Anyone reading this article from a phone connected to a high-speed network can thank an optical physicist. Research on how to convert electrical energy into light energy (and vice versa) has led to a number of innovations, including LEDs, solar cells, and telecommunication devices. But as the field of optics continues to push boundaries, there are still a number of unsolved problems, especially when it comes to areas like energy efficiency and signal control.
Bo Zhen and his research group are setting out to address a number of the unresolved questions. As an assistant professor in the Physics & Astronomy Department of the School of Arts and Sciences, Zhen’s research is focused on how to improve optoelectronic devices by first asking fundamental physics questions. “Those kinds of questions are usually interesting ones,” says Zhen. “You find something that’s very deep on the fundamental side and gives you performances of devices that are drastically better.”
One of his group’s research areas is nanophotonics: devices with features at the nanometer-scale that imbue them with incredible properties, like making mirrors out of glass and trapping individual light particles, known as photons. “If you have structures where a feature size is comparable to, or even smaller than, the wavelength of light, then you have interesting ways to control light,” explains Zhen.
Part of his work is focused on new ways to reduce energy waste in optical communication systems. A prime example is a server room, with numerous rows of powerful supercomputers stacked on top of one another that rely on optical fibers for communication. An electronic signal from a chip on one server is converted into a light signal, which is carried by an optical fiber through a network of interconnected junctions until the light reaches its destination and is converted back into an electrical signal by the receiving chip.
But because of how light travels, and how chips are designed, not all of the light that is transmitted can be captured. “Whenever light travels through materials, or more critically, interfaces, some part is transmitted and some part is reflected,” explains first-year graduate student Valerie Yoshioka. In server rooms, this translates to a minimum of 25 percent of the energy being wasted at each junction, with a typical server room having multiple junctions that add up to even more energy being lost in total.
Building off his previous research experience, and armed with insights from topological physics, Zhen showed that a particular type of photonic crystal slabs, materials with holes patterned on the surface at the same intervals as the wavelength of light, can radiate light in a single direction without having reflecting surfaces on the other side. In physics, this phenomenon of “unidirectional bound states in the continuum” has been long sought after, with Zhen’s group demonstrating it for the first time. These results can help engineers create new devices that have energy efficiency up to 99 percent, which are relatively easy to make since the devices are compatible with existing fabrication platforms.
Other members of the Zhen lab, including Yoshioka and post-doc Li He, are also working on fundamental questions in photonics and topological physics that have direct applications to optoelectronics. Yoshioka’s project will explore the use of atomically-thin quantum materials in optical isolators, which are devices that force light to travel in a single direction. These types of unidirectional devices currently exist, but many are too bulky or require magnetic fields, so it is difficult to directly integrate them onto electronic chips. Yoshioka’s project could lead to structures that work like traffic networks, with a system of one-way roads and roundabouts that can control the “flow” of photons.
Li He was attracted to Zhen’s lab so he could work on photonics and topological physics, where his research is currently focused on finding new theoretical approaches for the field. Half of his time is spent fabricating and characterizing new materials and the other half developing theoretical strategies in close collaboration with Zhen.
Both He and Yoshioka were attracted to Penn because of its condensed matter research program and the proximity of two of the “founding fathers” of topological insulators. “It’s really helpful to have them around. When you have questions, you can always go to their office and ask them,” says He.
As Zhen’s group continues to attract new talent, the lab’s physical space is also starting to take shape. Flanked by two tables of devices for measuring transmission, reflectance, and photoluminescence of the materials created for them by the Singh Center, a 30-feet-long optics table currently sits empty at the center of the lab. It’s a blank canvas for new equipment and tools that will help bring more of the group’s ideas to life in the coming months.
In the future, Zhen says that his research in optics will naturally have strong connections to engineering and that having a mixture of researchers from these two fields will likely be an ongoing theme of his group. But, regardless of what specific projects his group members work on, Zhen’s hope is that each researcher will find a balance between delving deep into fundamental research or working on things that can be applied in the near-term in a way that inspires them.
“Graduate study is always about finding interesting physical phenomenon and trying to explain it. In my later stages of study, and also as a postdoc, I started to appreciate the engineering side where you look at devices and appreciate the beauty of industry, and you know that there’s a lot of important questions to be answered,” says Zhen.
Part of this research was supported by the Army Research Office Early Career Award for Scientists and Engineers (ECASE) program and the Air Force Office for Scientific Research Young Investigator Program.
Bo Zhen is an assistant professor in the Department of Physics & Astronomy in the School of Arts and Sciences at the University of Pennsylvania.