Here's what micro-resonators in home gadgets can teach you about your health

Smart gadgets in the home might soon be able to tell you what’s wrong with you when you’re feeling unwell. What if this gadget could quickly tell you whether you have COVID, or the flu or just a cold? Or maybe it would even pick up that you have diabetes without knowing it. Such a device could figure all this out without you having to go to a doctor or a laboratory.

This technology could become a reality within a few years, thanks to electrical engineers using a key component called the whispering-gallery-mode microresonator. Such a microresonator is about 100 times better than what was available before for the longwave infrared spectrum. New technology is providing better optical sensors that are important for electronics, including devices that analyze chemicals using light.

“We’ve built the lowest loss-whispering gallery mode microresonator out there for the longwave infrared spectrum,” said Dingding Ren, a researcher at the Norwegian University of Science and Technology’s (NTNU) department of electronic systems. “Because the longwave infrared spectrum provides definitive information about chemicals, it provides [a] new possibility for sensing applications.”

He and his team published their research in the peer-reviewed, open-access journal Nature Communications under the title “High-quality microresonators in the longwave infrared based on native germanium.”

Microresonators, which a type of optical cavities, can store a high optical field inside a very small volume. They can be made into a trace or disk geometry, but they usually are at a micro-scale dimension, similar to the thickness of a hair. Light travels inside the microresonator in circles, so the optical field gets amplified.

The competition is fierce in this field,” Ren said.

The new microresonator is made using the element germanium. The material may sound exotic, but it was used in the world’s first transistor as early as 1947, before silicon took over that market. Today, germanium is frequently used in optical lenses in sensors and infrared cameras, so it is neither rare nor expensive.

Health technology for good

Ren’s microresonator, which can store light for certain wavelengths much longer in the resonance, can retain the light 100 times longer than previous versions. It amplifies the optical field inside and makes nonlinear processes much easier, such as frequency comb generation, he said.

Ren has worked closely with Prof. David Burghoff and his colleagues at the University of Notre Dame in Indiana.

Storing light waves in the infrared part of the light spectrum more effectively is good news for several types of new technologies, especially for particle sensing and spectroscopic chemical identification that analyze a gas/fluid sample to check for viruses and bacteria and other pathogens you might have. The new microresonator means that scientists can develop broadband frequency combs in the longwave infrared spectrum using these devices.

Frequency combs are laser lights whose spectrum consists of a series of discrete, equally spaced frequency lines. These can be found in various places, such as in your GPS, in atomic clocks and in fiber-optic equipment used in telephones and computers. The technology also opens the door to analyzing several chemicals at once if a broadband frequency comb is available at the longwave infrared spectrum.

“The technology is still in its initial stage when it comes to measurements in the longwave infrared spectrum of light,” Ren said. “But our improvement gives us the possibility to identify several different chemicals in real time in the near future.”

This kind of spectroscopic machine already exists, such as something called a Fourier-transform infrared interferometer, which is so big and so expensive that only hospitals and big-budget institutions can afford them. Other, slightly simpler machines might be able to analyze a few chemicals, but not many at once – unlike what the new technology could make possible.

“We can compare the microresonator to what happens with the sound in the whispering gallery in St. Paul’s Cathedral in London,” Ren said. This elliptical gallery has produced a famous phenomenon. You can whisper at one end of it, and people at the other end of the room can hear you, even though they wouldn’t normally be able to hear you at that distance. The sound waves are amplified by the shape of the room and the walls, which is how light waves behave in the microresonator.

Prof. Bjørn-Ove Fimland and Prof. Astrid Aksnes, who are both at NTNU’s department of electronic systems, have provided advice along the way.

“Ren has done excellent work, which is supported by the fact that he’s had an article published in Nature Communications,” Aksnes said.

“The fact that we can now measure in the longwave IR range [eight to 14 micrometers] of the light spectrum opens up many possibilities in relation to use in imaging and detection, environmental monitoring and biomedical applications, she added.