Physicists at Delft University of Techno­logy have built a new techno­logy on a microchip which could measure distances in materials at high precision, for example underwater or for medical imaging. Because the technology uses sound vibrations instead of light, it is useful for high-precision position measure­ments in opaque materials. The instrument could lead to new techniques to monitor the Earth’s climate and human health.

The microchip mainly consists of a thin ceramic sheet that is shaped like a trampoline. This trampoline is patterned with holes to enhance its inter­action with lasers and has a thickness of 80 nanometers. As a former PhD candidate in Richard Norte’s lab, Matthijs de Jong studied the small trampolines to figure out what would happen if they pointed a simple laser beam at them. The trampo­line’s surface started vibrating intensely. By measuring the reflected laser light from the vibra­ting surface, the team noticed a pattern of vibrations in the shape of a comb that they hadn’t seen before. They realised that the trampoline’s comb-like signature functions as a ruler for precision measure­ments of distance.

This new technology could be used to measure positions in materials using sound waves. What makes it special is that it doesn’t need any precision hardware and is therefore easy to produce. “It only requires inserting a laser, and nothing else. 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”, Norte says. “Once this happens, we could really put these microchip sensors anywhere, given their small size.”

The new techno­logy is based on optical trapping and frequency combs. “The interesting thing is that both of these concepts are typically related to light, but these fields do not have any real overlap. 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,” Norte said. When the researchers pointed a laser beam at the tiny trampoline, they realised that the forces that the laser exerted on it were creating overtone vibrations in the trampoline membranes.

“These forces are called an optical trap, because they can trap particles in one spot using light. This technique won the Nobel Prize in 2018 and it allows us to mani­pulate even the smallest particles with extreme precision”, Norte explains. “You can compare the overtones in the trampoline to particular notes of a violin. The note or frequency that the violin produces depends on where you place your finger on the string. If you touch the string only very lightly and play it with a bow, you can create overtones; 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.”

“Optical frequency combs are used in labs around the world for very precise measurements of time, and to measure distances”, Norte says. “They are so important to measure­ments in general that their invention was given a Nobel Prize in 2005. We have made an acoustic version of a frequency comb, made out of sound vibrations in the membrane instead of light. Acoustic frequency combs could for instance make position measurements in opaque materials, through which vibrations can propa­gate better than light waves. This technology could for example be used for precision measure­ments underwater to monitor the Earth’s climate, for medical imaging and for appli­cations in quantum technologies.” (Source: TU Delft)

Reference: M. H. J. de Jong et al.: Mechanical overtone frequency combs, Nat. Commun. 14, 1458 (2023); DOI: 10.1038/s41467-023-36953-8

Link: Dept. of Precision and Microsystems Engineering, Delft University of Technology, Delft, The Netherlands