Researchers develop novel way to shrink light to detect ultra-tiny substances

Researchers at UC Berkeley have created light-based technology that can detect biological substances with a molecular mass more than two orders of magnitude smaller than previously possible.

In a paper published this week in Nature Physics, Boubacar Kanté, associate professor of electrical engineering and computer sciences and faculty scientist at Lawrence Berkeley National Laboratory, describes a method to build a device that shrinks light while exploiting mathematical singularities known as exceptional points (EP).

The research could lead to the development of ultra-sensitive devices that can quickly detect pathogens in human blood and considerably reduce the time needed for patients to get results from blood tests.

“Our goal is to overcome the fundamental limitations of optical devices and uncover new physical principles that can enable what was previously thought impossible or very challenging,” Kanté said. “What I’m really excited about is the ability to implement such singularities at such a small scale. The results are both fundamentally exciting and practically important.”

Graduate student Junhee Park and post-doctoral researcher Abdoulaye Ndao led the work.

The wavelength of light is much larger than the size of most biologically relevant substances. For light to strongly interact with these small substances, its wavelength must be reduced.

Kanté and his group used plasmons, very small fluids of electronic waves that can move back and forth in metallic nanostructures.

The group placed two plasmonic nanoantenna arrays on top of each other with each array producing plasmon resonances that control light waves of a certain frequency. The researchers then “coupled” the nanoantenna arrays, pushing the two waves to come together until they finally resonated at the same frequency and, most critically, lost energy at the same rate — a  moment known as the exceptional point. This marked the first time researchers have used EPs for plasmons.

When an external substance comes into contact with the EP and disturbs the synchronized rates of lost energy, the device detects the substance with higher sensitivity.

The device detected anti-Immunoglobulin G in blood, the most common antibody in human blood to fight infection, at a molecular weight 267 times lighter than in previous reports using plasmonic arrays.

Adding additional plasmonic arrays to the original device could also further boost the sensitivity at the EP, Kanté said.

The National Science Foundation and the Office of Naval Research supported this research. The Defense Advanced Research Projects Agency and the Department of Energy also supported the work.