Plasmonic Interferometry Could Enable Compact Bio-, Environmental Sensors

A nanoscale plasmonic interferometry system that eliminates the need for an external coherent light source could enable accurate and compact bio- and environmental sensors.

In the device, fluorescent light-emitting atoms were integrated directly within a tiny hole in the center of an interferometer. An external light source was still necessary to excite the internal emitters, but it didn’t have to be a specialized coherent source. The system was developed in the lab of professor Domenico Pacifici at Brown University’s School of Engineering.

Plasmonic interferometers make use of the interaction between light and surface plasmon polaritons, density waves created when light energy rattles free electrons in a metal.

Plasmonic interferometers containing light emitters are being explored for compact biosensors. Courtesy of the Pacifici Lab/Brown University.

When light from an external source is shone onto the surface of an interferometer, some of the photons travel through a central hole, while others are scattered. The scattered photons generate surface plasmons that propagate through the metal inward toward the hole, where they interact with photons passing through the hole. That creates an interference pattern in the light emitted from the hole, which can be recorded by a detector beneath the metal surface.

When a liquid is deposited on top of an interferometer, the light and the surface plasmons propagate through that liquid before they interfere with each other, altering the interference patterns picked up by the detector depending on the chemical makeup of the liquid or compounds present in it. By using different sizes of groove rings around the hole, interferometers can be tuned to detect the signature of specific compounds or molecules. With the ability to put many differently tuned interferometers on one chip, engineers can hypothetically make a versatile detector.

Up to now, all plasmonic interferometers have required the use of highly specialized external light sources that can deliver coherent light, the researchers said. Without coherent light sources, the interferometers can’t produce usable interference patterns. Coherent light sources tend to be bulky, expensive, and require careful alignment and periodic recalibration to obtain a reliable optical response.

In the Brown team’s device, incoherent light was shone on the interferometer, causing the fluorescent atoms inside the center hole to generate surface plasmons. Those plasmons propagated outward from the hole, bounced off the groove rings and propagated back toward the hole after. Once a plasmon propagated back, it interacted with the atom that released it, causing an interference with the directly transmitted photon. Because the emission of a photon and the generation of a plasmon were indistinguishable — alternative paths originating from the same emitter — the process is naturally coherent. Interference can therefore occur even though the emitters were excited incoherently, the researchers said.

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

In addition to eliminating the need for specialized external light sources, the approach has several advantages, Pacifici said. Because the surface plasmons traveled out from the hole and back again, they probed the sample on top of the interferometer surface twice, making the device more sensitive.

Additionally, external light can be projected from underneath the metal surface containing the interferometers instead of from above, which eliminates the need for complex illumination architectures on top of the sensing surface, which could make for easier integration into compact devices.

The embedded light emitters also eliminated the need to control the amount of sample liquid deposited on the interferometer's surface. Large droplets of liquid can cause lensing effects, a bending of light that can scramble the results from the interferometer. Most plasmonic sensors make use of tiny microfluidic channels to deliver a thin film of liquid to avoid lensing problems. But with internal light emitters excited from the bottom surface, the external light never comes in contact with the sample, so lensing effects are negated, as is the need for microfluidics, the researchers said.

The internal emitters also produced a low intensity light, useful for probing delicate samples such as proteins than can be damaged by high-intensity light.

Pacifici and his team plan to continue to refine the idea. The next step will be to try eliminating the external light source altogether. It might be possible, the researchers said, to eventually excite the internal emitters using tiny fiber optic lines or perhaps electric current.

While still at the proof-of-concept stage, the team envisions the technique could be used to build hand-held environmental sensors that can instantly test water for lead, E. coli and pesticides all at the same time, or biosensors that can perform a complete blood workup from just a single drop.

The research was published in Nature Scientific Reports (doi: 10.1038/srep20836).