February 2, 2023 | Researchers at the Norwegian University of Science and Technology and the University of Notre Dame have developed a new microresonator for biochemical sensing. This is big news, according to Dingding Ren and David Burghoff, co-leaders in joint research between the two institutes.
Within the longwave infrared spectrum, many chemicals have distinct, characteristic patterns of absorbance associated with intramolecular vibrations. As a result, measuring the absorbance in this region allows researchers to reveal a sort of molecular fingerprint for a sample. For example, DNA and RNA absorbance appears in a region located from 823 to 1250 cm-1 (8 to 12 μm), whereas proteins show characteristic absorbance from 1250 to 1667 cm-1 (6 to 8 μm).
There are numerous applications for this kind of optical identification for particle sensing and chemical identification, such as analyzing a gas or fluid sample for the presence of viruses, bacteria, or other indicators of health problems. The optical microresonator Ren and Burghoff described in an article published last October in Nature Communications (DOI: 10.1038/s41467-022-32706-1) represents a key step forward, because their device is roughly 100 times better than existing components at detecting intramolecular rotation and vibration within the “golden spectrum,” the range from 6 to 14 μm, enabling label-free biomedical identification in a small, portable format.
As an example of the power of this technique, a successful early study was able to differentiate brain cancer from control patient blood samples at a sensitivity and specificity of 93.2% and 92.8%, triaging patients to allow their rapid access to imaging (Nature Communications, DOI: 10.1038/s41467-019-12527-5). Other potential applications include at-home detection of viral infections (e.g., colds, flu, and COVID) and in helping to triage serious diseases like diabetes and cancer at very early stages, says Ren, and this could all be a reality in a few short years.
Amplification Architecture
As Ren explains it, a microresonator is a type of optical cavity that is typically of microscale dimension. Light travels inside the cavity in circles, so the optical field gets amplified. The structure miniaturizes a series of bulky optical components and is the premier on-chip mechanism for magnifying an optical field in a small volume, “significantly lowering the threshold for nonlinear processes.”
The promising ongoing development by Ren and the Burghoff group is to demonstrate an on-chip frequency comb (aka “microcomb”) using these microresonators. These microcombs in principle will show broadband and bright coherent emission—a feat previously impossible at longwave infrared range, says Ren.
Frequency combs are laser lights in a series of discrete, equally spaced frequency lines. They’re found in GPS devices and fiber optic equipment used in telephones and computers, but when created with long-wave infrared light, they can enable the simultaneous analysis of several different chemicals on a chip, he explains.
As imagined in future home use, the gadgets would resemble the chip-scale, near-infrared photonic system described in an article last year in Nature (DOI: 10.1038/s41586-022-04579-3). “If we have a broadband frequency comb at home, we will be able to monitor our health daily for early disease detection,” says Ren. Combined with information technologies, data mining, and machine learning, the microresonator could produce hospital-level information from urine or other body fluids.
Competition in the field has been fierce to identify a suitable material for a microresonator that could overcome the one major problem working in the longwave infrared region of the spectrum: reliance on a fast and nondestructive analytical method known as Fourier transform infrared spectroscopy (FTIR) with a very dim longwave infrared light source, he explains. “Although FTIR is the workhorse in hospitals and pharmaceutical industries for chemical identification, it is bulky and slow, not suitable for integration on portable devices.”
Ren and Burghoff resolved the problem by fabricating a microresonator using native germanium (Ge), a material used in the world's first transistor made in the Bell lab in 1947 that marked the beginning of chips, computers, and the information age. Germanium shares several properties with silicon (Si), but it also has several notable advantages, says Ren.
“For longwave photonics, our fabricated Ge device outperforms what Si can perform in theory,” Ren continues. “But due to the similarity, the Ge fabrication process is compatible with the current dominant Si industry, making Ge photonics scalable easily.” These days, germanium is used in the optical lenses of sensors and infrared cameras. It is neither particularly rare nor expensive, he notes.
Ren calls the new device a “whispering gallery mode” microresonator, calling to mind what happens with sound in the Whispering Gallery of London’s St. Paul's Cathedral, where visitors on one end of the room can hear the whispers of people on the other end. The sound waves get amplified by the shape of the room and the walls, analogous to how light waves behave in the microresonator.
Amplification of the optical field within the whispering gallery mode microresonator is important in analyzing chemicals because of the significantly enhanced light retention. By storing light 100 times longer, the threshold for comb generation is effectively reduced by 10,000 times, says Ren. It is the lowest loss whispering gallery mode microresonator ever built for the longwave infrared spectrum.
The research was funded by a Fripro project grant from the Research Council of Norway. Burghoff, assistant professor of electrical engineering at the University of Notre Dame, is one of five fellows named in 2022 by the Gordon and Betty Moore Foundation for making the first broadband frequency comb in longwave infrared a reality.
Ren says that the team is interested in hearing from clinical experts on planning for practical biochemical features for early sensing experiments.