured by a cellphone camera, but that could be captured with imaging systems that include metamaterials. Access to this additional information could change how we use the images we take. For example, if you have a skin condition, a great deal more optical information of the skin could be extracted from a simple cellphone image and used to better assess your condition.
What excites you about metamaterials?
Metamaterials lead us to a completely different set of questions – metaquestions, if you will. For instance, are these nanoantennas even the best way to go about doing what we want to do? At this point in time, even that’s not clear. In addition, you get to the big questions of applications for these materials and devices. It’s just wide open. That’s why this is exciting to me.
Any early impressions to share as a new faculty member?
Stanford is a really special place. The people are top-notch and the environment is highly collaborative, not siloed. As an example, I have recently expanded into robotics, where I have been looking to apply concepts in radio frequency waves to create smarter soft robotic systems. In this effort, I’ve started a collaboration with Allison Okamura and Sean Follmer, who are mechanical engineers. It’s been fantastic so far, and I’ve been learning so much. People here are very open-minded and are inspired to do exciting interdisciplinary research to identify and solve big problems. I’m thrilled to be a part of that.
