The image shows an illustration of a triangline nanomechanical resonator realised in a piezoelectric material. The central part of the resonator is shaped like a triangle that moves up and down and at the same time acts as a mirror to reflect a laser beam.  The resonator is suspended by thin tethers, which are branched to minimise the loss of mechanical energy from the triangline’s movement. Credit: Chalmers University of Technology | Boid.

Researchers in Sweden and Germany have developed high-quality nanomechanical resonators with built-in piezoelectricity

Researchers at Chalmers University of Technology in Sweden and at the University of Magdeburg in Germany have developed a novel type of nanomechanical resonator that combines two important features: high mechanical quality and piezoelectricity.

This development could open doors to new possibilities in quantum sensing technologies.

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 and 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 nanometre scale.

At these tiny sizes, resonators oscillate at much higher frequencies and exhibit a greater sensitivity compared to their macroscopic counterparts.

Project leader of the study Witlef Wieczorek, Professor of Physics at Chalmers University of Technology, said: “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.”

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.

The quest

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 the University of Magdeburg have made a big leap as they demonstrated a nanomechanical resonator made of tensile-strained aluminium nitride, a piezoelectric material that maintains a high mechanical quality factor.

Lead author Anastasiia Ciers, research specialist in quantum technology at Chalmers, said: “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 utilised for interfacing mechanical and electric degrees of freedom, which is relevant in the transduction of information, even down to the quantum regime.”

The aluminium nitride resonator achieved a quality factor of more than ten million.

Wieczorek said: “This suggests that tensile-strained aluminium nitride could be a powerful new material platform for quantum sensors or quantum transducers.”

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.

The study is published in Advanced Materials.