High-quality microresonators in the longwave infrared based on native germanium

What if there were a gadget that could within short order check whether you have COVID or the flu—or maybe it would even pick up that you have diabetes without knowing it? The 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, and electrical engineers are some of people who make it possible to create such gadgets, which contain a key component called the whispering gallery mode microresonator.

New technology is providing better optical sensors, which 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. Because the longwave infrared spectrum provides definitive information about chemicals, it provides new possibility for sensing applications," says Dingding Ren, a researcher at NTNU's Department of Electronic Systems.

Ren and his colleagues have developed a new whispering gallery mode microresonator—which can store light for certain wavelengths much longer in the resonance.

"Our microresonator is about 100 times better than what was available before for the longwave infrared spectrum," says Ren.

"It can retain the light 100 times longer than previous versions, which amplifies the optical field inside and makes nonlinear processes much easier, such as frequency comb generation," he said.

Ren and his colleagues have contributed to developing a whole set of nanofabrication processes to make the microresonators. Their results were recently presented in Nature Communications.

Opens up great opportunities

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, which analyze a gas or fluid sample to check for viruses and bacteria and other nasties you might have.

The new microresonator means that scientists can develop broadband frequency combs in the longwave infrared spectrum using these devices. And just what might those be?

Frequency combs are laser lights whose spectrum consists of a series of discrete, equally spaced frequency lines. These can be found 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. But our improvement gives us the possibility to identify several different chemicals in real time in the near future," Ren said.

This kind of spectroscopic machine already exists, but they are 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.

Ren has worked closely with Professor David Burghoff and his colleagues at the University of Notre Dame in the U.S..

"The competition in this field is fierce," says Ren.

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, and it is consequently neither particularly rare nor expensive. These are also advantages when theory is going to be brought to the market.

What are microresonators anyway?

Microresonators, which are a type of optical cavity, can store optical fields inside a very small volume. They can be made in a race track or disk geometry, but they usually are microscale in size, similar to the thickness of a hair. Light travels inside the microresonator in circles, so the optical field gets amplified.

"We can compare the microresonator to what happens with the sound in the whispering gallery in St. Paul's Cathedral in London," says Ren.

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 to able 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.

"We promised that we would develop a better microresonator, and we've succeeded," Ren said.

Bjørn-Ove Fimland and Astrid Aksnes, both professors in NTNU's Department of Electronic Systems, have provided advice along the way.

"The fact that we can now measure in the longwave IR range (8-14 µm, or micrometers) of the light spectrum opens up many possibilities in relation to use in imaging and detection, environmental monitoring and biomedical applications," says Aksnes.

"Many molecules have fundamental vibrational bands in the mid-wave IR range (2-20 µm), the so-called 'molecular fingerprint region.' By measuring in this wave range, we achieve higher sensitivity," she says.