High-quality nanomechanical resonators with built-in piezoelectricity

Mechanical resonators have been used for centuries for a multitude of applications. A key aspect of these devices is their ability to vibrate at specific frequencies. A well-known example is the tuning fork. When struck, the tuning fork oscillates at its resonance frequency, producing a sound wave within our hearing range. With advancements in microfabrication techniques, researchers have been able to shrink mechanical resonators down to the micro- and nanometer scale. At these tiny sizes, resonators oscillate at much higher frequencies and exhibit a greater sensitivity compared to their macroscopic counterparts.

"These properties make them useful in precision experiments, for example for sensing minuscule forces or mass changes. Recently, nanomechanical resonators have raised significant interest among quantum physicists due to their potential use in quantum technologies. For example, the use of quantum states of motion would improve the sensitivity of nanomechanical resonators even further," says Witlef Wieczorek, Professor of Physics at Chalmers University of Technology and project leader of the study.

A common requirement for these applications is that nanomechanical resonators need to sustain their oscillation for long times without losing their energy. This ability is quantified by the mechanical quality factor. A large mechanical quality factor also implies that the resonator exhibits enhanced sensitivity and that quantum states of motion live longer. These properties are highly sought after in sensing and quantum technology applications.

In the quest for a material with a high-quality factor and built-in piezoelectricity

Most of the best-performing nanomechanical resonators are made from tensile-strained silicon nitride, a material known for its outstanding mechanical quality. However, silicon nitride is quite "boring" in other aspects: it doesn't conduct electricity, nor is it magnetic or piezoelectric. This limitation has been a hurdle in applications that require in-situ control or interfacing of nanomechanical resonators to other systems. To address these needs, it is then required to add a functional material on top of silicon nitride. However, this addition tends to reduce the mechanical quality factor, which limits the resonator's performance.

Now, researchers at Chalmers University of Technology and at the University of Magdeburg, Germany, made a big leap as they demonstrated a nanomechanical resonator made of tensile-strained aluminum nitride, a piezoelectric material that maintains a high mechanical quality factor.

"Piezoelectric materials convert mechanical motion into electrical signals and vice versa. This can be used for direct readout and control of the nanomechanical resonator in sensing applications. It can also be utilized for interfacing mechanical and electric degrees of freedom, which is relevant in the transduction of information, even down to the quantum regime," says Anastasiia Ciers, research specialist in quantum technology at Chalmers and lead author of the study published in Advanced Materials.

The aluminum nitride resonator achieved a quality factor of more than 10 million.

"This suggests that tensile-strained aluminum nitride could be a powerful new material platform for quantum sensors or quantum transducers," says Witlef Wieczorek.

The researchers now have two major aims: to improve the quality factor of the devices even further, and to work on realistic nanomechanical resonator designs that enable them to make use of the piezoelectricity for quantum sensing applications.

About the aluminum nitride-based nanomechanical resonators

The researchers used a highly stressed 295 nanometer-thin film of aluminum nitride for fabricating their nanomechanical resonators. The stress was about 1GPa, the equivalent of balancing two elephants on a fingernail. The researchers used this high stress in a technique called dissipation dilution, which boosts the mechanical quality factor. The aluminum nitride film was epitaxially grown on a silicon substrate, which ensures a high crystalline quality of the film to preserve the piezoelectricity of aluminum nitride. They created a novel resonator design, called triangline, that looks like a fractal-like structure with a central triangular-shaped pad. This triangline resonator can maintain a single quantum coherent oscillation at room temperature, which is an important benchmark for its application in quantum technology.