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Array of coupled nanoresonators

Final Report Summary - ACRES (Array of coupled nanoresonators)

Mechanical resonators have been an important part of our society for more than a century. Recently, a lot of interest on nanomechanical resonators has arisen. These mechanical resonators are nanoscale cantilevers or planks clamped at one or both ends. They act like tuning forks, vibrating at a signature resonant frequency or 'resonant peak'. When a particle becomes attached to the resonator the change in mass changes the resonant peak, allowing mass detection applications. The smaller the mass of the resonator the more sensitive or responsive to mass they become.

The 'Arrays of coupled nano-resonators' (ACRES) project has aimed to expand the typical geometries and configurations used as nanomechanical resonators in the past 20 years and start looking into arrays of coupled devices, operation beyond the linear regime and parametric excitation.

For example, two resonators can be 'coupled' by connecting them with a very narrow bridge. This project has looked at the behaviour of such systems, which indeed will have different resonant modes, i.e. collective modes. For two coupled resonators, they can either both move up and down simultaneously - 'in phase' - or one can move up as the other moves down - 'out of phase'. A mass landing on one resonator in a system of two resonators in phase will lead to uncoupled dynamics so that the two resonators no longer oscillate with the same amplitude. Alternatively, mass landing uniformly over both resonators can be detected twice better than using a single device (and this can be extended to N times better if using N coupled devices).

ACRES has identified a series of limiting factors in the use of coupled systems. The most important one is based on fabrication inaccuracies, which are unavoidable and a consequence of the type of machines and equipment used to perform the nanofabrication. In order to have a system highly responsive to mass, dimensions need to be reduced. But fabrication inaccuracies remain the same, therefore variations in the natural resonant frequency of individual devices are more pronounced when decreasing the dimensions. These variations in frequency require either a stronger coupling or active individual tuneability of each device in order to use coupled dynamics in the final system, in other words it makes much harder the use of coupled systems.

As a consequence of the difficulties encountered, ACRES has developed the fabrication of NEMS devices with piezoelectric actuation and piezometallic detection. These devices' frequency can be individually tuned via both piezoelectric load or via local heating using the metallic loop for detection. This tuneability, together with a very efficient transduction (actuation and detection) made this type of devices the chosen ones to realise most of the experimental work along the project.

Along the way, ACRES included in its research interests the study of not only coupled resonators, but also nonlinear and parametrically driven resonators. This has produced a number of interesting results, some of them defying conventional wisdom and others pioneering novel topologies.