On-Chip Metrology Links Nobel Prize-Winning Optical Techniques

A team from Delft University of Technology has developed on-chip technology to measure distances in materials at high precision — for example, underwater or for medical imaging. The technology relies on sound vibrations, and is useful for high-precision position measurements in opaque materials. Further, the instrument could lead to new techniques to monitor Earth’s climate and human health.

The researchers combined optical trapping and frequency combs — both Nobel Prize-winning techniques — to build their microchip, which consists mainly of a thin ceramic sheet shaped like a trampoline. The sheet is patterned with holes to enhance its interaction with lasers, and it has a thickness about 1000× smaller than the thickness of a human hair.

Upon contact with a laser beam, the surface vibrates intensely. By measuring the reflected laser light from the vibrating surface, the researchers noticed a pattern of vibrations in the shape of a comb that they hadn’t seen before.

The comb-like signature, the team concluded, functions as a ruler for precision measurements of distance.

So far, the researchers said in their paper, the main challenge to frequency comb operation has been strict requirements on drive frequencies and power. The researchers demonstrated a mechanism to create a frequency comb consisting of mechanical overtones of a single eigenfrequency. This was achieved by monolithically integrating a suspended dielectric membrane with a counter-propagating optical trap.

What distinguishes the technology is that it doesn’t require precision hardware and can be easily produced. The method uses only a milliwatt continuous-wave laser beam.

Artists’ impression of the trampoline-shaped sensor. The laser beam passes through the middle of the trampoline membrane creating the overtone vibrations inside the material. Courtesy of Sciencebrush.

“There’s no need for complex feedback loops or for tuning certain parameters to get our tech to operate properly. This makes it a very simple and low-power technology that is much easier to miniaturize on a microchip,” said research team leader Richard Norte, an assistant professor at TU Delft. “Once this happens, we could really put these microchip sensors anywhere, given their small size.”

The overtone vibrations in the trampoline membranes that resulted from contact with a laser beam, Norte said, can be compared to particular notes of a violin. The note or frequency produced by the violin depends on the finger placement. Touching the string very lightly and playing the note with a bow can produce an overtone, a series of notes at higher frequencies.

“In our case, the laser acts as both the soft touch and the bow to induce overtone vibrations in the trampoline membrane,” Norte said.

Optical frequency combs, which won a 2005 Nobel Prize, see use in labs around the world for very precise measurements of time and to measure distances.

“We have made an acoustic version of a frequency comb, made out of sound vibrations in the membrane instead of light,” Norte said.

Acoustic frequency combs could, for instance, make position measurements in opaque materials, which propagate acoustic vibrations better than lightwaves.

“The interesting thing is that both of these concepts are typically related to light, but these fields do not have any real overlap,” Norte said. “We have uniquely combined them to create an easy-to-use microchip technology based on sound waves. This ease of use could have significant implications for how we measure the world around us.”

The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-36953-8).