How proximity steals energy from nanoresonators Stephanie Baum Scientific Editor Andrew Zinin Chief Editor Nanomechanical resonators are miniature vibrating structures on chips that oscillate at frequencies ranging from a few kilohertz to gigahertz. They are used as ultrasensitive detectors of mass and force, temperature and pressure, and as components in radio frequency filters and on-chip clocks. Modern, state-of-the-art resonators are also used to create quantum states of macroscopic objects and test fundamental physics.

Many applications require placing the resonators close to other materials to read out the motion or interaction with other phenomena. The high coherence of these devices boosts the performance of most applications, but it also creates a new challenge: Even without physical contact, nearby dielectrics can introduce additional energy loss. This extra damping reduces the quality factor and sets practical limits on how close other structures can be brought without degrading performance.

Scientists in the group of Tobias J. Kippenberg at EPFL have now shown that simply bringing these resonators close to insulating materials can reduce their performance. The research is published in Nature Physics.

How trapped charges drain energy The reason lies in static electric charges that can be trapped in the resonator. As the resonator vibrates, it creates a changing electric field in the space around it. If a nearby material, such as silicon dioxide or silicon nitride, has small electrical losses, that field causes energy to dissipate inside it.

The two objects never touch, yet energy leaks away. This effect is related to so-called "noncontact friction," a phenomenon previously observed in atomic force microscopy. The researchers built a model that predicted a clear signature: Lower-frequency vibrations should lose more energy.

They tested this using silicon nitride strings suspended about 500 nanometers above a dielectric layer and measured how quickly different vibration modes faded. The lowest-frequency modes showed extra loss, exactly as predicted. A sharper limit at small gaps But a surprise came in a second experiment, where the scientists designed strings to have a high Q factor and placed them between photonic crystal cavities with gaps of a few hundred nanometers.

As the gap narrowed, the quality factor dropped, in some cases by up to a factor of 10. The techniques developed by the researchers in this work allowed them to accurately model noncontact friction from the trapped charges in the complex geometry. The findings set new design constraints for ultracoherent nanomechanical systems.

Devices that rely on close proximity to other components must account for noncontact friction caused by trapped charges, which can reduce mechanical coherence. A hidden loss that can be useful At the same time, the same mechanism can serve as a tool, helping probe dielectric losses in thin films or enabling controlled coupling to other electric systems. As these resonators move toward more advanced sensing and quantum technologies, understanding and controlling such hidden sources of loss will be essential.