Lan Yang Investigates How to Bend Two Laws of Physics

The concepts will introduce new photonics applications

24 October 2016

IEEE Senior Member Lan Yang’s project of bending two laws of physics that currently govern photonic systems recently received a US $2 million grant from the U.S. National Science Foundation, along with interest from industry. The light-wave-propagation research project focuses on the physical principles of time-reversal symmetry and reciprocity.

In an interview with The Source, Yang compared time-reversal symmetry to running a film from beginning to end and then rewinding it from the end to the beginning. In fundamental physics terms, such reciprocity as it applies to most types of materials and systems means an electromagnetic or acoustic wave has to travel the same way in both the forward and reverse directions. For example, in an optical fiber used for long-haul communication, light can propagate in both forward and backward directions.

But Yang is out to break that symmetry. “Exciting applications and technologies can emerge when such symmetries are violated,” she says. “We will achieve novel photonic devices with asymmetric and nonreciprocal light transport, including nanoscale optical diodes and unconventional lasers.” Several companies have expressed interest in Yang’s project, including Intel, Mediomics, and OEwaves.

As the principal investigator, Yang, professor of electrical and systems engineering at Washington University in St. Louis, is leading a team of researchers from Stanford, the University of Minnesota, the University of Wisconsin, and Wesleyan University.

It is not the first time Yang’s work has been recognized. She received the NSF Career Award in 2010, and was honored a year later by President Obama with an early career award. The Presidential Early Career Awards for Scientists and Engineers are the highest honors bestowed by the U.S. government on science and engineering professionals who are in the early stages of their independent research careers.

“Our project will investigate time and space symmetries in multi-length-scale photonic systems and explore the exciting applications and technologies that emerge when such symmetries are violated,” says Yang, who is a Fellow of the Optical Society and a member of the IEEE Photonics Society.

The Institute asked her a few questions about her project.

What’s the idea behind bending these laws?

When we talk about time-reversal symmetry, we think of fiber. In optical fiber, light can propagate in two directions: forward and backward. Today’s reliance on systems that are reciprocal, or that have time-reversal symmetry, limits the ways in which light, and high-frequency radio and sound waves, function. But imagine if the structure could be designed to allow the wave to pass more easily in one direction but not the other. The idea of the project is to take waves and make them do things that were seemed physically impossible, such as bending light, radio waves, or sound around an object, guiding them along a path, or completely absorbing them. To do that we have to break the old laws.

There are a number of structures that are nonreciprocal in electronic circuits and materials science, for example. The widely used electric diode is asymmetric because current flows in only one direction but is blocked in the other.

An application like, say, high-speed computing would prefer to use photons because they propagate much faster than electrons. Devices achieve nonreciprocal behaviors when strong external magnetic fields are applied. But the fields hinder integration of small devices such as silicon chips. By breaking the reciprocity, we can introduce the effect of a magnetic field without magnetic materials. That would simplify the silicon chip fabrication and cut costs.

What could benefit from your project?

In addition to photonic applications, our discoveries could also enable new technologies that will help with environmental monitoring.

For example, air-quality sensors that detect sub-micron-sized, 2.5-micrometer particles are used to check air-pollution levels. But according to recent reports, airborne ultrafine particles, with diameters in the range of 100 nanometers, have been found to be more damaging to humans. The human body’s internal filtering system can block micron-sized particles, but smaller ultrafine particles pass through and enter the bloodstream. These particles can even “parachute” into the human brain and potentially might impact and accelerate diseases, like Alzheimer’s.

By introducing the nonreciprocal light transport concept along with an exception-point concept, we could develop a new sensor that could detect and count nanoparticles, at sizes as small as 10 nanometers, one at a time.

An exception point arises in physical fields when two complex eigenvalues and their eigenvectors coalesce, or become the same. These are mathematical tools that describe a physical system. This physical phenomenon forces photons to go randomly either clockwise or counterclockwise instead of both directions. If you operate a sensor around an exception point, and particles the size of 20 nanometers enter it, the sensor will quickly break the barrier of the equilibrium of the state and the system will drift away from the exception point.

Describe one of the unconventional lasers and its use.

Consistency in light propagation is important to get a reliably strong photonic signal and light pulse for all lasing systems and applications. If we can break the symmetry of a photonics structure, we can bias the laser so its light prefers to go in only one direction. We call that the chiral modes, or the preferred, directional lasing emission. We can do this by operating close to exception points—which achieves dynamic control of the rotation direction of light in a whispering-gallery-mode (WGM) resonator. This resonator works similarly to the renowned whispering gallery in London’s St. Paul’s Cathedral, where a person on one side of the dome can hear a message spoken to the wall by a person on the other side. The WGM device does a similar thing with light frequencies rather than sound.

This enabled us to tune the emission direction of a WGM microlaser from bidirectional, when the system is away from the exception points, to unidirectional emission at the exception points and to reverse the emission direction by moving from one exception point to the other. This feature provides WGM resonators new functionalities useful for lasing, sensing, optomechanics, and cavity quantum electrodynamics.

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