News: Putting a damper on nanoresonators.

The harmonic oscillator holds a special place in the history of science and technology, having an important role in the development of both classical and quantum physics. Examples include Galileo’s pendulum, resonant electrical circuits and molecular vibrations. In micro and nanomechanical systems, the prototypical harmonic oscillator is a mechanical resonator — a beam of material that oscillates in response to an externally applied force. In an ideal world, or at least a world in which there is no dissipation of energy, the oscillations would continue indefinitely after energy had been supplied to the resonator. In the real world, however, energy is sucked out of the system, which leads to damping and the eventual disappearance of oscillations. For many systems this damping force is found to be linearly proportional to velocity and independent of the amplitude of the oscillations. Writing in Nature Nanotechnology, Adrian Bachtold and coworkers describe how damping in carbon nanotube and graphene resonators follows a different paradigm — it depends strongly on the amplitude of motion1. Mechanical resonators are potential candidates for applications such as mass sensing, quantum motion detection and radiofrequency signal processing. Device performance improves as the resonant frequency increases and the resonator mass decreases, so researchers have developed smaller and smaller resonators over the past several decades. This has now culminated with the development of carbon nanotube and graphene resonators — the ultimate limits of 1D and 2D nanomechanical structures. Not only are carbon nanotubes and graphene the thinnest materials in the world, they are also the stiffest and strongest, making them promising materials for a variety of mechanical applications2,3. However, energy losses in these atomically thin resonators are notoriously high at room temperature, so it is essential to understand and minimize the damping in these systems if we are to make the most of their remarkable potential4. Bachtold and co-workers — who are based at the Catalan Institute of Nanotechnology and the Technical University of Munich — measured how the damping changed as the amplitude of the oscillations was increased for three different resonator setups: nanotubes under tensile stress; nanotubes with slack; and graphene under tensile stress (Fig. 1). In all three cases, and for a range of temperatures from room temperature down to 90 mK, they found that the damping increased with increasing amplitude of motion. Before this work, there were theoretical results describing the importance of nonlinear dynamics on nanomechanical resonators5, and experimental results on the first graphene resonators had suggested that nonlinear damping might have an important role6. The present work puts the hypothesis of nonlinear damping in carbon nanotube and graphene resonators on a firm experimental footing, and represents an important step towards understanding damping in atomically thin resonators. However, the exact physical mechanism responsible for the damping is not fully understood. There are also practical obstacles to overcome before these nanoscale resonators can live up to their potential. Mass production of consistent and reliable devices is one challenge, although there has been significant progress in this area7–9, and detecting and actuating their motion — especially at higher resonant frequencies — is another. In the first experiments on nanotube resonators, the motion was detected in an electron microscope, whereas optical detection was used for the first graphene resonators6,10. However, both of these techniques are costly and bulky. Electrical actuation and detection is preferable, but small resonators with high resonant frequencies typically generate only small electrical signals, and this — combined with high resistances and large parasitic capacitances — makes electrical detection difficult. Bachtold and co-workers used an all-electrical electromechanical mixing technique that was developed at Cornell University for measuring carbon nanotube resonators11 and was later applied to graphene resonators by researchers at Columbia University12. Direct electrical readout of a graphene resonator operating at tens of megahertz was demonstrated by the Columbia team last year using a local gate to minimize the parasitic capacitance13. NEMS