Since 2012, IEEE Spectrum has been covering Domenico Pacifici at Brown University as he works to improve the capabilities of plasmonic interferometers. One major application would be glucose monitors that enable diabetics to check glucose levels through saliva instead of blood—no finger pricking necessary.

In his latest research, Pacifici and his team have developed a way to get a plasmonic interferometer to take measurements without the need for a coherent light source.

To have a coherent light, the light waves have to run in parallel, possess the same wavelength, and travel in-phase, which means the peaks and valleys of light waves are in alignment. Producing this kind of light demands expensive and bulky equipment. By eliminating that need, Pacifici’s team has created a far smaller and less expensive way to operate these devices.

“It has always been assumed that coherent light was necessary for plasmonic interferometry,” said Pacifici, in the press release. “But we were able to disprove that assumption.”

In research published in the Nature journal Scientific Reports, Pacifici and his team embedded light emitters in the form of fluorescent atoms directly into the sub-wavelength cavities of the plasmonic interferometers. The result was that even when the source light had very low coherence, the internal emitters could get the interferometer to operate as though the light was coming from a coherent light source.

Plasmonic interferometers operate on the same principles of plasmonics as every other plasmonic device does. When photons of light hit a metal surface, they rattle the electrons in the metal so much that they generate waves of electrons known as surface plasmons.

A plasmonic interferometer exploits this phenomenon by its very architecture, essentially a piece of metal that has a hole—or cavity—at its center and around that hole is carved co-centric grooves. The cavity is around 300 nanometers in diameter and the co-centric grooves are measured in microns.

When the light hits the surface of this device, some of the photons go into the cavity at the center of it while others hit the outer grooves and scatter. The scattered photons excite the electrons on the metal surface to the point where they become waves of surface plasmons. Just like waves on water, the waves move along the surface of the metal until they go into cavity at the center. Here they interfere with the photons that were originally drawn into the cavity generating an interference pattern: you can measure how the light weakens and strengthens coming out of the cavity.

By embedding fluorescent atoms in the cavity of the device, Pacifici’s team made the cavity produce its own surface plasmons. In this way, surface plasmons move out of the cavity onto the surface and then bounce off the co-centric grooves and back into the cavity. When these surface plasmons come in contact with the fluorescent atoms that were its source, it creates an interference with the directly transmitted photon. In this arrangement, the photons in the cavity and the plasmons are coming from the same emitter, so they are naturally coherent and interference occurs even though the emitters (the fluorescent atoms) are excited by incoherent light.

“The important thing here is that this is a self-interference process,” Pacifici said in the release. “It doesn’t matter that you’re using incoherent light to excite the emitters, you still get a coherent process.”

In addition to being able to use incoherent light sources, the architecture provides additional benefits, such as greater accuracy and the internal emitters mean that more delicate samples can be tested.

While this work is really just a test of concept, Pacifici believes that this is such a fundamentally different way for these devices to operate that it represents a significant breakthrough in the field.

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Restoring Hearing With Beams of Light

Gene therapy and optoelectronics could radically upgrade hearing for millions of people

13 min read
A computer graphic shows a gray structure that’s curled like a snail’s shell. A big purple line runs through it. Many clusters of smaller red lines are scattered throughout the curled structure.

Human hearing depends on the cochlea, a snail-shaped structure in the inner ear. A new kind of cochlear implant for people with disabling hearing loss would use beams of light to stimulate the cochlear nerve.

Lakshay Khurana and Daniel Keppeler

There’s a popular misconception that cochlear implants restore natural hearing. In fact, these marvels of engineering give people a new kind of “electric hearing” that they must learn how to use.

Natural hearing results from vibrations hitting tiny structures called hair cells within the cochlea in the inner ear. A cochlear implant bypasses the damaged or dysfunctional parts of the ear and uses electrodes to directly stimulate the cochlear nerve, which sends signals to the brain. When my hearing-impaired patients have their cochlear implants turned on for the first time, they often report that voices sound flat and robotic and that background noises blur together and drown out voices. Although users can have many sessions with technicians to “tune” and adjust their implants’ settings to make sounds more pleasant and helpful, there’s a limit to what can be achieved with today’s technology.

8 channels

64 channels

Since optogenetic therapies are just beginning to be tested in clinical trials, there’s still some uncertainty about how best to make the technique work in humans. We’re still thinking about how to get the viral vector to deliver the necessary genes to the correct neurons in the cochlea. The viral vector we’ve used in experiments thus far, an adeno-associated virus, is a harmless virus that has already been approved for use in several gene therapies, and we’re using some genetic tricks and local administration to target cochlear neurons specifically. We’ve already begun gathering data about the stability of the optogenetically altered cells and whether they’ll need repeated injections of the channelrhodopsin genes to stay responsive to light.

Our roadmap to clinical trials is very ambitious. We’re working now to finalize and freeze the design of the device, and we have ongoing preclinical studies in animals to check for phototoxicity and prove the efficacy of the basic idea. We aim to begin our first-in-human study in 2026, in which we’ll find the safest dose for the gene therapy. We hope to launch a large phase 3 clinical trial in 2028 to collect data that we’ll use in submitting the device for regulatory approval, which we could win in the early 2030s.

We foresee a future in which beams of light can bring rich soundscapes to people with profound hearing loss or deafness. We hope that the optical cochlear implant will enable them to pick out voices in a busy meeting, appreciate the subtleties of their favorite songs, and take in the full spectrum of sound—from trilling birdsongs to booming bass notes. We think this technology has the potential to illuminate their auditory worlds.

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